How Cable Crossing Angle and Separation Affect Underground Cable Ampacity
When two underground power cable circuits cross, the crossing point can become a local thermal constraint on the cable system. This matters because cable crossing ampacity is ultimately limited by conductor temperature: when nearby circuits add heat to the surrounding soil, the conductor may reach its permissible operating temperature at a lower current than it would in an isolated section of the route.
The important variables are not limited to current and cable size. Crossing angle, vertical separation, burial depth, cable arrangement, soil thermal resistivity, thermal backfill, conductor losses, screen losses, and the loading of adjacent circuits can all influence the resulting thermal condition.
IEC 60287-1-1:2023 provides the general equations for steady-state cable current rating and losses, while IEC 60287-3-3:2007 specifically provides a method for calculating the continuous current-rating factor when cables cross external heat sources. The IEC method is applicable to any type of cable, subject to the assumptions defined by the standard.
This guide explains the mechanism behind cable crossing ampacity, how crossing geometry affects thermal interaction, why separation is important, and how engineers can approach a cable crossing study without relying on oversimplified derating rules.
What Is Cable Crossing Ampacity?
Cable ampacity is the maximum continuous current that a cable can carry under specified conditions without exceeding its permissible conductor temperature.
For underground cables, ampacity is fundamentally a thermal problem.
The conductor produces heat because of electrical losses, with the basic resistive relationship expressed as:
P = I²R
where:
P = resistive power loss
I = current
R = conductor resistance
As current increases, conductor heating increases approximately with the square of current.
The heat then travels outward through the cable insulation, sheath or screen system, bedding, backfill, soil, and surrounding environment.
The steady-state ampacity is reached when the heat generated by the cable is balanced by the heat that can be dissipated while keeping the conductor at or below its maximum permissible temperature. IEC 60287-1-1:2023 defines its current-rating calculations for steady-state operation and includes cables buried directly in soil, installed in ducts, troughs or steel pipes, and cables in air.
Why a crossing changes the calculation
A cable in isolation has one primary heat source: itself.
At a crossing, there is another nearby source of heat.
That creates mutual heating.
A simple representation is:
Cable A → heat → surrounding soil ← heat ← Cable B
The additional heat changes the thermal environment around the crossing point.
As a result, the conductor in Cable A can reach its allowable temperature at a lower load current.
That is the fundamental reason a crossing can affect cable crossing ampacity.
Why Do Underground Cable Crossings Create Thermal Hotspots?
A cable crossing does not normally make the entire cable route uniformly hotter.
Instead, it can create a localized thermal hotspot around the point where the two circuits are closest.
Research published in IET Generation, Transmission & Distribution describes cable crossings as locations where hot spots can occur because the thermal environment becomes less favorable than along the rest of the route. The paper notes that these hot spots can become the limiting condition for ampacity and therefore require appropriate consideration in cable-rating calculations.
Heat generated by the first cable
The loaded cable produces conductor and other electrical losses.
Depending on the cable construction and installation, relevant losses can include:
Conductor losses
Metallic screen losses
Sheath losses
Dielectric losses where applicable
IEC 60287-1-1 provides the general framework for calculating cable current ratings and losses under steady-state conditions.
Heat generated by the second cable
The neighboring circuit behaves as an additional external heat source.
If both circuits are heavily loaded, the thermal interaction becomes stronger.
If one circuit is lightly loaded, its contribution may be smaller.
This means a cable crossing study should not automatically assume that all nearby cables operate at 100% load unless that is the specified design condition.
Why the crossing point matters
At the point of closest approach:
Thermal coupling is strongest
Away from that point:
Thermal interaction decreases
Practical engineering analyses describe the crossing temperature profile as a local peak with longitudinal heat flow carrying heat away from the crossing region.
This is why a relatively short crossing section can nevertheless become the controlling location for an otherwise much longer cable route.
How Does Crossing Angle Affect Underground Cable Ampacity?
