How to know how many breakers I can put into the main panel

circuit breakerelectricalelectrical-panel

The service to my home is 100A but the main panel is rated for 150A and has

  • one 40 amp breaker (stove)
  • one 30 amp breaker (dryer)
  • four 20 amp GFI (bathrooms, kitchen)
  • sixteen 20 amp breakers (lights, outlets)
  • two 40 amp breakers at the bottom

My Questions:

  1. I have a 150A panel on a 100A service. Is this okay?
  2. I have 510 amps worth of breakers in my 150A box. Is this okay?

main panel

Best Answer

  1. Yes, that's fine. This is a sub-panel and the 100A breaker must be elsewhere, perhaps outside near the meter. Equal to or greater than the rating is fine for any sub-panel. In the unlikely event that your loads draw more than 100A, the main breaker (apparently located elsewhere) will trip.
  2. Generally fine, specifically you need a load calculation (Per NEC Article 220) but assuming it was put in by a licensed electrician, and additional circuits have not been added willy-nilly without one, that's presumably been done. It's is quite normal for the sum of all the breakers to vastly exceed the incoming amperage, because not all circuits are fully loaded at all times - which the load calculation takes into account. Additionally, you have 100A at 240V coming in, so any single breakers (whether half-width or full width) only count for half, since they are 120V.

There only appears to be space for 2 additional half-width breakers, physically. There's also what appears to be a 40A half-width two-pole switched off on the lower right, which you appear (from your breaker count) to be counting as 2 20A, which it's not.

So what's in there in terms of breaker amperage at 240V is 20 120V 20A, (GFCI and Plain) that only count for 200A at 240V, Plus 70A of turned on two-pole, plus 40A of turned off two-pole, for a grand total of 310A (or 270 currently switched on.)

The load calculation takes into account all the loads, based on the size (square footage) of the house for some, and on the ratings of the load itself for others.

In the event that you require a service upgrade for additional loads you plan to add, an upgrade to 150A would not require changing out the panel (it might or might not need changing the wire between the main breaker and this panel, depending on the choices made at install time for the size of that wire, but nothing else.) You would, however, need another panel if the new loads don't fit this one, physically. That might look like 200A service and a 100A breaker to this panel, and a 100A breaker to a new sub-panel (which might be a 200A panel, or a 150, or a 125, or a 100 - the 200A usually gives the most physical spaces, and does not cost much more.)

If the panel labeling and your load calculation allow, you might be able to free up some space with more half-width breakers, but those can only go into spaces where the panel labeling permits them to go - which is often NOT all the spaces. So it may not be an option here.

Related Topic