How to Calculate Cable Cross-Section: A Practical Sizing Guide for B2B Buyers

Q: How do I calculate the cable cross-section for a motor?

For a motor feeder: calculate the design current Ib = P ÷ (√3 × V × PF × η), then multiply by 1.25 for the motor cable sizing factor. Look up the tabulated current rating It from IEC 60364-5-52 for the installation method. Apply derating factors for ambient temperature (Ca) and grouping (Cg). The derated current Iz = It × Ca × Cg must be greater than or equal to Ib. Finally check voltage drop for the cable run length.

Q: What is a derating factor in cable sizing?

A derating factor reduces the cable's allowable current below the tabulated reference value to account for real installation conditions. The main derating factors are: ambient temperature factor (Ca) — higher temperatures reduce allowable current; grouping factor (Cg) — cables running in a group impede each other's heat dissipation. The derated allowable current is Iz = It × Ca × Cg, and this must be greater than or equal to the design current.

Q: What is the maximum acceptable voltage drop for industrial cables?

IEC 60364-5-52 recommends a maximum voltage drop of 4% for lighting circuits and 5% for other circuits, measured from the supply point to the most remote outlet. For industrial motor circuits, some manufacturers specify a minimum voltage at the motor terminals — check the motor datasheet if a tighter limit applies. For a 400V system, 5% corresponds to a maximum of 20V drop.

Q: Why does the grouping factor make such a big difference to cable sizing?

When multiple cables run alongside each other, they share heat and each cable's ability to dissipate heat is reduced. With 6 cables grouped on a tray, the grouping factor Cg is approximately 0.57 — meaning the cable can only carry 57% of its tabulated rating. Combined with an ambient temperature correction, the effective current capacity can be less than 50% of the tabulated value, requiring a significantly larger cross-section than a thermal calculation without derating would suggest.

Q: Does voltage drop or thermal capacity usually determine cable size?

For short cable runs with high load currents — motor feeders under 30m, main switchboard feeders — thermal capacity (with derating applied) is usually the limiting factor. For long cable runs to lower-power loads — lighting circuits, instrumentation supplies, remote distribution boards — voltage drop often requires a larger conductor than the thermal calculation alone. Both constraints must be checked independently, and the larger cross-section required by either is the one to specify.

Q: Can I use the same cable sizing for copper and aluminum conductors?

No. Aluminum has approximately 61% of the electrical conductivity of copper, so an aluminum conductor requires approximately 1.6 times the cross-section of copper to carry the same current. The tabulated current ratings in IEC 60364-5-52 are given separately for copper and aluminum conductors. Always use the correct table for the conductor material specified — applying copper ratings to an aluminum cable will result in an undersized conductor.

A step-by-step explanation of cable cross-section sizing — how design current, installation derating, and voltage drop determine the conductor size required for an industrial or infrastructure cable installation.

One of the most common questions in B2B cable procurement is: how do I know what cross-section I need? The answer depends on four factors — the load current, how the cable is installed, the ambient temperature and grouping conditions, and whether the cable is long enough for voltage drop to be a constraint.

This guide walks through the cable sizing process step by step, using IEC standards as the reference framework. It is written for procurement managers, project engineers, and B2B buyers who need to understand the sizing logic — either to write a specification, to check a supplier’s proposal, or to verify that a cable schedule produced by a designer is reasonable before ordering.

Note: Cable sizing for safety-critical installations should always be confirmed by a qualified electrical engineer. This guide explains the methodology and gives buyers the tools to understand and check sizing — it is not a substitute for professional engineering sign-off on a project specification.

Why Cable Cross-Section Matters

The cross-sectional area of a cable conductor (expressed in mm²) determines how much current the cable can carry continuously without the conductor temperature exceeding the insulation’s rated limit. Too small a cross-section and the cable overheats — insulation degrades, protection devices may not trip fast enough, and in severe cases fire results. Too large a cross-section and the cable is more expensive and heavier than necessary.

