What size electric service do I need?

Question

My son and I are building a house at the beach. I currently have a 100 amp temp service and my question is “can I get everything I need on the service to pass code if I keep it at 100 or 125Amps”?

The inspector is telling me that I can only use 80% or 80 amps if I keep the box at 100 amps but that doesn’t seem right because the load center that I bought has a 100 amp main breakers and 145amps worth of branch breakers that came with it when I bought it new.

I have been reading the NEC codes and the way I understand it I need 2ea 20 Amp 120 circuit plus 1ea 50 Amp 240 circuit for the kitchen, 30 Amps 240 for heating and I have an AC unit that runs off of 20 Amps 240. Which adds up to 110 Amps before I do an outlet or light circuit so I must be figuring this wrong.

Should I be figuring by watts or volts instead?

Answer 1

My advice would be to install the largest service that you can get regardless of how much you plan on using in the short term. 200amps is pretty typical for a lot of residential areas in North America. Reasons:

1. In most residential areas, you pay for what you use, not overall capacity
2. There might not be capacity on the transformer later when you need it
3. Replacing the feeder cables can be very expensive
4. Replacing the meter and panel later is expensive
5. You won't have to worry about it again

Years ago, a lot of houses only had 60amp service. Consumption increased for a while especially with the increase in electronics and lighting, but now with LED, it looks like it might be going the other way again.

Also consider what amenities you might add later. A pool, hot tub and second kitchen can easily put you over capacity if only have a small service.

Answer 2

It's best to size load by Watts (or Volt-Amps). The NEC has some minimums that must be included that often push homes over 100 Amps. I'm assuming electric applinaces.

220.82(B)(1) = 3VA per square foot. Assume 1,500 sf home = 4,500VA

220.82(B)(2) = 4,500VA (2 kitchen circuits + 1 laundry at 1,500VA each)

220.82(B)(3) = 5,000VA clothes dryer; (220.54) = 4,500VA water heater; = 8,000VA range. (220.55)

220.82(B)(4) = any other motors like pumps.

220.82(B) = 4,500 + 4,500 + 5,000 + 4,500 + 8,000 = 26,500VA = 40% anything above 10,000VA = 16,600VA = 69.2 Amps @ 240V

220.82(C) = Add A/C nameplate. 2-ton is about 17.6 Amps

69.2A + 17.6A = 86.8 Amps @ 240 Volts. This is the bare minimum for a 1,500sf house. And other known loads connected need to be added. I'd suggest new homes get a 200A service size.

The 80% rule is from 210.20. It is often misunderstood, even by inspectors and electricians. Loads are separated into two categories, continuous and noncontinuous loads. All breakers and fuses are rated for 100% of noncontinuous loads + 125% of continuous loads.

The inverse of 125% is 80%. So a 100A panel can only hold 80A of continuous loads. Section 100 defines continuous load as any load that runs at peak amperage for 3 hours or more. This is different than being "on" for 3 hours. I contacted a kiln manufacturer when I was green because their listed amperage was 49.5A on a 50A outlet. I thought I knew more than them and pointed out the 80% rule, kilns run all night, well more than 3 hours. They pointed out to me that the kiln cycles on and off to maintain the temperature inside. The peak amperage is not continuous. I had a "duh" moment.

There's not a lot of stuff that runs at peak amperage for 3 hours or more.

Answer 3

I cannot think of any reason not to install a 200 amp service.

Answer 4

It is an old post, but I thought it might be important to note this for others that might come upon this looking for answers. I believe the initial poster was confused about what a 100 amp service actually is, and nobody seemed to catch it, he was confusing his panel with the service. The service size has everything to do with the size of the cable that comes from the street to your house, and nothing to do with panel size. Regardless of how big a panel you think you have, if you have a 100 amp service running to your house that is all you get! If you have a 100 amp service and you need more, you can't just buy a new panel, you need to upgrade the service to your house!