The cable crossing angle changes the geometry of the two circuits and therefore changes the length over which they remain thermally close to each other.
Consider two simplified cases.
Small-angle crossing
If two cables cross at a shallow angle, they can remain close over a relatively long distance.
That creates a larger zone of thermal interaction.
Conceptually:
Small angle → longer interaction region → greater mutual heating potential
This is one reason shallow-angle crossings are often examined carefully in cable crossing studies.
Near-perpendicular crossing
At approximately 90°, the cables pass one another over a much shorter interaction zone.
The strongest thermal interaction is concentrated near the crossing location.
Conceptually:
Larger angle → shorter interaction region → more localized thermal interaction
A practical cable-engineering analysis from ELEK describes this relationship and notes that small-angle crossings can produce a longer thermal interaction length, while the interaction becomes more localized as the crossing approaches 90°.
Is a 90-degree crossing always dramatically better?
No.
This is an important qualification.
The effect of crossing angle is configuration-dependent. A 2022 peer-reviewed study examining underground cable crossings found that, under its particular modeled conditions, the crossing angle did not significantly affect ampacity once the angle was at or above 30°. The authors also found that cable arrangement and bedding conditions had important effects on the thermal solution.
Therefore, it would be inappropriate to publish a universal rule such as:
“90° always gives X% more ampacity than 45°.”
The correct engineering conclusion is:
Crossing angle changes the thermal geometry, but its quantitative effect must be evaluated for the actual cable configuration and thermal environment.
That distinction is important for technically credible content.
How Does Cable Separation Affect Cable Crossing Ampacity?
Cable separation controls how strongly two nearby circuits exchange heat through the surrounding medium.
The basic principle is straightforward:
Greater separation → weaker thermal coupling
Smaller separation → stronger mutual heating
This can make vertical separation an effective mitigation measure, particularly when cables would otherwise remain thermally close over a significant interaction region.
However, separation is not an isolated parameter.
Vertical separation
In a typical buried crossing, one circuit is often installed above the other.
Increasing the vertical distance reduces the direct thermal interaction between them.
But moving one circuit farther downward also changes its burial depth.
That creates a second thermal effect.
The separation-depth trade-off
Suppose a new cable is installed below an existing circuit.
Increasing vertical separation may:
Reduce mutual heating from the existing cable.
Increase the burial depth of the new cable.
The first effect can improve ampacity.
The second can alter heat dissipation to the surrounding environment.
A 2022 optimization study specifically examined this interaction and found that the effect of separation depends on the crossing geometry and thermal environment. It reported that optimized bedding and cable arrangement could significantly mitigate hotspot effects, while conductor and screen dimensions and screen-bonding methods also influenced the solution.
Therefore:
More separation is not automatically equivalent to proportionally higher ampacity.
The entire thermal geometry has to be evaluated.
Why Does Burial Depth Affect Cable Ampacity?
Burial depth matters because underground cable heat must travel through the surrounding thermal environment.
A deeper cable has a different thermal path to the surface and may operate under a different thermal resistance condition.
At a cable crossing, depth also interacts with separation.
For example:
Increase vertical separation
→ crossing cables are farther apart
but also potentially:
→ lower circuit is deeper
The final effect on cable crossing ampacity depends on which thermal mechanism dominates.
This is why a cable-crossing study should record both:
Vertical separation between circuits
Absolute burial depth of each circuit
and not treat them as interchangeable parameters.
How Does Soil Thermal Resistivity Affect Cable Crossing Ampacity?
Soil is part of the cable's thermal circuit.
The cable generates heat, and the surrounding soil determines how effectively that heat moves away.
The key parameter is soil thermal resistivity, generally expressed in:
K·m/W
A higher thermal resistivity means that heat encounters more resistance as it moves through the soil.
That can increase cable temperature and reduce allowable current.
A lower thermal resistivity generally provides a more favorable heat-transfer environment.
Thermal backfill
Engineered thermal backfill can be used to provide a more predictable thermal environment around underground cables.