The sizing process is a balance between three constraints:

Current-carrying capacity: the cable must carry the design load current without overheating
Voltage drop: for long cable runs, the resistive voltage drop along the cable must stay within acceptable limits at the load terminals
Short-circuit withstand: the cable must survive the fault current flowing until the protection device operates — this is typically checked by the protection engineer rather than the cable buyer

In practice, for most B2B industrial and commercial cable specifications, current-carrying capacity and voltage drop are the two constraints that determine the final cross-section. Short-circuit withstand is usually handled automatically by the protection design.

Step 1: Determine the Design Current (Ib)

The design current Ib is the maximum continuous current the cable is expected to carry under normal operating conditions. For different load types, Ib is calculated differently:

Motors (single motor feeder)

For a motor feeder, the design current is the motor’s full-load current (FLC), which can be read from the motor nameplate or calculated from the motor rated power:

Three-phase motor: Ib = P ÷ (√3 × V × PF × η)
P = rated motor power in watts (W)
V = line-to-line supply voltage (V)
PF = power factor (typically 0.80–0.90 for industrial motors)
η = motor efficiency (typically 0.88–0.95 for modern IE3 motors)
Example: 75kW motor, 400V, PF 0.85, efficiency 0.92 → Ib = 75000 ÷ (1.732 × 400 × 0.85 × 0.92) = 138A

Tip: For motor feeders, add a 25% margin to the calculated FLC when specifying the cable — motor cables are sized to 125% of FLC per IEC 60364-4-43 to account for starting current thermal effects and to match the motor protection device setting.

Distribution feeders (multiple loads)

For a feeder supplying multiple loads, the design current is the sum of the individual load currents multiplied by a diversity factor — typically 0.5–0.8 depending on how many loads are likely to operate simultaneously:

Ib = (sum of individual load currents) × diversity factor
Diversity factor of 1.0: all loads operate simultaneously at full load — use for safety-critical circuits or where simultaneous operation is confirmed
Diversity factor of 0.7–0.8: typical for general industrial distribution where partial simultaneous loading is expected
Diversity factor of 0.5–0.6: commercial or office distribution where load diversity is high

Transformer secondary feeders

For the main LV feeder from a transformer, the design current is the transformer’s full-load secondary current:

Ib = transformer kVA rating × 1000 ÷ (√3 × secondary voltage)
Example: 1000kVA transformer, 400V secondary → Ib = 1,000,000 ÷ (1.732 × 400) = 1443A

Step 2: Select the Tabulated Current Rating (It)

The tabulated current rating It is the maximum current a cable of a given cross-section can carry under standardized reference conditions — in free air at 30°C ambient temperature, with cables not grouped. These values are published in IEC 60364-5-52 (wiring installations) for each combination of cable type, conductor material, insulation, and installation method.

Cross-Section (mm²)	Method B — Conduit/enclosed (A)	Method C — Clipped to surface (A)	Method E — Free air / tray (A)	Method D — Direct burial (A)
16mm²	76	87	103	95
25mm²	99	114	136	122
35mm²	119	138	164	146
50mm²	141	163	195	173
70mm²	179	207	247	218
95mm²	216	249	298	261
120mm²	249	286	346	298
150mm²	283	328	395	338
185mm²	323	371	450	382
240mm²	375	436	538	444
300mm²	430	500	621	506

Reference values for 4-core copper XLPE cable, 30°C ambient, no grouping. Source: IEC 60364-5-52. Apply derating factors for actual installation conditions. Always verify against the applicable edition of the standard for project use.

To select the correct tabulated value:

Select the installation method reference letter from IEC 60364-5-52 Table B.52.1 (e.g. Method B = enclosed in conduit in thermally insulating wall, Method C = clipped to a surface, Method E = free air)
Look up the tabulated current It for the required installation method and conductor type
The selected cross-section must have It ≥ Ib before derating is applied

Key Point: The installation method letter changes the tabulated current significantly. The same 95mm² copper XLPE cable has a tabulated rating of approximately 238A on a cable tray in free air (Method E) but only about 167A when enclosed in conduit (Method B). Always use the tabulated value for the actual installation method — not the highest available value.

