Electrical Service Size

One thing that all home inspections need to include is a description of the home’s electrical service size, including the voltage and the amperage.  In this blog post I’ll give some more information about the electrical service size, what it means, and how it’s determined.

The vast majority of single family homes in the United States have 120/240 volt service.  Voltage is a measure of how hard electricity is being pushed through the wires or how hard it wants to move between two points.  Using the analogy of water, voltage describes the water pressure – how hard water is being pushed through the pipe.  There are three wires coming into the house from the utility transformer – two hots and one neutral.  Each of these two hot wires is at 120 volts to the neutral and 240 volts to each other.  The utility is the one pushing electrical voltage into the house, so you have no say in your voltage.

As a quick aside, a very few houses but many high-rise apartments will have 120/208 service instead of 120/240.  The difference is very minor, and those details are for a different time and different blog post.

From here you can connect to one hot leg and the neutral for a 120 volt circuit for most things in the house like lights and ordinary outlets.  Or you can connect to both of the hot legs for a 240 volt circuit for the things in your house that use a lot of electrical power, like your central air conditioner, electric oven, electric water heater, etc.

Then there’s the question of how many amperes (or just amps) you have coming into the house.  To return to the water analogy, current is like how much water will flow into the house – how fast you can fill up your bathtub for example.  The more current you have the more electrical power you have and the more things you can power at the same time.  Larger houses need more amps than smaller houses, and some apartment units need even fewer amps than a house.  Most new houses these days have 200 amps, but many houses and most smaller houses get by just fine on only 100 amps.  Very large houses might have 400 amp service.  Small apartments might have only 60 or 70 amps.  The minimum code-approved service size for a single family home today is 100 amps.

As I said before, the utility is the one pushing voltage into the house and everyone has the same voltage: 120/240.  You have no say in the matter.  But you do have a say in how many amps are coming into the house.

And here are the things that determine what your service size is.

First, the wire.  The service entrance cable is the main set of wires coming into the electrical panel from the utility.  Based on its material (either copper or aluminum) and its size, the service entrance cable can safely carry a certain amount of electrical current.  That’s the cable’s rating.  If too much current flows through the cable then it can overheat and that’s a fire hazard.  The cable might be rated for 100 amps, or 125 amps, or 150 amps, or 200 amps.

Second, the electrical panel itself.  The panel’s manufacturer specifies the maximum service size that the panel can safely handle, and you’re not allowed to run more current through the panel than that.  The panel might be rated for 100 amps, or 125 amps, or 150 amps, or 200 amps.

Third is the main circuit breaker or main fuse.  This main breaker should be rated high enough to protect the wire and the electrical panel from having too much current flow through them.

The service size is the smallest rating of these three items – the service entrance cable, the electrical panel, and the main breaker.  That’s it.  That’s the rule!

In most houses these are all the same rating.  The service entrance cable might be 2/0 copper, so rated for 200 amps.  The electrical panel would be rated for 200 amps, and the main breaker would be 200 amps.  Sometimes I see cable and breaker rated for 100 amps, but the panel rated for 125 amps.  That’s fine, and so the service size is 100 amps.  If I were to see a panel rated for 200 amps, with cable rated for 150 amps, and a main breaker rated for 100 amps, then the service size is 100 amps – the smallest of those three items.

A problem comes up when the main breaker size is too big to protect the service entrance cable or the panel.  For example, if the panel is rated for 100 amps, and the service entrance cable is rated for 100 amps, but the main breaker is rated for 200 amps then you have 100 amp service by definition.  But the main breaker is too big to protect the wire and the panel, so that’s a big problem.  Not only is it unsafe, but it tells me that the person who installed it didn’t know what the heck he was doing.  The main breaker has to be sized properly to protect the cable and the panel.

In summary, the utility provides you with 120/240 volts.  Everybody has that.  Your amperage service size is the smallest of the ratings for the service entrance cable, the electrical panel, and the main circuit breaker.

A very common question is, “Does this house have a big enough electrical service?”  To determine for sure what service size a house should have requires doing a load calculation.  The method for doing this is detailed in the National Electrical Code, but it’s basically just a matter of adding up all the electrical loads in the house and making allowances based on the size of the house for general lighting and outlet loads.  So of course the answer will be different depending on whether the house has a gas furnace or an electric furnace, a gas oven or an electric oven, a gas water heater or an electric water heater, a gas clothes dryer or an electric dryer.  You get the idea.  The load calculation spits out a number of amps required, and then you just move up to the next higher standard service size.  So if the load calculation determined that you need 157 amps, you would just move up to 200 amps.

Another common questions is, “How do I increase my service size?”  And of course how much will it cost?  To increase your service size you need to increase the rating of each of the three components – service entrance cable, electrical panel, and main breaker.  The panel and breaker generally come together as a package, and you’ll need to replace them.  You’ll also need to replace the cable that runs into the electrical panel from the service point, which is the place where the utility’s wires connect to yours.  There’s a good chance that you’ll also need to replace the metal conduit that these cables run through.  How hard all this will be depends on many variables, so it’s best to call an electrician to get an estimate.  But today for a single family home upgrading from 100 amp to 200 amp service might cost anywhere from $ 3,000 to $ 6,000 or more.