The thermal properties of:
Native soil
Bedding material
Thermal backfill
Moisture conditions
can therefore influence the cable rating.
The 2022 IET study specifically investigated the influence of cable-bedding dimensions and thermal properties in the crossing region and showed that optimizing the thermal environment could substantially mitigate the hotspot effect.
What Is Mutual Heating Between Underground Cables?
Mutual heating is the temperature interaction that occurs when the heat generated by one cable raises the temperature of a neighboring cable.
It can occur between:
Parallel circuits
Crossing circuits
Adjacent cable systems
Power cables and other external heat sources
IEC 60287-3-3 specifically addresses cable current-rating calculations when cables cross external heat sources and states that the method is based on uniform thermal characteristics of the surrounding region and the principle of superposition. The standard also warns that this principle does not strictly apply to touching cables and that applying the method to touching cables can produce an optimistic result.
That last point is especially important.
Touching cables are a special case
If cables physically touch, the thermal boundary conditions are different from those assumed in a simple superposition approach.
Consequently, engineers should not blindly apply a standard crossing formula outside its stated assumptions.
For close or unusual geometries, a more detailed thermal model may be appropriate.
How Is Cable Crossing Ampacity Calculated?
The general calculation begins with the normal cable-rating problem and then accounts for the additional thermal interaction caused by the crossing.
IEC 60287-1-1:2023 provides the general current-rating equations and loss calculations for steady-state operation. IEC 60287-3-3:2007 then addresses the additional situation where cables cross external heat sources.
A practical calculation sequence is:
Define the cable construction.Record conductor material, conductor size, insulation, screen/sheath characteristics and relevant electrical losses.
Define the operating condition.Establish load current, load factor and any required contingency condition.
Define the installation geometry.Record cable formation, burial depth, crossing angle, vertical separation and horizontal spacing where relevant.
Define the thermal environment.Establish soil thermal resistivity, thermal backfill properties, ambient ground temperature and other relevant conditions.
Calculate the baseline cable rating.Determine ampacity away from the crossing.
Model the crossing condition.Include the neighboring cable or external heat source according to the applicable calculation method.
Locate the thermal hotspot.Identify the position with the highest conductor temperature.
Determine the limiting current.Reduce the current rating if necessary so the conductor remains within its permissible temperature.
Compare all route sections.The lowest valid ampacity among the analyzed conditions becomes the controlling rating for the circuit.
This is more robust than applying a generic “20% derating” or “50% derating” rule.
What Is a Cable Crossing Derating Factor?
A cable derating factor represents the reduction in current-carrying capability caused by conditions that are less favorable than the reference installation.
In a crossing study, the factor can be understood conceptually as:
Derated ampacity ÷ Reference ampacity
For example, if an isolated cable could carry 800 A under a defined reference condition and the crossing condition limits it to 680 A:
680 ÷ 800 = 0.85
The corresponding derating factor would be 0.85, or an effective 15% reduction relative to the reference rating.
This is an illustrative calculation only.
Actual derating must come from the applicable thermal model and specified installation conditions.
Why fixed percentages are risky
A published engineering article reports that cable crossing ampacity reductions can be substantial in some practical cases, but its case-study values should not be generalized to every cable system.
Different results can arise from:
Cable size
Voltage level
Conductor losses
Screen bonding
Cable arrangement
Crossing angle
Separation
Burial depth
Soil thermal resistivity
Load current
External heat sources
Therefore, a credible cable crossing study should report the assumptions behind the calculated derating factor.
How Cable Arrangement Affects Thermal Performance
Cable geometry matters even before two systems cross.
Common arrangements include:
Flat formation
Trefoil formation
Single-circuit arrangements
Multiple circuits
Different vertical and horizontal spacing configurations
The spacing between individual single-core cables affects their mutual thermal interaction.
At a crossing, these formation details continue to matter.
A crossing between two individual cables is thermally different from a crossing between complete multi-cable circuits.
