Size the phase, neutral and earth (PE) busbars of a switchboard from the rated current or transformer kVA, in copper or aluminium, using the current-density method.
The current-density method is the standard first step in switchboard design. The final rating of a busbar system depends on the enclosure, ventilation, bar spacing, painting and ambient temperature, and must be verified by temperature-rise testing or calculation to IEC 61439, together with the short-circuit withstand of the bars and their supports.
| Flat bar (mm) | Area (mm²) | Cu @ 1.2 A/mm² | Cu @ 1.6 A/mm² | Al @ 0.8 A/mm² |
|---|---|---|---|---|
| 12 × 3 | 36 | 43 | 58 | 29 |
| 15 × 3 | 45 | 54 | 72 | 36 |
| 20 × 3 | 60 | 72 | 96 | 48 |
| 25 × 3 | 75 | 90 | 120 | 60 |
| 30 × 3 | 90 | 108 | 144 | 72 |
| 20 × 5 | 100 | 120 | 160 | 80 |
| 40 × 3 | 120 | 144 | 192 | 96 |
| 25 × 5 | 125 | 150 | 200 | 100 |
| 30 × 5 | 150 | 180 | 240 | 120 |
| 50 × 3 | 150 | 180 | 240 | 120 |
| 60 × 3 | 180 | 216 | 288 | 144 |
| 40 × 5 | 200 | 240 | 320 | 160 |
| 80 × 3 | 240 | 288 | 384 | 192 |
| 50 × 5 | 250 | 300 | 400 | 200 |
| 30 × 10 | 300 | 360 | 480 | 240 |
| 60 × 5 | 300 | 360 | 480 | 240 |
| 100 × 3 | 300 | 360 | 480 | 240 |
| 120 × 3 | 360 | 432 | 576 | 288 |
| 40 × 10 | 400 | 480 | 640 | 320 |
| 80 × 5 | 400 | 480 | 640 | 320 |
| 50 × 10 | 500 | 600 | 800 | 400 |
| 100 × 5 | 500 | 600 | 800 | 400 |
| 60 × 10 | 600 | 720 | 960 | 480 |
| 120 × 5 | 600 | 720 | 960 | 480 |
| 80 × 10 | 800 | 960 | 1,280 | 640 |
| 100 × 10 | 1000 | 1,200 | 1,600 | 800 |
| 120 × 10 | 1200 | 1,440 | 1,920 | 960 |
Single bar per phase. Manufacturer ratings for bare bars in free air can be higher; enclosed switchboards in hot rooms can be lower.
Size the incoming cable with the Cable Size Calculator, the transformer with the Transformer Sizing Calculator, and the fault level with the Short Circuit Calculator.
Divide the rated current by the allowed current density. For 1,250 A in copper at 1.6 A/mm², the cross-section must be at least 781 mm², so an 80 × 10 mm (800 mm²) bar per phase is used. Verify temperature rise and short-circuit strength to IEC 61439.
Typically 1.2–1.6 A/mm² for copper in enclosed switchboards and 0.8–1.0 A/mm² for aluminium. Bare bars in well-ventilated free air can carry more; hot plant rooms call for the lower end.
Usually 50% of the phase busbar. Use 100% (or more) where the load includes many non-linear loads such as LED drivers, IT equipment and VFDs, because third harmonic currents add up in the neutral.
IEC 61439-1 relates the PE cross-section to the phase cross-section S: equal to S up to 16 mm², 16 mm² up to 35 mm², S/2 up to 400 mm², 200 mm² up to 800 mm², and S/4 above 800 mm², unless a fault calculation gives a different size.
The bars heat each other and the proximity effect pushes current to the outer surfaces, so two bars carry roughly 1.6–1.8 times one bar. This calculator applies a factor of 0.90 for two bars and 0.80 for three.
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