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Generator Output Conductor at 115% (NEC 445.13)
The conductor rule from a generator to its first overcurrent device, which estimators routinely get wrong.
Example
You enter
- Generator real power (kW, used if nameplate is 0) 150
- Voltage (V, line-to-line 3ph or line-to-neutral 1ph) 480
- Phase 3
- Power factor 0.8
- Generator design false
You get
- Nameplate current 225.5 A
- Applied basis 115% of nameplate
- Required conductor ampacity 259.4 A
Checked against published worked examples from: IAEI Magazine.
Details, formula, and sources
ScopeNEC 445.13(A) sets the ampacity at not less than 115% of the generator NAMEPLATE current -- not 125% of a computed load, not the connected running load. A 150 kW, 480 V three-phase generator at 0.8 pf draws 225.5 A nameplate, so the conductor must carry 1.15 x 225.53 = 259.4 A. Where the design prevents output above nameplate (overload-limited), the basis drops to 100% and it is just 225.5 A. Enter the nameplate current directly or derive it from kW, voltage, phase, and pf. The 110.14(C) termination limit and 310.15 adjustments still apply. A design aid; the AHJ governs.
Formula or tablenameplate = given, or kW x 1000 / (sqrt(3) x V x pf) three-phase, or kW x 1000 / (V x pf) single-phase; basis = overload_limited ? 1.00 : 1.15; required_ampacity = basis x nameplate.
Source and editionThe generator output-conductor ampacity rule of NEC 2023 445.13(A) -- not less than 115% of the generator nameplate current, or 100% where the design prevents output above the nameplate -- by name. A computational aid; the AHJ-adopted NEC edition governs.
Free accessNEC is free to read at nfpa.org/freeaccess. The 115% basis is on the nameplate current, not 125% of a computed load; the 110.14(C) termination limit and 310.15 ambient/fill adjustment are stated as out of scope, not modeled.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesgen_kw, voltage_v, phase, power_factor, overload_limited, nameplate_a, basis, required_ampacity_a
- Basis 115% of the generator nameplate current for output conductors; 100% where the design prevents output above nameplateNEC 445.13(A)
- Nameplate supplied directly, or derived from kW, voltage, phase, and power factorgenerator nameplate
- Downstream the conductor still must satisfy the 110.14(C) termination limit and any 310.15 adjustmentNEC 110.14(C) / 310.15