The 2022 IET study specifically considered cable arrangement within the bedding and found that optimized cable placement could improve the thermal environment and increase ampacity relative to less favorable conventional arrangements.
How Conductor and Screen Design Affect Cable Crossing Ampacity
Cable crossing studies should not look only at conductor size.
Electrical losses also arise in metallic screens and sheaths depending on cable construction and bonding arrangement.
The IET research on cable crossings found that:
Conductor cross-sectional area
Metallic screen cross-sectional area
Screen bonding methods
can influence the optimal thermal solution.
This matters because two cables with similar conductor sizes can have different thermal performance when their overall construction and loss mechanisms differ.
For high-voltage and medium-voltage projects, cable construction should therefore be part of the thermal model rather than treated as a generic “240 mm² cable,” for example.
How Loading of the Existing Cable Affects the Crossing
An important question in a cable crossing study is:
How heavily loaded is the existing cable?
A new cable crossing an unloaded circuit does not have the same thermal environment as one crossing a neighboring circuit operating continuously near its design rating.
The heat source created by the neighboring circuit depends on its actual electrical losses.
Therefore, engineers may need to analyze:
Existing cable at normal load
Existing cable at design load
New cable at design load
Possible simultaneous maximum loading
Relevant contingency conditions
The appropriate loading assumptions should be defined by the project design basis rather than selected arbitrarily.
Why a Crossing Can Limit the Ampacity of an Entire Cable Route
Imagine a 500-meter underground cable route.
Most of the route is installed in favorable soil with ample thermal capacity.
One section crosses another heavily loaded circuit.
That short crossing becomes the hottest section.
The cable therefore cannot simply be rated according to the 499 meters of favorable installation.
The controlling condition is the worst thermal location.
This is a fundamental principle of cable rating:
The allowable current is governed by the most thermally restrictive part of the installation.
The IET research describes the ampacity as being limited by the most unfavorable thermal conditions along the cable route and identifies cable crossings as potential hotspot locations.
90° vs 45° vs 30° Cable Crossings
The angle comparison is frequently discussed, but it needs to be handled carefully.
Crossing Configuration | Typical Thermal Consideration |
Small angle, such as 15° | Longer region of close thermal interaction may occur |
30° | Geometry may still create appreciable interaction depending on separation and installation |
45° | Intermediate configuration; actual effect depends on thermal geometry |
60° | More localized interaction than shallow crossings in many configurations |
90° | Near-perpendicular crossing; interaction tends to be concentrated around the crossing point |
These are geometric tendencies, not universal derating values.
The peer-reviewed 2022 study cited earlier found that, under its particular assumptions, crossing angles of 30° and above did not significantly alter ampacity.
At the same time, practical cable-crossing engineering analyses emphasize the role of interaction length and geometry.
The correct takeaway is therefore:
Crossing angle matters, but its numerical effect depends on the complete thermal configuration.
How to Reduce Thermal Derating at a Cable Crossing
If a calculation shows inadequate ampacity, engineers can examine several mitigation options.
Increase the crossing angle
Where route constraints permit, increasing the angle can reduce the length over which the circuits remain close.
Increase vertical separation
Greater separation can reduce mutual thermal coupling, although its interaction with burial depth must also be evaluated.
Improve the thermal environment
Options can include appropriately designed thermal backfill or bedding.
Reconfigure cable placement
Changing the arrangement within the trench can improve heat dissipation.
Increase conductor size
A larger conductor can reduce electrical resistance and increase available current-carrying capacity, subject to the complete design.
Review screen or bonding configuration
Where applicable, reducing additional losses can improve thermal performance.
The 2022 optimization research found that thermal-environment and cable-arrangement optimization could increase the ampacity of the studied crossing systems by about 15% on average compared with the referenced conventional arrangement, without additional cooling equipment. That result is specific to the study's modeled conditions and should not be treated as a universal performance gain.
What Information Is Needed for a Cable Crossing Study?
A reliable cable crossing ampacity calculation needs substantially more information than cable size and voltage.
