What Size Cable Do You Need for a 400 Amp Supply?

For a 400 amp supply, use 185mm² copper cable with PVC insulation under BS 7671 Table 4D1A, rated at 415A clipped direct. Under NEC, 500 kcmil at 75°C provides 380A — parallel runs of 2×250 kcmil are common. At this current level, parallel cable runs or busbar trunking are often more practical than single large conductors.

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Quick Reference: Cable Size for 400 Amp Industrial Mains by Standard

Standard	Cable Size	Conditions	Reference
BS 7671	185mm²	Method C or E, PVC insulation	Table 4D1A
NEC	500 kcmil	THWN-2, 75°C column	Table 310.16
AS/NZS 3008	150mm²	Clipped direct, PVC insulation	Table 13
IEC 60364	185mm²	Reference method C or E, PVC	Table B.52.4

Values shown are for copper conductors in typical domestic/commercial conditions. Actual size depends on installation method, ambient temperature, and cable length. Use our free cable sizing calculator for your exact scenario.

Typical Applications for a 400 Amp Industrial Mains

Industrial main switchboard
Large commercial building supply
Data centre main feed
Manufacturing plant mains

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These are general guidelines. Your specific installation needs exact calculation accounting for cable route length, installation method, ambient temperature, and cable grouping.

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Frequently Asked Questions

Is a single 185mm² cable practical for 400 amps?▼

While technically possible, 185mm² cable is heavy and has a large bending radius. Parallel runs of 2×95mm² or 2×120mm² per phase are often preferred for easier installation and termination.

Can I use aluminium cable for 400 amps?▼

Yes. Aluminium 300mm² PVC cable is rated at approximately 400A and is significantly lighter and cheaper than copper. At 400A, aluminium is the industry standard choice for long cable runs.

What is 500 kcmil in mm²?▼

500 kcmil equals 253mm², between 240mm² and 300mm² in metric sizing. In practice, NEC installations at 400A often use parallel 250 kcmil conductors (2 per phase) rather than a single 500 kcmil.

Should I use busbar trunking instead of cable for 400A?▼

For short runs within a building (under 30m), busbar trunking is often more cost-effective and flexible than 400A cable. It offers easy tap-off points for sub-distribution and simpler future modifications.

What size earth conductor for a 400 amp circuit?▼

Under BS 7671 Table 54.7, the minimum CPC for 185mm² line conductors is 95mm². For parallel cable runs, each cable requires its own CPC sized to its line conductor. The adiabatic equation confirms the final size.

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