Your electrical service size is how many amperes and how many volts you have coming into your house.  This determines how much electrical power you have to run your electrical devices.  Here are some things to know about service size.

The overwhelming majority of residential dwellings in the United States has 120/240 voltage.  Some dwellings, particularly in large multi-family buildings, have 120/208 voltage.  But these are the only two options for a residential dwelling – everybody has 120 volts for their basic lighting and outlet circuits and either 240 or 208 volts for their larger appliances like central air conditioners and electric water heaters.  There are no exceptions.

In the analogy of water flow to electricity flow, voltage is like the water pressure – how hard the water is being pushed through the pipe.  Or how hard the electricity is being pushed through the wire.  The voltage is determined entirely by the utility.  You have no say in the matter.

What differs from house to house is how much electrical current capacity you have.  In the water analogy, current is like the total amount of water that can flow through the pipes.  Many houses and most apartments and condominiums have 100 ampere service.  Some houses, especially newer houses, have 200 amp service.  Some smaller apartments have only 60 amp service.  And some very large houses have 400 amp service, or even higher.

More amps means that you can power more things at the same time.  More amps means that you can have your air conditioner conditioning and your coffee maker making coffee and the toaster toasting and the hair dryer drying all at the same time.

To determine what your service size is you look at three things: (1) the size of the main electrical breaker for the house, (2) the rated size of the service wire coming into the house, and (3) the rated size of the main electrical panel.  The main breaker is stamped by the manufacturer with the current that will cause it to trip off if exceeded.  The service wire has a certain capacity of current that it can handle safely based on its size and material (either copper or aluminum), and that’s determined by the National Electrical Code.  The manufacturer of the electrical panel lists the maximum current that the equipment can safely handle.

So you look at these three numbers – the current rating of the main breaker, the current rating of the service wire, and the current rating of the main panel, and you take the smallest of these three numbers.  That’s your service size.

As an example, if the main breaker is 100 amps, the wire is rated for 100 amps, and the panel is rated for 125 amps then your service size is 100 amps.

The big question of course is whether or not the service size is big enough.  Is 100 amps enough for this house, or should it be 200 amps?  And of course the answer is: it depends.

Most houses in my area use natural gas to power the furnace or boiler, which is generally the biggest energy user in the house.  Most houses in my area also use natural gas for the water heater and natural gas for the kitchen range and even for the clothes dryer.  In this case 100 amps is almost certainly enough for the house because all of the large energy loads are being handled by natural gas.

The only way to know for sure what your electrical service size should be is to do a load calculation using the standard process as outlined in the National Electrical Code.  You start with the size of the dwelling for typical lighting and outlet circuits, and then you add up all the electrical appliances you have, including for space heating, water heating, clothes drying, cooking, even the dishwasher.  The formula spits out the minimum number of amps the dwelling should have, and you go up from there to the next biggest standard size.  So for example if the load calculation determines that the house needs 85 amps then 100 amp service is fine.  If the load calculation determines that the house needs 132 amps then you can go up to the standard of 150 amp service, or you’ll probably just go to 200 amps.

Doing a load calculation is far beyond the scope of a standard home inspection, but any home inspector worth anything should be able to give a rough estimate of whether a house’s service size is big enough.

A word about power

This blog post started out by defining your electrical service size as how much electrical power you have coming into your house.  So let’s talk a little bit about power.

Here’s a pile of bricks.

Now let’s say that you want to move this pile of bricks from one side of the room to the other.  That’s the definition of “work”.  Work is force times distance.  So the work of moving this pile of bricks is the force required to do the work multiplied by the distance that the pile moves.

Now, my mother is approaching 90 years old, but she could move this pile of bricks.  Seriously, she could.  It might take her four weeks, and she’d need somebody to break the bricks into smaller pieces first, but she could do it.

My son, who goes to the gym six times a week, could also move this pile of bricks, and it would take him about 10 minutes.

Power is how fast you can do work.  My son can generate more power so he can do the same amount of work in a shorter time.  Now, let’s say that your job is to keep the pile of bricks very small, but someone keeps adding a brick every so often.  My mom would quickly get overwhelmed, and the pile would grow.  But with my son’s power he could easily keep up with the brick addition and the pile would remain small.

Now let’s think about this another way.  Suppose it’s a hot, sunny day and you need your air conditioner to cool your house.  It’s not just a matter of removing the heat in your house, because you also have Heat Miser constantly dropping little heat bombs in your house, adding to the heat load.  So what you need is an air conditioner with enough power to remove heat at a rate faster than heat is being added.

Electrical power is calculated as voltage times current.  The electrical power you have coming into your house is the service size in amperes times the voltage, which is probably 240 volts (but it might be 208 volts in some cases).

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