At minimum, collect:
Cable type
Number of cores
Conductor material
Conductor cross-sectional area
Insulation system
Screen or sheath construction
Screen bonding method
Operating voltage
Normal and design current
Load factor
Cable arrangement
Horizontal spacing
Vertical spacing
Burial depth
Crossing angle
Soil thermal resistivity
Backfill thermal properties
Ground temperature
External heat sources
Existing cable loading
For large projects, it may also be necessary to define seasonal ground conditions, wet and dry states, contingency operation, and multiple nearby circuits.
IEC 60287 and Cable Crossing Calculations
For engineers and technical buyers, the relationship between the main IEC documents is useful.
IEC 60287-1-1
IEC 60287-1-1:2023 provides the general equations for cable current rating and loss calculations under steady-state conditions. IEC states that the standard applies to AC cables and DC cables up to 5 kV under the specified installation conditions.
IEC 60287-3-3
IEC 60287-3-3:2007 is specifically titled:
Electric cables — Calculation of the current rating — Part 3-3: Sections on operating conditions — Cables crossing external heat sources
It describes a method for calculating continuous current-rating factors when cable crossings with external heat sources are involved. IEC lists it as a 2007 publication with a stability date of 2030.
The current IEC 60287 series page published in 2026 still includes IEC 60287-3-3:2007 in the series, alongside the newer 2023 editions of the general rating documents.
This distinction is useful for an evergreen engineering article because it shows that the general cable-rating equations and the specialized crossing methodology are related but serve different roles.
When Is an Advanced Thermal Model Needed?
IEC-based analytical calculations are highly useful, but unusual or congested cable routes may justify additional numerical modeling.
Examples include:
Multiple crossing circuits
Complex trench geometry
Multiple external heat sources
Nonuniform thermal backfill
Strongly varying soil properties
Closely spaced circuits
Touching or near-touching cables
Complex three-dimensional geometry
Recent CIGRE technical work continues to examine advanced 3D finite-element modeling for underground cable systems and crossings, reflecting the growing use of detailed thermal models for complex installations.
A practical approach is often:
IEC-based calculation for standard conditions → detailed numerical model where geometry or thermal interactions become too complex for simplified assumptions.
Common Mistakes in Cable Crossing Ampacity Calculations
Treating all crossings as parallel cables
This can produce unnecessarily conservative results because a true crossing is not thermally equivalent to two cables running side by side for the entire route. Practical engineering tools explicitly distinguish crossing geometry from long parallel interaction.
Using one fixed derating percentage
A 20%, 30%, or 50% reduction is not a universal rule.
The result depends on the thermal model.
Ignoring burial depth
Increasing separation can also increase the depth of one circuit.
Ignoring the existing cable's loading
A neighboring cable is not the same heat source at 20% load and 100% load.
Using nominal soil properties without checking the project
Thermal resistivity can materially affect heat dissipation.
Ignoring screen and sheath losses
For many MV and HV cable systems, these losses can contribute to the thermal balance.
Treating touching cables as an ordinary crossing
IEC 60287-3-3 explicitly notes a limitation of the superposition principle for touching cables.
Looking only at the cable crossing
The crossing may not be the only thermal bottleneck.
A complete route study should compare:
Normal sections
Crossing points
Duct sections
Trench sections
High thermal-resistivity areas
Other external heat-source locations
Practical Cable Crossing Design Checklist
Before approving an underground cable crossing, verify:
Crossing angle
Vertical separation
Horizontal spacing
Burial depth
Cable formation
Conductor size
Cable construction
Screen/sheath losses
Screen bonding
Existing-circuit loading
Soil thermal resistivity
Thermal backfill
Ground temperature
External heat sources
Maximum conductor temperature
Applicable IEC calculation method
A documented calculation should also state its assumptions so another engineer can reproduce or review the result.
How FRCABLE Can Support Cable Specification
For FRCABLE manufacturers, a cable-crossing problem is not only a question of selecting a conductor size.
