What good is a 100 amp electric service?
Will your home inspector tell you if your electric service is too small?
While it goes beyond the scope of a home inspection to perform load calculations, occasionally I’ll do a rough calculation if I get concerned that an electric service is too small for a house. If the service size from my rough calculation comes up too close to the actual service size, I’ll recommend having an electrician perform an official load calculation… and I think I’ve done this twice. Ever.
At nearly every home inspection, I find one of two things: either the electric service is outdated and obviously too small, such as the 30 amp service pictured at right, or the service has been upgraded or over-sized and is plenty large enough for the home. I don’t find much in-between those two.
I got to thinking about this while inspecting a 3,600 sf house in Plymouth for an old friend from high school. This home had a 100 amp electric service, which seemed too small for that size of a house. I considered recommending an electrician to do a load calculation on the house, but I first plugged in a few numbers at an online load calculator – http://www.electricalknowledge.com/SFDLoadCalc.asp .
I didn’t have all of the exact numbers that I needed, so I guessed on a bunch of them, such as the VA ratings on the garage door openers, garbage disposer, dishwasher, and microwave. I put in 1800 VA for each one of these, which is certainly way too high, but it makes me feel better about guessing – at least I’m not guessing on the low side.
This home was heated with a gas furnace, had a gas clothes dryer, a gas water heater, and a gas oven. The only major 240 volt appliance at this home was the air conditioner. This is pretty common for a home in Minnesota.
Can you guess what the calculated service size was? 68 Amps.
Granted, my calculation probably wasn’t perfect, and I’m not sure that this online load calculator was completely accurate, but this was enough to make me not worry about the service size. When I took an electrical inspection class many years ago, we had to perform a lot of load calculations for fictional houses, and I learned enough to know that if most of the major appliances are gas, a 100 amp service is probably plenty enough.
I invite you to plug in the numbers from your own home at the online load calculator that I linked to above – you might be surprised at how small of a service you could actually get away with. For the record though, the smallest allowable service for a new home today is 100 amps.