Generator & Transfer Switch Voltage Drop
Size generator conductors and transfer-switch feeders with voltage drop, start-up sag, and NEC/IEC checkpoints instead of guessing from breaker size alone.
Generator runs look simple until source impedance, motor starting current, and transfer-switch location are included. A 30A portable generator 85 ft from a manual transfer switch behaves very differently from a 22 kW standby set or a 208V three-phase generator feeding an ATS.
For US work, check the National Electrical Code in NEC 445.13 for generator conductors, NEC 702.5 for transfer equipment, and the familiar 3% / 5% design targets in NEC 210.19(A)(1) and 215.2(A)(1) informational notes. For IEC-style work, IEC 60364-5-52 and table G.52.1 are the usual references for grouping and steady-state voltage-drop limits. NEC IEC
“When a generator run is long, the first bad assumption is usually that the breaker decided the wire size. It didn't. The start-up voltage decided it.”
— Hommer Zhao, Technical Director
Why Generator Circuits Need a Separate Voltage-Drop Check
- Generator voltage can sag during motor starting, so cable drop stacked on top of source sag can pull a 240V load below its usable range.
- Conductor sizing must survive ampacity math and start-up performance, not just breaker size or nameplate kW.
- The generator is often remote from the transfer switch, so the generator-to-switch conductors become part of the critical path.
- Portable sets, permanent standby sets, and 3-phase systems change current, fault level, and installation method enough that one rule of thumb is not reliable.
Code Checkpoints Worth Marking on the Print
- NEC 445.13: if overcurrent protection is not provided at the generator, conductors to the first distribution device are generally sized at not less than 115% of generator nameplate current.
- NEC 702.5: optional standby systems need transfer, interlocking, or equivalent equipment that prevents unintended interconnection with the normal source.
- NEC 210.19(A)(1) and 215.2(A)(1) informational notes: 3% branch-circuit drop and 5% combined feeder plus branch circuit remain the common US design targets.
- IEC 60364-5-52 / table G.52.1: steady-state limits are typically 3% for lighting and 5% for other loads on public LV supplies, with grouping factors applied when conductors share an enclosure.
Worked Examples with Numbers You Can Recheck
Portable generator, 240V, 30A, 85 ft to manual transfer switch
With copper 8 AWG, the round-trip drop is about 3.2 V, or roughly 1.3%. With 10 AWG, the same run is about 5.1 V, or 2.1%. Both may run, but 8 AWG leaves better margin when a well pump or air compressor starts.
22 kW standby set, 240V, 91.7A, 220 ft to ATS
At this distance, 2 AWG copper lands near 7.8 V drop, about 3.3%. Upsizing to 1/0 copper cuts the drop to about 4.9 V, around 2.1%. That extra margin matters when the generator is carrying HVAC and refrigerator compressors at the same time.
45 kW generator, 208V three-phase, 125A, 160 ft to ATS
Using the three-phase formula, 2 AWG copper is about 6.7 V drop, roughly 3.2%. 1/0 copper is about 4.2 V, or 2.0%. If the downstream load includes motors, the larger conductor usually gives the cleaner start.
Field Checklist Before You Lock the Conductor Size
- Use full-load current or nameplate current, then verify any 115% generator-conductor rule that applies to the equipment arrangement.
- Measure one-way distance from generator terminals to the transfer switch or first protected distribution point, not just to the building wall.
- Check start-up conditions for pumps, compressors, and air handlers instead of designing only for steady-state current.
- Apply conductor material, temperature, and grouping corrections before calling the design finished.
- Run the final numbers through the calculator and compare the result with the load voltage tolerance, not only with the breaker rating.
Run the Generator Scenario in the Calculator
Enter the generator voltage, load current, conductor size, and one-way distance to verify the drop before you buy cable or set the ATS location.