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I've replaced several meter panels and subpanels over the years but I never had to figure out amp ratings for a new dual meter main. Here is what is existing:

125A meter main panel feeding house

1 1/4" pvc underground pipe to garage feeding 50A sub

Here is a description of the finish product:

  • Main panel that feeds front house is replaced
  • Garage is converted to a single family dwelling unit, will now call it "rear unit"
  • Rear unit new 125A sub panel is feed on its own meter from new dual meter main panel at front unit

Questions:

How do I size the main panel? 200A or 400A?

Does the optional ampacity calculation in the NEC give a more reliable answer?

What is the best formula for converting the total wattage of the front unit to find the amp rating for this new main panel?

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  • Hello, and welcome to Home Improvement. Good question; let's see what our pros say. And, you should probably take our tour so you'll know how best to participate here.
    – Daniel Griscom
    Jul 27, 2019 at 23:31
  • How many square feet are the two units? Does the existing unit have electric or gas for the "three sisters" (range/cooker, hot water heater, dryer), and what does it use for an HVAC system for that matter? Is replacing the existing conduit between the two structures in the cards? Are there grounds (i.e. in the middle of the yard, not on either structure) lighting/receptacle loads to be concerned about here? Are there other outbuildings on the property for that matter?
    – ThreePhaseEel
    Jul 27, 2019 at 23:50
  • 1100 sq ft total
    – user104380
    Jul 28, 2019 at 2:19
  • 950 front unit, 150 rear unit, there are no plans to run gas so range and hot water and heating and AC are all electric. I am suggesting replacing the pvc run with 2" pvc to the back unit, its less than 150ft so I suggested 3/0cu to feed the 125A panel for the rear. Nothing between buildings is happening, nothing in the way other than the water line which has been located
    – user104380
    Jul 28, 2019 at 2:30
  • Will the rear unit have its own laundry facilities? Also, how many kitchen appliance branch circuits does the existing (front) unit have, and is the front unit set up with a heat pump, or electric heat + a straight air conditioner?
    – ThreePhaseEel
    Jul 28, 2019 at 3:02

1 Answer 1

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Power: Electric houses just need a lot

Generally speaking, all-electric houses get 200A, 320A or 400A service. Now you are adding a second dwelling that's also all-electric. So we can definitely disqualify 200A (for both) as inadequate.

Therefore your best bet is a 400A meter-main. Even if the power company won't provision that much, they could upgrade the service later if your usage warrants it.

The size of the main breakers will be decided by the power company.

Subpanels: Think big

As for the subpanels, **the single most important thing about panels is to provision enough spaces. 40 spaces per panel is not too many, especially in an all-electric house where many loads will be 240V and gobble up 2 panel spaces. 30 spaces is a bare minimum that will likely cause you trouble later, but it may be suitable for the auxiliary dwelling. Given the small cost to upgrade to a 40-space, there's no reason to paint yourself into a corner.

We certainly do not want you back here later going "my panel is full, how do I add this breaker?"

And double especially, don't make the most common mistake I see: chintzing out on the panels, and then spending a fortune on copper wire when aluminum is the right stuff, and 1/3 the price.

You may be accustomed to using "double-stuff" breakers to economize on spaces. These are no longer practical, due to the need for almost every breaker to be AFCI or dual mode AFCI/GFCI. These are not made in double-stuffs.

You mentioned a 125A subpanel, and that will surely be much too small in terms of numbers of spaces. The subpanels can be any size you please; that is merely a current limit for the buses and it doesn't need to match the official main breaker in the meter-main.

If the subpanel has its own main breaker, there's no need for it to match either. So you can feed a 225A subpanel from a 125A or 200A main - no trouble at all. The panel in the house doesn't even need a main breaker since it's in the same building as the meter-main. If it's not attached (e.g. via a breezeway), the auxiliary dwelling would need a "main breaker"; actually what it needs is a main disconnect switch, but using a "main breaker panel" is usually the cheapest way to do that.

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  • In Florida and in Illinois the power company doesn't determine main breaker sizes. It is always included in the electrical specks provided by the builder.... and yes definitely, the customer might need a 400 amp main because of connected load but the power company will probably only supply half of that.
    – JACK
    Jul 28, 2019 at 16:55
  • @Harper... Can you give me a reference in the NEC that will help me make this determination? I appreciate opinions and advice but I need to back up the decision I make with something the inspector will pass the first time
    – user104380
    Jul 29, 2019 at 13:57

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