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How Do You Calculate the Right Bus Duct Size?

By DHYN Engineering Team  ·  September 14, 2026

Quick answer: A bus duct sizing calculation has five steps. Work out the design current from your real load, pick a catalog rating above it, apply derating factors for ambient temperature, mounting position and harmonics, check the short-circuit withstand against the available fault level, and confirm voltage drop over the full run length. Skipping any one of these is how undersized or overspecified busways end up on site. (Answer time: September 14, 2026)

We get drawings every week where someone picked a busway rating off the transformer nameplate and called it done. Sometimes that works. Sometimes it produces a 2000 amp duct for a 700 amp load, or worse, a 1000 amp duct that cooks at 40 °C ambient. A proper bus duct sizing calculation takes about twenty minutes once you know the order of operations. Here is the sequence we use internally.

Step 1: Get the design current, not the transformer rating

Start from the load list. Add up the connected loads, apply the demand factor your local code or standard allows, and you get the design current IB. For three-phase loads the conversion is IB = P / (√3 × U × cos φ). A 630 kW load at 400 V and 0.9 power factor draws about 1010 A, not the 1443 A a 1000 kVA transformer could theoretically push. The transformer rating is a ceiling, not the requirement.

One adjustment we always argue for: add growth margin at this stage, not later. Twenty to twenty-five percent on the design current is typical for commercial buildings, less for fixed industrial processes. A busway run costs far more to replace than to oversize by one frame on day one.

Step 2: Pick the frame size, then derate it

Choose the smallest standard rating above IB. Standard low-voltage busway ratings step through 400, 630, 800, 1000, 1250, 1600, 2000 A and up. Then cut that rating down to reality. The catalog number assumes 35 °C average ambient, horizontal mounting, sea level and a clean sinusoidal load. Every deviation has a factor, and the factors multiply:

Condition	Typical correction
Ambient 40 °C instead of 35 °C	× 0.93 to 0.95
Vertical riser mounting	× 0.95
Altitude above 2000 m	× 0.95 per additional 1000 m (approx.)
Heavy harmonic load (UPS, VFD, LED)	Upsize neutral to 200% and/or derate per maker

The full logic of reading these numbers off a datasheet is in our guide to bus duct ampacity tables. The point here is the order: nominal rating × all applicable factors = usable ampacity, and usable ampacity must exceed IB with your growth margin included.

Step 3: Check the short-circuit rating

Ampacity is about heat in normal service. Short-circuit rating is about surviving the worst five cycles of the system's life. Compare the busway's rated short-time withstand current (Icw, usually for 1 second) and peak withstand (Ipk) against the fault level at the point of installation, which you get from the utility data or a fault study. Close to a large transformer, 50 kA is ordinary; on big industrial services 65 kA and above shows up regularly. Our article on bus duct short-circuit ratings explains why the bracing, not the conductor cross-section, usually sets this limit. Never assume a higher ampacity frame automatically carries a higher fault rating; check the table per size.

Step 4: Verify voltage drop over the actual route

A duct can pass every thermal check and still deliver a saggy voltage at the far end. For three-phase busway, ΔV = √3 × I × L × (R cos φ + X sin φ), with R and X per meter from the manufacturer. Keep feeders within about 3 percent at full load, 5 percent as an absolute ceiling including downstream runs. Long runs above 60 to 80 meters are where this check starts to bite, and where stepping up one frame size is cheaper than accepting losses for thirty years. We walked a full worked example in the voltage drop calculation guide.

Step 5: Confirm the physical envelope

The last check is embarrassingly practical: does it fit? A 1600 A sandwich busway is roughly 130 to 180 mm tall and 400 to 600 mm wide depending on the maker, plus joint clearance and bend radius. Compare that against the shaft drawing and the ceiling void, including space for maintenance access at joints and tap-offs. More projects than you would expect have been value-engineered into a duct that only fits if installed before the pipework.

A worked example

Say the load study gives IB = 1010 A, you add 20 percent growth, so design basis is about 1210 A. Nearest frame up: 1600 A. The run is a vertical riser in a shaft that averages 40 °C in summer, so usable ampacity is 1600 × 0.95 × 0.93 ≈ 1414 A. That still clears 1210 A comfortably. Fault level at the switchboard is 42 kA, so specify 50 kA / 1 s minimum. The run is 45 meters, and the maker's data gives R = 0.030 mΩ/m and X = 0.017 mΩ/m for this size; at 1210 A and 0.9 pf the drop works out near 1.6 percent, well inside limits. Physical check confirms the 1600 A section fits the shaft with joint access. Done: 1600 A, 50 kA, copper, specified with the derating written into the schedule so nobody swaps it for a 1250 A frame at procurement.

The mistakes that keep coming back

Sizing straight off the transformer kVA. Forgetting that derating factors stack. Checking ampacity but never fault level, because "the breaker will protect it" (the breaker limits let-through, it does not cancel the first half-cycle). Ignoring the neutral on harmonic-heavy floors, which is how neutral conductors end up running hotter than the phases. And treating voltage drop as somebody else's problem until commissioning day. Our roundup of common bus duct problems shows where each of these ends up in service.