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Calculating electric Load for 2 structures on same service panel

AdamPNW | Posted in Building Code Questions on
Hi all, I’m familiar with Jon Harrod’s article about using the NEC 220.53 to determine if a service upgrade is needed. 
But what if you want to add a new 2400sqft structure (our future primary residence) to our existing 200 amp service (which currently serves our 900 sqft manufactured home)?
To calculate the extra load, do we simply add the 2 structures together, treating them as one big house?  Or is there a demand factor % that comes into play when calculating separate structures?
Some load info:  both structures will be all electric with ductless mini-splits. New house will have Heat pump water heater, high performance envelope, 10kw PV array, and a single level 2 car charger.  Mild climate (4c)

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  1. gusfhb | | #1

    REally your local building inspector is the only one who can answer, whatever anyone tells you, even if it is 'correct' can be overruled by them.
    If it is temporary it would probably be fine, but permanent I would think needs its own service

  2. nynick | | #2

    I'm going through this exact same issue with our home renovation and detached garage addition with an apartment above. We currently have 200 Amp service and want to convert the house to all electric and then have the garage/apartment all electric. The electricians all recommend going with a new/separate 200 for the garage or a 400 in the house that feeds the garage. The 400 amp panels have a 1 year plus waiting list. Good luck with that. Even getting a new 200 with new meter is problematic potentially with the utility.

    You're especially loaded up with those car chargers, BTW.

    If I can't get a new 200 amp service, I'm investigating just using gas for the garage/apt. and a 100 amp sub-panel fed from the house. I've done this before and it worked fine.

  3. Expert Member
    BILL WICHERS | | #3

    With these two structures being on the same property (presumably), I would calculate loads by treating them as "one big house", which is essentially how loads are calculated for multitenant occupancies like apartment buildings. The issue you run into is that if both dwellings will be simultaneously occupied, demand factors don't really help you much with only two of things. It's very likely, for example, that both dwellings would be cooking something in the oven at the same time, or running a clothes dryer at the same time, etc.

    The easiest way to deal with this is to use a subfeed type of arrangement, so you'd have a main "panel" that would have breakers feeding only the main panels in each of the structures and no branch circuits. In a system like this, you would have a 300A-400A main panel fed from the service, and you'd have 200A circuit breakers in that panel to feed the two structures, so that each structure was served by a seperate 200A breaker and feed from that main panel. If you're using Siemens equipment, you'd use either type MD-T or type MPP breakers in that panel, and you'd be using a panel capable of supporting multiple breakers of that type. You don't even need a "main" breaker in this case, since the code allows for up to six "mains", so each of the two 200A breakers could act as seperate "mains". My own recommendation is to put in a proper "main" main breaker though ahead of those 200A breakers to make service work easier.

    With a system like I describe, you still have only one service from the utility, but each structure gets it's own seperate 200A feed. This is usually a better way to go than a "400A" panel. Keep in mind that there are all kinds of lead time issues with electrical switchgear right now. I have several projects currently delayed by exactly these kinds of problems. It's very frustrating to schedule jobs right now.


    1. AdamPNW | | #5

      Thanks Bill, great to know about the subfeed idea. So to clarify, this design would start with a service upgrade to 320 amp. This service would feed a 400 amp main panel with two 200 amp breakers (each feeding a structure). Then each structure has a 200amp sub panel with individual circuit breakers.

  4. Expert Member
    Akos | | #4

    Since this is an ADU, as Bill said these are treated as a multiplex.

    Your electrician should be able to do a demand factor calculation for them. The way it works in our code is heating load from both structures are treated at 100% demand and the rest is factored for the demand (65% of demand for ADU in our code). Large continuous loads such as EV charge or hot tub are than added at 100% on to this combined load to get service size. This generally means that it is not a problem to have a 200A service feed multiple all electric places.

    This does take a bit of work to calculate and some people take the easy way out (ie 2x200A panel, so 400A service). This is not needed as the demand calculations are conservative, you are paying more for a service that you never use.

    1. AdamPNW | | #6

      Excellent, I’ll look into local treatment of ADU demand. Also, I plan to use load sharing between the EV charger and the dryer, for example. And, when available, some intelligent load shedding to interrupt certain circuits when getting close to maximum.

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