What Size Breaker Do I Need for a Detached Garage? (A Quick Guide!)

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What Size Breaker Do I Need for a Detached Garage?

Is your new detached garage almost finished? Maybe you just have a few final things to take care of, before you can start using it. Surely getting power to the detached garage would be among your top priorities. For that you need to buy a breaker. But what size breaker do you need for a detached garage?

A detached garage with normal electric load needs a 50 A or 60 A breaker. A garage with a well equipped workshop, a space heater, a mini split ac/heating, etc. will need a 100 A breaker. A 100 A breaker will also give you free capacity for adding more branch circuits and load in the future.

The size of the breaker required for a detached garage is a function of the proposed electrical load. Having said that, it is always better to plan for 20 to 30% additional load for future expansion.

What Size Breaker Do I Need for a Detached Garage?

How To Calculate the Breaker Size for a Detached Garage?

To calculate the breaker size for a detached garage you need to add up the power requirement of all lights, appliances, machinery & equipment that will be connected to the garage subpanel.

The breaker size in Amps is calculated using the formula:

Current (Amps) = Power (Watts) / Voltage (Volts)

Example 1 (Normal Garage):

Power (Watts) = 10,000 W

Voltage (Volts) = 240 V

Then the Current (Amps) = 10,000 W / 240 V = 41.67 A

You must add a safety factor of 25% to allow for smooth operation of the circuit breakers even in high temperatures. Therefore, the breaker size should be rated at 125% X 41.67 A = 52.09 A.

This would be rounded up to 60 A as breakers are available either as 50 A or 60 A. This is good as it gives you room to add to the garage load in future, should the need arise.

Example 2 (High Load Garage):

Power (Watts) = 18,000 W

Voltage (Volts) = 240 V

Then the Current (Amps) = 18,000 W / 240 V = 75.00 A

You must add a safety factor of 25% to allow for smooth operation of the circuit breakers even in high temperatures. Therefore, the breaker size should be rated at 125% X 75.00 A = 93.75 A.

This would be rounded up to 100 A, a size that is quite popular and easily available.

NOTE: In this context, the terms, “breaker”, “breaker box” and “subpanel” are used interchangeably and mean the “subpanel assembly”.

Article 220 Branch-Circuit, Feeder, and Service Load Calculations of 2020 National Electric Code (NEC) provides requirements for calculating branch-circuit, feeder and service loads

How Many Circuits Are Allowed in a Panel?

Until 2008 NEC permitted a maximum of 42 circuits on a panel. However this restriction has now been lifted and a larger number of circuits are allowed on a main panel or a subpanel.

The number of circuit breakers depends on the subpanel rating in amps as well as the specific design. The specifications of a subpanel will state, both the number of spaces and the number of circuits that the panel can support.

The number of circuits can be higher than the number of spaces. This is because a standard space can use a tandem (twin) breaker. A tandem (twin) breaker allows two circuits to fit in the space of a one full size breaker.

So it is quite common to find subpanels with twice as many circuits as spaces. Circuit breakers can be single pole or double pole.

Single pole circuit breakers in a subpanel will provide 120 V and are usually rated as 15 A or 20 A. They are ideal for receptacles and lighting.

Double pole circuit breakers in a subpanel will provide 240 V and are usually rated from 15 A to 50 A. They are ideal for appliances, such as heaters or air conditioners.

Moreover a breaker can be GFCI type or AFCI type. Some breakers combine the AFCI & GFCI function and are known as Dual Function Circuit Breakers.

The table below is just to give you an idea of some of the options available:

Brand with Amazon LinkPanel Rating (Amps)# of Spaces# of Circuits
Siemens 60 amps Surface Mount Circuit Breaker  60 A  2  4
Square D by Schneider Electric QO  60 A  4  8
Square D – HOM816M100PC Homeline100 A  816
Siemens SN2040B1100 SN Series100 A2040
Siemens SN1632L1125 SN Series125 A1632

What Gauge Wire Do I Need for a Subpanel?

To determine the wire size from the main panel to the subpanel you need to have the following information and facts:

  1. Rating of the garage subpanel in amps
  2. Safety factor as per 2020 NEC : You must add 20% to the subpanel rating
  3. Voltage loss for every hundred feet of the wire: Voltage typically drops 20% for every 100’ of wiring
  4. Ampacity of the feeder wire

Sample calculations for a 60 A and a 100 A subpanel that is 100’ away from the main panel is as under:

Feeder Wire for 60 A Subpanel

For a 60 amp subpanel box in a detached garage that is 100’ away from the main panel use #3 THHN/THWN or THWN wire with an ampacity of 100 amps. 

Feeder Wire for 100 A Subpanel

For a 100 amp subpanel box in a detached garage that is 100’ away from the main panel use #0 (1/0) THHN/THWN or THWN wire with an ampacity of 150 amps. 

Please refer to AWG Wire Gauge Chart

Formulas for calculations to comply with NEC code.

Min. Wire Ampacity = (Subpanel Rating) Amps / 80%

Min. Wire Ampacity (100 ft away) = Min. Wire Ampacity Amps × 120%

NOTE: Maximum loading for any branch circuit is 80% of the rating of the circuit for ampacity of wire for any load. Voltage drops by 20% over a 100’ distance. 

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Thank you very much for reading the post. I do hope you found it informative and useful.

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