If you have electrical questions you’d like answered in a future edition of this column, send them to the Editor at northshorejournal@gmail.com, or email John directly at john@clovervalleyelectric.com.
By Clover Valley Electric’s Master Electrician, John Christensen
Running Power to a New Garage
A reader wrote in with a question about his new garage project. He just built a new 24 x 30-foot garage, and he wants to run power to it from his house. He has a 200-amp panel with some open breaker spaces and plans to use the garage for woodworking and storage. He also wants to install a gas heater to keep it warm in the winter. He was looking at putting a 100-amp subpanel in the garage and wanted to know whether to run copper or aluminum wiring to the new garage. He noted that copper seems expensive, but he had heard it is better for voltage drop when running power tools.
I’ll start with the metallurgy question first. For a feeder run from a house panel to a detached garage over a short distance, aluminum is the standard choice, and there is nothing wrong with it. Our typical approach is to run #2 aluminum Mobile Home Feeder in PVC conduit, landed on a 90-amp breaker in the house panel. That feeds a 100- amp rated subpanel in the garage with plenty of capacity for lighting, receptacles, and woodworking equipment. The general equivalent in copper, which is #3 AWG, costs nearly five times as much for the same project right now at our wholesale pricing.
I think the concern about aluminum underground usually comes from the way it corrodes and oxidizes when damaged. When the insulation jacket on aluminum feeder wire is damaged underground, the aluminum starts to corrode and kind of “turn to dust”. However, with good installation techniques and preferably conduit for protection, aluminum is a great economic choice for this type of application. In fact, utility companies run aluminum for almost every service entrance in our area. When buried, protected, and terminated properly, aluminum feeders are reliable and have been standard practice for decades.
Our reader also asked about voltage drop, which is how much voltage is reduced over a distance due to the current in the circuit trying to overcome the resistance of the wire – similar to the transmission line discussion from a couple of weeks ago. I ran the numbers for a 75-foot feeder at 85 amps of load and 240 volts. The #2 aluminum drops about 3.7 volts, or roughly 1.56 percent, using Southwire’s online calculator. The #3 copper drops about 3 volts, or roughly 1.25 percent. Both are well under the NEC-recommended 3 percent for feeders. The difference between them is less than half a percent, which is not nearly enough to justify the cost difference. The reader’s table saw is not going to know the difference.
Since our reader mentioned a gas heater, that simplifies the electrical side. Most likely, a propane gas unit needs a general circuit for the blower and controls, which is normally just a 15-amp circuit. An all-electric heater would require a much larger circuit, possibly 50 or 60 amps. That is one reason the 90-amp feeder with a gas heat plan works well. I will also mention that using a ductless mini-split is also a potentially good idea for a garage/shop, with the gas heater as backup.
Beyond the feeder itself, there are a few pitfalls worth knowing about. A detached structure fed from the house needs its own grounding electrode system, typically a couple of ground rods at the garage, and the neutral and ground buses in the subpanel must be kept separate. I think a good practice is to remove the bonding screw and leave it in the bottom of the enclosure so the inspector and future electricians can see it came out.
Since most detached garages on the North Shore stay unfinished inside, the wiring will be exposed, which is fine as long as it is protected. NM cable should be run along the sides of studs and joists, not stretched across open spaces. A simple test is if you can hang a coat hanger on the wire, it is not protected well enough.
Finally, the code has some minimums for garages: at least one wallswitched light, a receptacle in each vehicle bay on a dedicated 20-amp circuit, and GFCI protection on all receptacles. In our reader’s case, this woodworking shop will exceed those anyway, so think through where the table saw, dust collector, and air compressor will live before the layout is set. Planning the circuits around how the shop will actually be used saves a lot of frustration later.
John Christensen is a licensed Master Electrician in Minnesota and has a bachelor’s degree in electrical engineering from the University of Minnesota – Duluth. If you have electrical questions you’d like answered in a future edition of this column, send them to the editor, or email John directly at john@clovervalleyelectric.com.
The advice provided in this column is intended for general informational purposes only. If you have specific concerns or a situation requiring professional assistance, you should consult with a qualified professional for advice or service tailored to your individual circumstances. The author, this newspaper, and publisher are not responsible for the outcomes or results of following any advice from this column. You are solely responsible for your actions.
