Voltage drop is the steady loss of voltage along a cable, caused by conductor resistance. Voltage dip is the brief sag in a generator's output the moment a large motor starts, caused by the alternator's reactance. They are easy to confuse — and often happen at the same time.
The steady loss of voltage along a cable, caused by conductor resistance (V = I × R). It is constant while the load runs and you fix it by using a larger cable.
The brief sag in a generator's output the moment a large motor starts, caused by the alternator's reactance. It lasts only a second or two and you fix it by choosing a generator with enough starting kVA.
When a motor starts, the surge produces both a dip at the generator and extra drop in the cable. A motor that is fine on a short, thick feeder can stall on a long, thin one — because the cable is too small.
This page is about voltage drop — the steady loss in the cable. The voltage dip side of the problem is handled by our companion motor starting generator sizing calculator, which sizes for the starting kVA. Size the cable here, check the motor start there, and you have both halves of the installation covered.
Voltage drop is steady, caused by cable resistance, and fixed by a larger conductor. It affects the load continuously while it runs.
Voltage dip is brief, caused by alternator reactance, and fixed by generator starting kVA. It only occurs during the first second of motor startup.
When a motor starts, the dip and the extra drop happen together. The load sees the sum of both — which is why a long cable can cause a motor to stall.
Size the cable here using the voltage drop calculator. Check the motor start using the motor starting generator sizing calculator. Together they cover the whole installation.
A generator feeder must satisfy two independent constraints, and the final conductor is whichever is larger. Choose Quick Mode for a fast estimate based on generator kVA, or Advanced Mode for detailed calculations with conductor material, temperature, and NEC derating factors.
Enter generator kVA and run length. The tool returns load current, ampacity-based size, voltage-drop-based size, and the final conductor — with percentage voltage drop for each option.
Enter generator power, conductor details, installation conditions, and derating factors for a complete NEC-compliant cable sizing.
Generator feeder sizing is governed by a few hard rules. NEC 445.13 requires conductors to have an ampacity of at least 115% of the generator's nameplate current rating. Voltage-drop limits follow common design practice: 3% for branch circuits, 5% combined for feeder and branch.
| Rule / Standard | Requirement | Application |
|---|---|---|
| NEC 445.13 | ≥ 115% nameplate current | Generator feeder conductors |
| Branch circuit drop | ≤ 3% | Individual branch circuits |
| Feeder + branch combined | ≤ 5% | Total from generator to load |
| Generator to switchboard (marine) | ≤ 1% | Shipboard installations |
| Manual rule of thumb | ≤ 5% | General generator feeders |
| Motor starting allowance | Extra margin | Voltage sag stacks on cable drop |
The table below shows the drop for real generator scenarios, calculated with the formulas above (copper, K = 12.9). Use it as a sense check before you run the calculator.
Voltage: 240 V, Single‑phase
Current: 91.7 A
Run: 220 ft
Conductor: 2 AWG Cu
Voltage drop: ≈ 3.3%
Voltage: 240 V, Single‑phase
Current: 91.7 A
Run: 220 ft
Conductor: 1/0 Cu
Voltage drop: ≈ 2.1%
Voltage: 208 V, Three‑phase
Current: 125 A
Run: 160 ft
Conductor: 2 AWG Cu
Voltage drop: ≈ 3.2%
Voltage: 208 V, Three‑phase
Current: 125 A
Run: 160 ft
Conductor: 1/0 Cu
Voltage drop: ≈ 2.0%
Voltage: 240 V, Single‑phase
Current: 30 A · Run: 85 ft
10 AWG Cu: ≈ 2.6% drop
8 AWG Cu: ≈ 1.7% drop
8 AWG gives cleaner margin
1. Distance drives the decision. At 160–220 ft, the difference between 2 AWG and 1/0 is the difference between borderline and comfortable — the voltage drop check, not the ampacity check, forces the upsize.
2. Motor loads push you bigger. The 22 kW and 45 kW sets run HVAC and pump motors; staying near 2% (1/0) rather than 3% (2 AWG) leaves starting margin.
3. Even small runs deserve a check. A 30 A portable generator at 85 ft is fine on 10 AWG, but 8 AWG gives a cleaner margin if a compressor or pump is on the circuit.
Two choices recur on every generator installation: the conductor material and the phase arrangement — and each changes the calculation. The calculator reports both so you can weigh the upsize against the material cost.
