Whole-House Surge Protection: Complete Wiring Guide
Bill of materials
NEC 2023 requires surge protective devices (SPDs) for all dwelling unit services per 230.67(A). A Type 2 SPD mounts at your main panel and protects against voltage spikes from lightning, utility switching, and motor loads. Lead length must be as short as possible - under 6 inches is ideal for best clamping performance.
NEC 2020 made SPDs mandatory for new dwelling services. Type 1 mounts before the service disconnect. Type 2 mounts at the panel on a dedicated 2-pole breaker. Keep conductor leads under 6 inches for best protection.
Type 2 SPD (panel mount)
| Component | Specification | NEC basis |
|---|---|---|
| Wire (copper) | 12 AWG THHN | Table 310.16, 110.14(C) |
| Breaker | 20A double-pole | 240.4 |
| Voltage | 240V single-phase | |
| Conduit (EMT) | 1/2" EMT | Chapter 9 Table 4 |
| Max distance (3% VD) | 999 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 | 12 AWG Co | 10 AWG Co |
|---|---|---|
| 25 ft | 0.0% | 0.0% |
| 50 ft | 0.0% | 0.0% |
| 75 ft | 0.0% | 0.0% |
| 100 ft | 0.0% | 0.0% |
| 150 ft | 0.0% | 0.0% |
| 200 ft | 0.0% | 0.0% |
Common products for 12 AWG Copper circuits
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Related wire data
Detailed ampacity: 12 AWG copper. Wire size lookup: 20A circuit. Calculators: voltage drop · panel load calculator.
How NEC edition affects this project
Not every state uses the same NEC edition. The table below shows how requirements for this project differ depending on which edition your state has adopted. Find your state's edition here.
| Requirement | NEC 2023 | Earlier editions |
|---|---|---|
| SPD requirement | Type 1 or Type 2 SPD required for dwelling unit services per 230.67 | Surge protection is optional under NEC. Your state may still require it by local amendment. |
Currently, 17 states enforce NEC 2023, 21 use NEC 2020, 7 use NEC 2017, and 2 are still on NEC 2008. Three states (Arizona, Mississippi, Missouri) defer to local jurisdictions. Find your state to see which specific rules apply to your location.
Frequently asked questions
What wire size do I need for whole-house surge protection?
Use 12 AWG copper or 12 AWG aluminum on a 20A 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 surge protection require GFCI?
No. GFCI is not required for this application.
NEC 2023 references verified April 2026