Step 3: Apply Derating Factors

The reference conditions for tabulated current ratings (30°C ambient, cables not grouped) rarely match the actual installation conditions. Correction factors — called derating factors — are applied to reduce the tabulated rating to the actual allowable current under real conditions.

Ambient Temperature Factor (Ca)

If the ambient temperature around the cable is higher than 30°C (for XLPE cable), the cable’s allowable current is reduced. IEC 60364-5-52 Table B.52.14 gives correction factors for different ambient temperatures:

XLPE cable: reference temperature 30°C, rated conductor temperature 90°C
Ca at 35°C = 0.96, at 40°C = 0.91, at 45°C = 0.87, at 50°C = 0.82
PVC cable: reference temperature 30°C, rated conductor temperature 70°C
Ca at 35°C = 0.94, at 40°C = 0.87, at 45°C = 0.79, at 50°C = 0.71

For installations in hot climates (Middle East, tropical regions) or near heat-generating equipment (furnaces, boiler rooms), ambient temperature correction is significant. A 50°C ambient reduces an XLPE cable’s allowable current to 82% of the tabulated value.

Grouping Factor (Cg)

When multiple cables run alongside each other on a tray or in a duct, they impede each other’s heat dissipation. The grouping factor Cg reduces the allowable current for cables in a bundle or group:

2 cables: Cg = 0.80
3 cables: Cg = 0.70
4 cables: Cg = 0.65
6 cables: Cg = 0.57
9 cables: Cg = 0.50

Note: These grouping factors apply when cables are touching or in close contact. Cables spaced by one cable diameter or more have reduced mutual heating — IEC 60364-5-52 provides separate factors for spaced installation. For large cable schedules with many circuits on a common tray, grouping derating is often the dominant factor in the sizing calculation.

Combining Derating Factors

The corrected allowable current Iz for the installed cable is:

Iz = It × Ca × Cg
The cable cross-section must be selected such that Iz ≥ Ib (design current)
Example: 4-core 95mm² XLPE copper cable, Method C (clipped to surface), It = 220A, ambient 40°C (Ca = 0.91), 4 cables grouped (Cg = 0.65)
Iz = 220 × 0.91 × 0.65 = 130A
If design current Ib = 127A → 95mm² is adequate (130A ≥ 127A, margin is small but sufficient)
If design current Ib = 135A → upsize to 120mm² (recalculate Iz for 120mm²)

Step 4: Check Voltage Drop

For longer cable runs, the resistive drop along the cable may cause the voltage at the load terminals to fall below acceptable limits — even if the cable is thermally adequate. Voltage drop is checked after the thermal sizing is completed.

The voltage drop in a three-phase cable run is calculated as:

ΔV (volts) = √3 × Ib × L × (r × cosφ + x × sinφ) ÷ 1000
Ib = design current (A)
L = cable run length (m)
r = conductor resistance at operating temperature (mΩ/m per phase) — from cable datasheet
x = conductor reactance (mΩ/m per phase) — from cable datasheet, typically 0.07–0.09 mΩ/m for LV cables
cosφ = load power factor, sinφ = √(1 – cos²φ)

The percentage voltage drop is:

ΔV% = ΔV ÷ nominal voltage × 100
Acceptable limits: IEC 60364-5-52 recommends maximum 4% voltage drop from the supply point to the most remote outlet for lighting circuits, and 5% for other circuits
For motor starters: some manufacturers specify a minimum voltage at the motor terminals during starting — check if a tighter limit applies

Cross-Section (mm²)	Resistance r (mΩ/m)	Reactance x (mΩ/m)	VD at PF 0.85 (mV/A/m)	Max. run at 100A, 5% VD (m)
16mm²	1.15	0.082	1.70	~118m
25mm²	0.727	0.080	1.09	~183m
50mm²	0.366	0.079	0.563	~355m
95mm²	0.193	0.076	0.308	~649m
150mm²	0.124	0.074	0.210	~952m
240mm²	0.0778	0.074	0.153	~1307m
300mm²	0.0620	0.073	0.128	~1563m

Approximate values for copper XLPE cable at 90°C operating temperature, three-phase system. VD calculated at PF 0.85. Max. run at 100A is indicative for 400V system with 5% voltage drop limit (20V). Verify against cable datasheet for actual resistance and reactance values.

