Wiring a Garage: Subpanels, Circuits and Wire Gauge

Garage wiring has to carry compressors, welders, heaters and lighting at the same time. This guide explains how wire gauge, breaker size and distance relate, how to plan circuits and a subpanel, and where the job should go to a licensed electrician.

Safety first

Electrical work can kill and starts fires. Most places require a permit and inspection for new circuits, and many require a licensed electrician for service and subpanel work. This guide helps you plan and understand the numbers; it is not a substitute for your local code, the National Electrical Code as adopted in your area or an electrician's judgment.

Wire gauge and current

Wire thickness is given as American Wire Gauge (AWG), where a smaller number is a thicker wire. The thicker the wire, the more current it can carry safely without overheating. For ordinary copper cable in a home, the common pairings are:

Breaker	Minimum copper wire	Typical use
15 A	14 AWG	Lighting
20 A	12 AWG	General receptacles
30 A	10 AWG	Welder or small 240 V loads
40 A	8 AWG	Larger 240 V loads
50 A	6 AWG	Welder, large compressor or EV charger

These are common textbook pairings for cable in typical dwelling installations; actual requirements depend on the insulation type, temperature rating, conduit and the equipment terminals. Aluminum wire needs to be thicker. Never put a larger breaker on a wire to stop it tripping, because the breaker exists to protect the wire.

Voltage drop over distance

Wire has resistance, so voltage falls along its length. A common guide is to keep the drop below 3 percent on a branch circuit. Long runs from the house to a detached garage may need thicker wire than the table suggests. The drop in volts is approximately 2 × length (one way, in feet) × current × resistance per foot. For 12 AWG copper, resistance is about 1.6 ohms per 1,000 feet, so a 100 foot run at 16 amps drops about 2 × 100 × 16 × 0.0016 = 5.1 volts, or 4.3 percent on 120 volts. That exceeds the guide, so you would step up to 10 AWG. The Wire Gauge Calculator runs this for your own current, distance and voltage.

120 volts or 240 volts?

Large motors and heaters are more efficient on 240 volts. The same power draws half the current, so the wire can be thinner and voltage drop is less. Compressors over about 2 horsepower, welders and many heaters expect 240. Check the nameplate and plug type before you plan.

Plan by loads, not by outlets

List what you expect to run. A typical home shop might include:

General-purpose 120 V receptacles on two or more 20 A circuits so a tripped breaker does not shut everything off.
Lighting on its own circuit. LED fixtures use little power, but you want light even when a tool trips a breaker.
A dedicated 240 V circuit for the compressor.
A dedicated circuit for a welder, typically 30 to 50 A at 240 V.
A circuit for a heater; see Heating and Cooling a Garage Workshop.
GFCI protection on garage receptacles, which codes generally require.

When you need a subpanel

If your garage is detached, or the main panel has no spare capacity, a subpanel is the standard fix. It is a smaller breaker panel fed by a feeder from the main panel. The feeder breaker, wire size and the subpanel's rating must match, and the feeder needs four conductors: two hots, a neutral and a separate ground. In a subpanel, the neutral and ground bars are kept separate, unlike in the main panel where they are bonded. A detached building also needs its own grounding electrode. These are the details that inspectors check, which is another reason to use a professional.

Sizing the feeder

Add up the loads you might use at the same time rather than every circuit's breaker rating, and apply any demand factors your code allows. Many home garages are well served by a 60 or 100 amp subpanel. A tradesperson will calculate the load properly. Before you buy anything, make sure the main service has enough capacity, because a house with a 100 amp service may not have much to spare.

Conduit and cable choices

Cables used outdoors or underground must be rated for it, and many installations use individual conductors in conduit rather than cable. Buried cable needs a minimum depth that varies with the wiring method. Exposed garage wiring often has to be in conduit or protected from damage. Use the right box sizes, staples and connectors, and keep splices inside boxes.

Plugs and receptacles

Match the receptacle to the breaker and the plug. NEMA 6-50 is common for welders and NEMA 14-50 for ranges and EV chargers. Using adapters to get around a mismatch is a common cause of overheated plugs.

Common mistakes

Using a larger breaker than the wire can handle.
Ignoring voltage drop on long runs.
Putting everything on one circuit.
Bonding neutral and ground in a subpanel.
Skipping the permit, which can cause trouble with insurance or when you sell.

Frequently asked questions

What wire gauge do I need for a 50 amp circuit?

6 AWG copper is the usual pairing for a 50 amp breaker in typical home wiring. The exact requirement depends on the insulation rating, whether it is in conduit and the equipment terminals, so check your local code or ask an electrician.

How much voltage drop is acceptable?

A common guide is to keep voltage drop to 3 percent or less on a branch circuit. Long runs, such as to a detached garage, often need thicker wire than the same current would need over a short distance.

Can I wire a garage subpanel myself?

Rules vary. Many jurisdictions require a permit and inspection, and some require a licensed electrician for service or feeder work. Check with your local building department before starting.

Tools used in this guide

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Figures in this guide are general guidance for planning. Always follow your vehicle, tool and product manufacturers' instructions and local codes, and read our disclaimer.