How can I find out the size of the electric service to a house?
Thursday, July 12, 2018
You can get a clue as to the size of the electric service before you even set foot inside a home by looking at the electric meter outside. A home with the meter glass dome mounted on a square metal base probably has 100 amp. service, and a newer 125 to 200 amp. service will have a rectangular base that extends further below the glass dome than above. There are very few 60 amp. services left nowadays, but they will have the glass dome mounted on a round base that is the same diameter as the dome, or the meter will be behind a flat glass window that is flush with the front of a metal box enclosure.
To verify the exact size of the service, find the main electric panel and look for a breaker switch or fuse marked “MAIN”. It will usually be separate from the rows of the other breakers, above or below them, unless the panel is backfed, in which case it will be at the top of one of the rows of breakers. The amperage rating will be marked on the side or front of the switch.
If the panel has multiple breakers (up to six) that are marked “MAIN,” then you are looking at a split-bus panel from the mid-20th century. All of the “MAIN” breakers must be shut off to disconnect power to the home and none of them reflect the rating of the electric service. There are two ways you can figure out the service rating of a split bus panel. One way is to look carefully at the paper manufacturer’s data plate on the inside of the panel door for the amperage rating of the panel, which will show the maximum service size it is rated for.
But the paper may be missing and the rating might be more than the size of the service—which can only be verified by determining the size of the service wires inside the panel box. We do not recommend the you remove the dead front (cover panel) to examine the wiring, so leave this to an electrician. Here is a chart of the rating of a service based on wire size in AWG (American Wire Gauge). Lower numbers indicate a larger wire.
To learn more about split bus electric panels, go to our blog post ”What is a split bus electric panel?”
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