Generator Transfer Switch: Complete Wiring Guide
Bill of materials
A transfer switch safely connects a generator to your home wiring without backfeeding the utility - which is illegal and dangerous. NEC 702 governs optional standby systems. You can choose between a manual interlock kit (cheaper, requires you to select circuits) or an automatic transfer switch (switches on its own when power fails).
Transfer switches prevent backfeeding to utility lines. NEC 702.5 requires a transfer switch for all permanent generator connections. The generator neutral may or may not be bonded depending on the transfer switch type (switched-neutral vs non-switched).
Manual interlock (30A portable)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 10 AWG THHN | Table 310.16, 110.14(C) |
| Wire (aluminum alt.) | 8 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 30A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 1/2" EMT | Chapter 9 Table 4 |
| Max distance (3% VD) | 120 ft at 240V copper | 210.19(A) Note 4 |
Automatic transfer switch (50A)
| 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 | 50A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 3/4" EMT | Chapter 9 Table 4 |
| Max distance (3% VD) | 182 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 | 10 AWG Co | 8 AWG Al | 8 AWG Co |
|---|---|---|---|
| 25 ft | 0.6% | 0.6% | 0.4% |
| 50 ft | 1.2% | 1.3% | 0.8% |
| 75 ft | 1.9% | 1.9% | 1.2% |
| 100 ft | 2.5% | 2.6% | 1.6% |
| 150 ft | 3.7% | 3.8% | 2.3% |
| 200 ft | 5.0% | 5.1% | 3.1% |
Common products for 10 AWG Copper circuits
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Related wire data
Detailed ampacity: 10 AWG copper, 8 AWG aluminum. Wire size lookup: 30A circuit. Calculators: voltage drop · panel load calculator.
Frequently asked questions
What wire size do I need for generator transfer switch?
Use 10 AWG copper or 8 AWG aluminum on a 30A 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 generator transfer switch require GFCI?
No. GFCI is not required for this application.
NEC 2023 references verified April 2026