Copper carries more current per cross‑section and drops less voltage for a given size. It costs more and weighs more than aluminum. For a short, tightly packed terminal box, copper usually wins.
Aluminum is lighter and cheaper than copper, but you need a larger conductor for the same ampacity and voltage drop — its resistivity is 0.0282 Ω·mm²/m versus copper's 0.0175. For a long outdoor feeder, the aluminum upsize is often the economical choice.
A three‑phase generator is three single‑phase circuits sharing one engine. The current formula changes accordingly (÷ 1.732). The terminal box, busbar, and recommended feeder are engineered together.
A motor‑heavy load with a power factor below 0.8 draws more apparent power than its kilowatts suggest, which raises the current — and the voltage drop — for the same run. The calculator's three‑phase mode takes your power factor and reflects it in the result.
Exact results shift slightly with the resistivity constant you use — 12.9 is standard for larger conductors, while finer conductors run closer to 10.8. Treat every figure here as a planning value, not a spec. The calculator uses 12.9 for copper and 21.2 for aluminum.
For a Huali three‑phase set, the terminal box, busbar, and recommended feeder are engineered together. The cable size you calculate here is the one our engineers will specify on the nameplate drawing.
Most cable sizing problems come from a small number of recurring assumptions. Avoiding these errors helps prevent both undersized conductors and unnecessary excess cost.
A conductor that meets the ampacity requirement can still drop too much voltage over a long run. The final size is always the larger of ampacity and voltage drop — never assume ampacity alone is enough.
The generator's output sags during motor starting, and that sag stacks on top of the cable drop. If you size the cable right up to the 5% limit, a motor start can push the combined figure well past it.
A motor‑heavy load at 0.7 PF draws more current than the same kW at 0.9 PF — which means higher voltage drop. Always enter the actual power factor of your load, not the generator's default.
Aluminum has higher resistivity — you need a larger conductor for the same ampacity and voltage drop. The calculator reports the answer in both materials so you can weigh the upsize against the material cost.
The K value changes with conductor size and temperature. 12.9 is standard for larger copper conductors; finer conductors run closer to 10.8. The calculator uses 12.9 for copper and 21.2 for aluminum — consistent with standard feeder design.
Final generator cable sizing should always check both ampacity (NEC 445.13 — 115% of nameplate current) and voltage drop. Use the cable size calculator to run both checks automatically, and confirm the final conductor with your project's local code requirements.
We build diesel generator sets from 8 kVA to 4000 kVA — on Cummins, Perkins, Weichai, and Yuchai engines with Stamford alternators — and we publish the rated current and terminal-box data for every model, so the feeder you size here is the one our engineers draw on the nameplate.
A: Size the conductor for 115% of the generator’s nameplate current (NEC 445.13), then check voltage drop over the run length and upsize if it exceeds 3% (feeder) or 5% (total). The final size is the larger of the ampacity and voltage-drop results.
A: Use VD = 2 × K × I × D ÷ CM for single-phase, or VD = 1.732 × K × I × D ÷ CM for three-phase, with K = 12.9 for copper. Divide the result by the system voltage for the percentage drop.
A: NEC informational notes recommend a 3% maximum for a branch or feeder circuit and 5% total for feeder plus branch. Generator manuals commonly keep total drop under 5%, and marine practice holds the generator-to-switchboard run to about 1%.
A: There is no fixed distance — the limit is set by voltage drop and ampacity, not an arbitrary length. A long run is fine if the conductor is upsized to keep drop within 3%, but distance drives cost and material size, so the calculator returns the conductor for your exact run.
A: Yes. Excess cable drop means the equipment at the load runs on reduced voltage, which causes motors to draw more current, run hotter, and struggle to start. It can also trip undervoltage protection even when the generator itself is healthy.
A: NEC 445.13 requires that conductors from a generator’s output terminals have an ampacity of at least 115% of the generator’s nameplate current rating, unless overcurrent protection is provided directly at the generator. It ensures the feeder is not the weak point in the circuit.
A: Copper carries more current per size and drops less voltage but costs more. Aluminum is lighter and cheaper but needs a larger conductor for the same ampacity and drop. For long outdoor feeders aluminum is often economical; for short terminal-box runs copper usually wins.
A: Voltage drop is the steady loss of voltage along a cable, caused by resistance, and you fix it with a larger cable. Voltage dip is the brief sag in a generator’s output when a motor starts, caused by alternator reactance, and you fix it with a larger generator or a soft-start.
