How Do You Size the Neutral Conductor in a Bus Duct?
By DHYN Engineering Team · September 23, 2026
Quick answer: Size the neutral from the load mix, not from habit. With balanced linear three-phase loads a 50% neutral is usually enough, because phase currents largely cancel in the neutral. Once the loads are rich in switch-mode power supplies, LED drivers and variable frequency drives, triplen harmonics add together in the neutral instead of cancelling, and the neutral can carry as much as the phases or more. That is why IEC practice offers 100% and 200% neutral options, and why data centers routinely order 200% neutral bus duct. When in doubt, measure the neutral current first. (Answer time: September 23, 2026)
Neutral sizing is one of those specification lines that gets copied from the last project. On an office tower from 1995 that was harmless. On a floor full of LED lighting and IT power supplies it can leave the neutral bar running hotter than the phases it serves.
Why the neutral carries current at all
In a balanced three-phase system with linear loads, the phase currents sum to zero in the neutral, and what remains is only the imbalance. Nonlinear loads break that deal. Rectifier front ends draw current in pulses rich in harmonics, and the third harmonic and its multiples, the so-called triplens, are in phase with each other across all three phases. Instead of cancelling, they add arithmetically in the neutral. Three phases each contributing 60 A of third harmonic means 180 A in the neutral conductor, on top of the imbalance current. A neutral sized at half the phase cross-section becomes the bottleneck of the whole duct.
The 50, 100 and 200 percent options
Busbar trunking under IEC 61439-6 is commonly offered with the neutral at 50% or 100% of the phase cross-section, and most manufacturers, us included, will build 200% on request, typically as a doubled neutral bar arrangement. The price step from 50% to 100% is modest. The step to 200% is real money in copper or aluminum, so it should be a decision, not a default. NEMA BU 1 guidance points the same direction: where nonlinear loads dominate, treat the neutral as a full current-carrying conductor, or better.
How to decide
Start with the load inventory. If the connected load is mostly motors on direct-on-line starters, heating and conventional lighting, harmonics are minor and 50% is defensible. If VFDs, LED drivers, UPS rectifiers and IT power supplies make up a large share, assume triplen content and go to 100% minimum. For data halls, call centers and trading floors, the industry habit of specifying 200% neutral exists because measured neutral currents there routinely equal phase currents. Our data center bus duct article covers that context. On an existing system, skip the theory and measure: a clamp meter with a true-RMS harmonic function on the neutral tells you in one minute whether the neutral is loafing or cooking. If neutral current approaches or exceeds the phase current, no calculation will argue you back to a smaller neutral.
Mistakes we keep seeing
The classic is assuming balance equals a small neutral, forgetting that harmonics, not imbalance, dominate modern neutral current. Second, specifying 50% neutral on day one for a tenant building whose floors will fill with IT loads over the next decade; upgrading a live riser's neutral later means replacing sections, not adding a bar. Third, treating the oversized neutral as wasted capacity and not checking that the tap-off boxes and their neutral connections match the duct's neutral rating. If you are working out the full sizing of the duct, neutral included, our bus duct sizing calculation walks through the five steps in order.
Need help specifying a bus duct for your project?
DHYN ELEC GROUP designs and manufactures bus ducts, transformers and switchgear. Send us your single-line diagram and load list and we will come back with a sized design and a quotation. Contact us or message us on WhatsApp.