Key Point: Voltage drop is most likely to be the limiting factor on long cable runs supplying low-power loads. For a 100m run to a 10kW load, voltage drop may require a larger conductor than the thermal calculation alone. For a 10m run to a 200kW motor, thermal sizing dominates. Always check both constraints — they apply independently and the larger cross-section required by either constraint is the one to specify.

Worked Example: Sizing a Motor Feeder Cable

To illustrate the four-step process, here is a complete worked example:

Scenario: 90kW induction motor, 400V three-phase, power factor 0.85, motor efficiency 0.92. Cable run length 60m. Installation: clipped to surface (Method C). Ambient temperature 40°C. Cable runs with 3 other cables of similar size. Conductor: copper, XLPE insulation.

Step 1: Design current

Ib = 90,000 ÷ (1.732 × 400 × 0.85 × 0.92) = 165A
Apply 125% motor cable factor: Ib(design) = 165 × 1.25 = 207A

Step 2: Tabulated current

Method C (clipped to surface), 4-core copper XLPE: select 150mm² → It = 261A from IEC 60364-5-52

Step 3: Apply derating

Ca (40°C, XLPE) = 0.91
Cg (4 cables grouped) = 0.65
Iz = 261 × 0.91 × 0.65 = 154A
154A < 207A (Ib) → 150mm² is insufficient. Upsize to 185mm²
185mm² It = 300A → Iz = 300 × 0.91 × 0.65 = 177A
177A < 207A → still insufficient. Upsize to 240mm²
240mm² It = 340A → Iz = 340 × 0.91 × 0.65 = 201A
201A < 207A → marginally insufficient. Upsize to 300mm²
300mm² It = 380A → Iz = 380 × 0.91 × 0.65 = 225A
225A ≥ 207A → 300mm² passes thermal check

Step 4: Voltage drop check

Using 300mm² copper: r ≈ 0.075 mΩ/m, x ≈ 0.074 mΩ/m at operating temperature
ΔV = 1.732 × 165 × 60 × (0.075 × 0.85 + 0.074 × 0.527) ÷ 1000
ΔV = 1.732 × 165 × 60 × (0.0638 + 0.039) ÷ 1000 = 1.76V
ΔV% = 1.76 ÷ 400 × 100 = 0.44% — well within 5% limit
Result: specify 4-core 300mm² copper XLPE cable (Method C, 60m run, 40°C ambient, 4 cables grouped)

Note: This worked example demonstrates why grouping derating is so significant — the required cross-section jumped from what might initially seem a 120mm² thermal calculation to 300mm² once ambient temperature and grouping are properly applied. Buyers should be cautious of cable schedules that do not show the derating calculation — a schedule showing only ‘load current’ without confirming the derated capacity may be undersized.

Common Sizing Mistakes to Avoid

Using free-air tabulated values for buried or conduit installations — always match the tabulated value to the actual installation method
Ignoring grouping derating for cables on a shared tray — a tray with 8 cables may require double the cross-section of an isolated cable for the same load
Not checking voltage drop on long runs — thermal adequacy does not guarantee acceptable voltage at the load terminals
Specifying cross-section without specifying installation conditions — a cable schedule that shows only ‘mm²’ without recording the installation method and ambient temperature cannot be verified
Using the same cross-section for copper and aluminum feeders without adjusting for conductivity difference — aluminum requires approximately 1.6× the cross-section of copper for the same current
Forgetting to apply the 125% factor for motor feeders — motor cables sized at exactly the FLC will be undersized for protection device coordination

The Sizing Checklist

When providing a cable specification for a B2B inquiry, include the following sizing parameters to allow the supplier to confirm the cross-section is appropriate and to compare quotations on a like-for-like basis:

Design current Ib (A) — the maximum continuous load current
Installation method (IEC 60364-5-52 method letter or description)
Ambient temperature at installation location (°C)
Number of cables grouped on the same tray or in the same duct
Cable run length (m) — for voltage drop check
Load power factor (for voltage drop calculation)
Maximum acceptable voltage drop (% of supply voltage)
Conductor material: copper (Cu) or aluminum (Al)
Insulation type: XLPE or PVC

Providing these parameters with the cable inquiry allows RichingPower to confirm the specified cross-section is correct, or to advise on a different cross-section if the sizing is found to be inadequate or oversized for the application.

