Whole-House Generator: Complete Wiring Guide
Bill of materials
Whole-house standby generators require a transfer switch, properly sized feeder, and compliance with NEC Article 702 (Optional Standby Systems). The transfer switch must be rated for the full generator output and prevents backfeeding the utility grid. Wire sizing depends on the generator output amperage.
The transfer switch rating must match or exceed the generator output. A 22kW generator at 240V produces about 92A, requiring a 100A transfer switch and 3 AWG copper feeder. The generator must have a disconnecting means per 445.18. A grounding electrode is required at the generator per 250.34 for separately derived systems. Never connect a generator without a transfer switch - backfeeding kills utility workers.
22kW standby generator (100A)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 3 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 1 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 100A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 1" EMT | Chapter 9 Table 4 |
| Max distance (3% VD) | 184 ft at 240V copper | 210.19(A) Note 4 |
14kW standby generator (60A)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 6 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 4 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 60A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 3/4" EMT | Chapter 9 Table 4 |
| Max distance (3% VD) | 153 ft at 240V copper | 210.19(A) Note 4 |
Installation notes
GFCI is not specifically required for this application by NEC, though local amendments may differ. Check with your local building department before starting work.
For the wire run, you have two options: NM-B cable (Romex) for interior runs through framing, or individual THHN conductors in conduit for exposed runs, outdoor installations, or runs through unfinished spaces. NM-B is faster to install but cannot be used outdoors or in conduit. For conduit, use EMT (metallic) or PVC Schedule 40.
Run length matters: every foot of wire adds voltage drop, reducing power to your equipment. The table below shows the exact voltage drop at common distances. If your run exceeds the 3% threshold, upsize one gauge.
Voltage drop by distance
Percentage voltage drop at common run lengths. NEC recommends 3% maximum for branch circuits and 5% for combined feeder plus branch.
| Distance | 3 AWG Co | 1 AWG Al | 2 AWG Co |
|---|---|---|---|
| 25 ft | 0.4% | 0.4% | 0.3% |
| 50 ft | 0.8% | 0.8% | 0.7% |
| 75 ft | 1.2% | 1.3% | 1.0% |
| 100 ft | 1.6% | 1.7% | 1.3% |
| 150 ft | 2.5% | 2.5% | 1.9% |
| 200 ft | 3.3% | 3.4% | 2.6% |
Common products for 3 AWG Copper circuits
As an Amazon Associate we earn from qualifying purchases.
Related wire data
Detailed ampacity: 3 AWG copper, 1 AWG aluminum. Wire size lookup: 100A circuit. Calculators: voltage drop · panel load calculator.
Inspection preparation
Before your service upgrade inspection, verify these frequently-checked items:
| Check | NEC Ref | Common Fail |
|---|---|---|
| Service entrance conductors properly sized | NEC 230.42 | Undersized SE conductors for panel rating |
| Main breaker matches panel rating | NEC 230.90 | Main breaker exceeds panel bus bar rating |
| Grounding electrode conductor properly sized | NEC 250.66 | GEC too small for service size |
| Grounding electrode system complete | NEC 250.50 | Missing required electrodes (water pipe, ground rods, Ufer) |
| Two ground rods (if no Ufer/water pipe) | NEC 250.53(A)(2) | Single ground rod without resistance test or supplemental electrode |
See the full inspection checklist tool for all 13 items with progress tracking.
Frequently asked questions
What wire size do I need for whole-house generator?
Use 3 AWG copper or 1 AWG aluminum on a 100A breaker at 240V. This is based on NEC 2023 Table 310.16 ampacity with 110.14(C) termination limits and 125% continuous load sizing per 210.19(A)(1).
Does whole-house generator require GFCI?
No. GFCI is not required for this application.
NEC 2023 references verified April 2026