Generator Conductor Size Calculator

The 445.13 115 percent rule

A generator is a source, so the conductors leaving its terminals are protected at their load end by the first overcurrent device, not at their supply end. NEC 445.13(A) compensates by tying their minimum ampacity to the machine's own rating: at least 115 percent of the nameplate current. The extra 15 percent gives headroom for the generator to deliver current above its continuous rating for short intervals without overheating the run.

The rule covers the conductors from the generator output terminals to the first distribution device that contains overcurrent protection. Downstream of that device, ordinary feeder and branch-circuit rules take over, and the transfer switch, feeder taps, and load calculations are sized by their own articles.

Worked example: 200 A nameplate generator

1. The generator nameplate current rating is 200 A.
2. Required conductor ampacity is 115 percent of nameplate: 200 x 1.15 = 230 A.
3. In the 75 degrees C column of Table 310.16, 3/0 copper is 200 A, which falls short of 230 A.
4. 4/0 copper is 230 A, which meets it exactly, so the minimum conductor is 4/0 copper.
5. Protect the output per 445.12, and size the neutral, if any, from the unbalanced load per 220.61.

Nameplate current versus kW

The 445.13 basis is the nameplate current rating. When you work from a power rating instead, use apparent power in kVA rather than real power in kW, so power factor does not change the answer: the conductor carries the full line current whatever the load's power factor. For single-phase, current is kVA times 1000 divided by the voltage; for three-phase, it is kVA times 1000 divided by the square root of 3 times the voltage. If a nameplate lists kW at a power factor, its apparent power is kW divided by that power factor. When in doubt, read the amps directly off the nameplate.

445.13(B) and the 100 percent reduction

NEC 445.13(B) allows the conductors to be sized at 100 percent of the nameplate current where the design and operation of the generator prevent overloading, such as when overcurrent protection limits the output to the conductor ampacity. That is an engineering determination about a specific machine and installation, so this calculator does not take the reduction: it always applies the 115 percent path of 445.13(A), which never undersizes. If your installation qualifies for the 100 percent path, size the conductor to the nameplate current directly.

Why this tool may size one step larger than others

This calculator holds the 115 percent figure to its exact value and never rounds it down to the nearest standard conductor. Take an 80 kW, 208 V, three-phase generator: its 222 A nameplate needs 222 x 1.15 = 255.3 A. Some published generator-wire tools round that to 255 A and report 250 kcmil copper, which is rated 255 A at 75 degrees C. Because 250 kcmil is 0.3 A short of the true requirement, this tool steps up to 300 kcmil. The gap only appears when the required ampacity lands just above a standard conductor's rating; it is the never-undersize choice of the accuracy principle this site is built on, not a disagreement about the NEC method.

Frequently asked questions

What size wire do I need for a generator?

Take 115 percent of the generator nameplate current, then pick the smallest conductor whose Table 310.16 ampacity meets it. A 200 A nameplate needs 230 A, which is 4/0 copper at 75 degrees C.

What is the generator 115 percent rule?

NEC 445.13(A): the conductors from the generator terminals to the first overcurrent device must have an ampacity of at least 115 percent of the nameplate current rating, giving headroom for short-term output above the continuous rating.

Do generator conductors follow the 125 percent continuous-load rule?

No. They use the 445.13 115 percent factor against the generator nameplate current, not the 125 percent continuous-load rule for feeders and branch circuits.

How do I size the generator conductor from kW?

Convert to apparent power in kVA (kW divided by the power factor), compute the current from the voltage and phase, then apply the 115 percent rule. Using kVA keeps power factor out of the conductor current.

Does the generator neutral have to be full size?

No. 445.13(A) permits the neutral to be sized per 220.61 from the maximum unbalanced load. This tool sizes the ungrounded conductors; size the neutral separately.

Related calculators

• Wire Ampacity Calculator: the Table 310.16 ampacity of a chosen conductor, with ambient and fill derating.
• Wire Size Calculator: size a conductor for a load by ampacity and voltage drop together.
• Transformer Sizing Calculator: rated current and maximum overcurrent device for a transformer per Table 450.3(B).
• Feeder / Service Neutral Load Calculator: the maximum unbalanced neutral load per NEC 220.61.