Quotation Requirements

RichingPower supplies industrial power cables in a full range of cross-sections from 1.5mm² to 630mm² in both copper and aluminum conductors. To receive an accurate quotation confirming that the specified cross-section meets your installation requirements, please provide:

Design current (A) and load type (motor, distribution feeder, transformer secondary)
Installation method and ambient temperature
Grouping conditions (number of cables on common tray or in duct)
Cable run length and maximum acceptable voltage drop
Conductor material, insulation type, armoring requirement, and voltage grade
Applicable standard and total quantity

Submit your cable sizing requirement via the RichingPower contact page. If you are unsure of the correct cross-section, provide the load data and installation conditions and our technical team will advise on the appropriate sizing before preparing a quotation.

Conclusion

Cable cross-section sizing is a four-step process: calculate the design current, look up the tabulated current rating for the installation method, apply derating factors for ambient temperature and grouping, then verify that voltage drop is within limits for the run length. The cross-section selected must satisfy both the thermal constraint (derated current ≥ design current) and the voltage drop constraint independently.

For B2B procurement, understanding the sizing methodology allows buyers to write complete specifications, check existing cable schedules for adequacy, and compare supplier quotations on a technically consistent basis. A specification that includes design current, installation conditions, and cable run length gives suppliers all the information needed to confirm or adjust the cross-section — and gives buyers a basis for rejecting under-specified proposals.

For guidance on reading cable specification fields, see How to Read a Cable Specification Sheet. For current-carrying capacity reference values comparing copper and aluminum, see

Copper vs Aluminum Cable Conductor: Which Should You Specify?. Contact RichingPower with your project specification for a quotation.

Frequently Asked Questions

QHow do I calculate the cable cross-section for a motor?

ACalculate design current Ib = P ÷ (√3 × V × PF × η), then multiply by 1.25 for the motor sizing factor. Look up the tabulated rating It from IEC 60364-5-52 for your installation method. Apply derating factors: Iz = It × Ca × Cg. Iz must be ≥ Ib. Finally verify voltage drop for the cable run length.

QWhat is a derating factor in cable sizing?

AA derating factor reduces the cable's allowable current below the tabulated reference value to account for real installation conditions. Main factors: ambient temperature (Ca) — higher temperatures reduce capacity; grouping (Cg) — cables in a bundle impede heat dissipation. Derated current Iz = It × Ca × Cg must be ≥ design current.

QWhat is the maximum acceptable voltage drop for industrial cables?

AIEC 60364-5-52 recommends 4% maximum for lighting circuits and 5% for other circuits. For a 400V system, 5% = 20V maximum drop. Some motor manufacturers specify tighter limits at the motor terminals during starting — check the motor datasheet if a tighter limit applies to your application.

QWhy does the grouping factor make such a big difference to cable sizing?

AGrouped cables share heat and each cable's ability to dissipate it is reduced. With 6 cables grouped on a tray, Cg ≈ 0.57 — the cable carries only 57% of its tabulated rating. Combined with ambient temperature correction, effective capacity can fall below 50% of the tabulated value, requiring a much larger cross-section than a simple load calculation would suggest.

QDoes voltage drop or thermal capacity usually determine cable size?

AFor short runs with high currents (motor feeders under 30m, main feeders), thermal capacity with derating is usually the limiting factor. For long runs to lower-power loads, voltage drop often requires a larger conductor. Both constraints must be checked independently — the larger cross-section required by either is the one to specify.

QCan I use the same cable sizing for copper and aluminum conductors?

ANo. Aluminum has about 61% of copper's conductivity and requires approximately 1.6× the cross-section for the same current. IEC 60364-5-52 tables are given separately for copper and aluminum. Always use the correct table for the conductor material — applying copper ratings to an aluminum cable produces an undersized conductor.

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