Surmising a Storage Solution: Installing electrical power from an existing home to a new garage

Shutterstock / Dmitry Natashin
Shutterstock / Dmitry Natashin
Published On
Sep 15, 2022

With home prices going up astronomically and rising interest rates, buying a new or new-to-you home may not be in the cards. Vicki and I are very happy with our home here in Peoria, Ariz. However, we don’t have enough storage space, so we rent a unit at a storage facility. We almost always search the house before remembering that what we need is in storage.

Garages and storage

We have a 2½-car garage at our home, which was supposed to be a 3-car garage. You can technically fit three cars in there, as long as the cars are convertibles, or each one has a sunroof to climb out of, or you don’t mind sleeping in the car because you can’t open the doors.

The answer may be in buying only small sports or subcompact cars, or using one of the spaces for storage and the other two for vehicles. The garage is also too short to park my truck in without notching the garage door, so if it is shut, the door leaves the back end of the truck bed sticking out. I could design a two-car garage door with a smaller garage door built into it so I can close both doors when the truck is not in there, maintaining the garage’s security and integrity.

I believe the answer to the garage and storage problem is to build another, larger garage with ample storage space for all of our stuff, including our boat (which is kept at the lake right now due to lack of space) and the truck (without worrying about the end of the bed sticking out). It could also have an upstairs space, with an art room for Vicki and a woodworking space for me. We can both explore and enjoy our hobbies without bothering each other.

What work needs to be done

Now we have to address the electrical, plumbing and air-conditioning planning for the new garage. Since it is extremely warm in the Phoenix area, we would need an air conditioning split unit that will provide conditioned air for both the garage and upstairs areas. Between the existing home and the new garage, a 400A, 120/240V, single-phase service at the house and a 125A with main panelboard for the garage would be nice, with ample power for both loads.

Plumbing is necessary for a small kitchen and bathroom in the upstairs part of the garage. The plumbing and air conditioning will be left to the experts.

My existing 400A, 120/240V, single-phase service panelboard is on the east side of the house, and the new garage will be on the west side where there is a driveway, front patio, planter and walkway pavers leading to the front door. A professional core driller can install a conduit under the existing concrete drive, front patio and pavers.

A new 1½-inch PVC conduit (I am intentionally sizing up from a 1-inch conduit for future use) with three No. 1 XHHW-2 copper conductors and a No. 6 AWG equipment grounding conductor would be installed from the service to the garage panelboard. Using 215.2(A)(1)(a) and (b) with the 90°C column in Table 310.15(B)(1) requiring an ambient temperature correction factor of a 0.87 multiplier for a maximum temperature of 113°C, the No. 1 AWG XHHW-2 conductors are good for 145A in the 90°C column in Table 310.16, or 0.87 times 145A= 126.15A.

Since the new garage is a separate building, 250.32(A) requires a grounding electrode system for the garage, such as a metal underground water pipe electrode, a concrete-encased electrode or a set of driven ground rods. The grounding electrode must be connected to the equipment ground bar in the 125A panelboard, with the neutral isolated from ground in the panel. Section 250.32(B) requires “an equipment grounding conductor be run with the supply conductors and be connected to the garage main disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of any equipment required to be grounded or bonded.” The No. 6 AWG equipment grounding conductor provides the proper grounding for the system.

Easy-peasy, right? Of course, it is for a qualified electrical contractor, so always use one for this type of project.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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