Project GuideNEC 2023 · 2026 · Updated Apr 2026

Subpanel Installation: Complete Wiring Guide

Q: What wire size do I need for subpanel installation?

Use 6 AWG copper or 4 AWG aluminum on a 60A 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).

Subpanel Installation: Use 6 AWG copper on a 60A breaker at 240V.

Wire (Cu)6 AWG

Wire (Al)4 AWG

Breaker60A

GFCINot required

Bill of materials

A subpanel is a secondary distribution panel fed from your main panel. The critical rule: in a subpanel, the neutral bus and ground bus must be separate (not bonded). This is the most common DIY mistake and an automatic inspection failure. The feeder must be 4-wire: two hots, one neutral, one ground.

Subpanels require separate neutral and ground buses per NEC 250.24(A)(5). The feeder must be 4-wire. For detached buildings, a grounding electrode is required per 250.32. Size the feeder breaker for the actual load, not the panel rating.

60A subpanel

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

100A subpanel

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

125A subpanel

Component	Specification	NEC basis
Wire (copper)	2 AWG THHN	Table 310.16, 110.14(C)
Wire (aluminum alt.)	1/0 AWG THHN	Table 310.16, 110.14(C)
Breaker	125A double-pole	240.4
Voltage	240V single-phase
Conduit (EMT)	1-1/4" EMT	Chapter 9 Table 4
Max distance (3% VD)	185 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	6 AWG Co	4 AWG Al	4 AWG Co
25 ft	0.5%	0.5%	0.3%
50 ft	1.0%	1.0%	0.6%
75 ft	1.5%	1.5%	0.9%
100 ft	2.0%	2.0%	1.2%
150 ft	3.0%	3.0%	1.9%
200 ft	3.9%	4.1%	2.5%

Common products for 6 AWG Copper circuits

🧵Wire Roll6 AWG copper, 100ftCheck prices on Amazon (paid link) ⚡60A BreakerStandard pairing for 6 AWGCheck prices on Amazon (paid link) 🔋EV ChargerLevel 2, 48A for 6 AWG circuitsCheck prices on Amazon (paid link) 📏Conduit1" EMT for 6 AWG runsCheck prices on Amazon (paid link)

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Detailed ampacity: 6 AWG copper, 4 AWG aluminum. Wire size lookup: 60A 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 subpanel installation?

Use 6 AWG copper or 4 AWG aluminum on a 60A 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 subpanel installation require GFCI?

No. GFCI is not required for this application.

Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.

Important: For reference only. Not a substitute for a licensed electrician. Electrical work can cause serious injury, death, fire, or property damage if performed incorrectly. Always hire a licensed electrician for electrical work, especially for service upgrades, panel work, and 240V circuits. Values are derived from NFPA 70 (NEC) for educational purposes. Always verify against your locally adopted NEC edition and amendments - local jurisdictions may enforce stricter requirements. WireRef provides reference information only and is not responsible for work performed based on this content. NEC® is a registered trademark of the National Fire Protection Association. Free NEC access via NFPA · OSHA Electrical Safety · Terms of use.

NEC 2023 references verified April 2026