In short
- I = P ÷ (√3 · U · cos φ): a 1 MW feeder at 400 V and PF 0.95 draws 1,519 A.
- Keep the run within about 80 % of its rating: 1,519 A needs a 2,000 A run.
- Correct for ambient above the reference (usually 35 °C) and use a 200 % neutral where IT loads create harmonics.
- Check voltage drop at the far end: ΔU = √3 · I · L · (R cos φ + X sin φ).
- The run's short-time rating Icw must exceed the prospective fault current where it is installed.
Design current
For a three-phase load the current is the power divided by √3, the line voltage and the power factor:
Then choose the next standard rating that keeps the load at or below about 80 % of it. That margin covers growth, unbalance and the fact that ratings assume a reference ambient. For 1,519 A the result is a 2,000 A run (76 % loaded).
Corrections: ambient, mounting and harmonics
- Ambient: ratings are usually given for 35 °C average over 24 hours. Above that, apply the maker's correction factor; in a hot riser or plant room it can cost a full rating step.
- Mounting: some ventilated designs are rated for one orientation; edge-wise or vertical mounting can reduce the rating. Sandwich designs are usually less sensitive.
- Harmonics: single-phase IT loads produce third-harmonic currents that add in the neutral. Specify a 200 % neutral for runs that feed racks or LED lighting.
Voltage drop at the far end
Worked example. A 2,000 A copper run, 100 m long, carrying 1,519 A at PF 0.95. Taking a conductor resistance of about 0.022 mΩ/m at 85 °C and ignoring reactance (low in sandwich designs):
| Quantity | Value |
|---|---|
| ΔU = 1.732 × 1,519 A × 100 m × 0.0000216 Ω/m × 0.95 | 5.4 V |
| As a share of 400 V | 1.35 % |
IEC 60364-5-52 suggests limits of about 3 % for lighting and 5 % for other loads from a public supply, and 6 % / 8 % from a private transformer, across the whole installation. Leave room for the cables and tap-offs after the run.
Fault rating
The run must survive the prospective short-circuit current at its feed point. A quick upper bound for a run fed directly from a transformer is the transformer's rated current divided by its impedance voltage:
| 2,500 kVA transformer, 400 V, uk 6 % | Value |
|---|---|
| Rated current = 2,500,000 ÷ (1.732 × 400) | 3,608 A |
| Short-circuit current ≈ 3,608 A ÷ 0.06 | ≈ 60 kA |
Choose a run whose Icw (1 s) and Ipk exceed this, and check that the upstream breaker clears the fault within the run's withstand time. Further from the transformer the fault level falls, so plug-in runs downstream often need a lower rating.
Tap-offs: size each one to its load
Size every tap-off unit to its own load with the same 80 % rule. A 40 kW rack at 400 V and PF 0.95 draws 40,000 ÷ (1.732 × 400 × 0.95) = 61 A. 61 ÷ 0.8 = 76 A, so an 80 A unit is enough, or 100 A where the range steps from 63 A to 100 A.
Spread single-phase tap-offs across L1, L2 and L3 so the run stays balanced. Plan a whole row in the run builder.
What the run costs to operate
Losses grow with the square of the current: P = 3 · I² · R · L. For the example run at full load that is about 15 kW, 1.5 % of the power it carries. At an average 75 % load the losses fall to 56 %, about 74 MWh a year, or roughly $8,900 at $120 per MWh. A larger rating or copper instead of aluminium lowers R and pays back through these losses.
Questions about sizing
Why size busbar at 80 % of its rating?
It leaves margin for growth, phase unbalance and warmer conditions than the reference ambient, and it keeps the conductors cooler, which lowers losses.
Does aluminium need a larger rating?
No, the rating is the same; an aluminium run of the same rating simply has larger bars. Its resistance per metre is similar, but the housing is bigger, so check the space.
How do I treat many tap-offs along the run?
For loads spread evenly along the run, the voltage drop is about half of what it would be with the whole load at the end. For uneven loading, add the drop section by section.
Where do I get R and X values?
From the maker's type-tested datasheet for the exact rating and conductor. Generic values are only good for screening.
Sources
- IEC 61439-6, busbar trunking systems; IEC 60364-5-52, selection and erection, Annex G (voltage drop).
- Resistivity of copper 1.724 × 10⁻⁸ Ω·m at 20 °C, temperature coefficient 0.00393 per K.
- Worked figures are screening calculations; design with the maker's data.