Project engineers and procurement teams may need to coordinate:
Cable construction
Conductor cross-section
Voltage class
Screen construction
Technical data
Loss characteristics
Installation requirements
Test documentation
Project-specific specifications
For complex underground applications, FRCABLE can provide project-specific cable technical information so engineers can evaluate the cable's electrical and thermal characteristics within the overall installation design.
For procurement, buyers should provide the manufacturer with the intended voltage level, conductor size, installation environment, required standard, operating conditions, and project-specific cable configuration before requesting a final technical recommendation.
Frequently Asked Questions
What is cable crossing ampacity?
Cable crossing ampacity is the continuous current-carrying capability of a cable under the thermal conditions created when it crosses another cable or external heat source. The crossing can increase local temperature through mutual heating and reduce the available current rating.
How does crossing angle affect cable ampacity?
Crossing angle changes the geometry and length of thermal interaction between two cable circuits. Shallow angles can produce a longer region of close proximity, while near-perpendicular crossings tend to localize the strongest interaction. The actual impact depends on cable configuration, separation and thermal conditions.
Does a 90-degree cable crossing have lower thermal interaction?
A 90-degree crossing generally creates a shorter interaction region than a shallow-angle crossing. However, the resulting ampacity depends on cable spacing, burial depth, soil thermal properties, loading and the specific calculation method.
Does increasing cable separation increase ampacity?
Increasing separation generally reduces mutual heating between nearby cables. However, increasing vertical separation can also increase burial depth, so the net effect must be evaluated as part of the complete thermal model.
How far apart should underground power cables be?
There is no universal separation distance that guarantees a particular ampacity. Required spacing depends on cable construction, current, soil thermal resistivity, burial depth, cable arrangement, crossing geometry and the applicable design method.
Why does a cable crossing create a thermal hotspot?
A crossing creates a localized region where two cable heat sources are close together. The additional heat raises the local thermal environment and can cause conductor temperature to peak at or near the crossing.
What is the role of soil thermal resistivity in cable ampacity?
Soil thermal resistivity determines how easily heat travels away from an underground cable. Higher thermal resistance generally makes heat dissipation more difficult and can reduce cable ampacity.
What is IEC 60287-3-3?
IEC 60287-3-3:2007 is the IEC document titled Electric cables — Calculation of the current rating — Part 3-3: Sections on operating conditions — Cables crossing external heat sources. It provides a method for calculating continuous current-rating factors in cable-crossing situations.
Is IEC 60287-3-3 still relevant?
Yes. The current IEC 60287 series published in 2026 continues to include IEC 60287-3-3:2007, while IEC 60287-1-1:2023 is the current general standard for current-rating equations and loss calculations.
Can I use one derating factor for every cable crossing?
No. Derating is configuration-dependent. Cable size, construction, loading, separation, angle, burial depth, soil properties and external heat sources can all change the result.
Does cable size affect crossing ampacity?
Yes. Conductor resistance, conductor temperature and heat generation all influence the thermal rating. The cable's complete construction, including screens or sheaths where applicable, should also be considered.
Can cable crossings be analyzed using software?
Yes. Specialized cable-rating and thermal-analysis software can model complex cable arrangements. For difficult three-dimensional geometries or multiple thermal interactions, numerical methods such as finite-element analysis may also be considered. CIGRE's recent technical program includes advanced 3D FEM work on underground cable systems and crossings.
Conclusion
Cable crossing ampacity is fundamentally a thermal interaction problem.
When underground power cables cross, each circuit can act as an external heat source for the other. The resulting temperature rise depends on the complete thermal environment, including crossing angle, cable separation, burial depth, cable arrangement, soil thermal resistivity, thermal backfill, conductor and screen losses, and circuit loading.
Planning an underground cable project? Contact FRCABLE with your cable voltage, conductor size, installation arrangement, crossing geometry, separation distance and applicable standard requirements for a project-specific cable specification.
Comments