Adding Additional Load to Service Panel for Accessory Dwelling Unit

W

Will Wire

Senior Member
Location
California: NEC 2020
Occupation
Electrical Contractor
I am doing various load calculations potentially replacing existing electrical appliances with gas to see if I can add an ADU (Accessory Dwelling Unit) to the existing 200 Amp Service Panel.

I am following the guidelines of NEC section 220.83 for an Existing Dwelling Unit.

What is the highest load I can have on this panel within the proper safety limit and good practice?
 
Without knowing the loads on the existing panel, and loads on the added ADU, how could we possibly answer that?
However, you can load a panel up to it's rating, which in your case would be 200A.
 
Thanks. I have done the real load calcs for the existing dwelling and the proposed ADU, which at present puts the load over 200 amps. The customer wanted me to see if they replaced their existing electric appliances with gas, if it would be possible to add the ADU on the existing Panel. I was wondering if the panel could or should be loaded to its rating. I always want to not max out the panel. Is it common practice to load a panel to its rating?
 
The NEC load calcs are way over conservative. Just look at the size of the transformer. The utility is more realistic.
 
I agree with the others. For the most part, I wouldn't even bother doing any calcs adding an ADU off a 200A panel, unless the AHJ was asking for them. If you are required to do calcs then I would have no problem with it being right at 200 amps, as the real load would realistically only be half of that tops.
 
Are you calculating both dwellings separately then adding them together? If so that will give you and inflated load. You need to do the service calculating combining all loads of both units.
 
Another thing to note. The loads would never all be on at the same time.

I had to repair a 100A feeder to a machine shop that was fed from a house with 200A service. With loads on in the house, and machines running, the customer said he has never had a problem. The feeder was repaired due to being hit with a backhoe.
 
Will Wire said:
I am doing various load calculations potentially replacing existing electrical appliances with gas to see if I can add an ADU (Accessory Dwelling Unit) to the existing 200 Amp Service Panel.

I am following the guidelines of NEC section 220.83 for an Existing Dwelling Unit.
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220.83 can only be used with one dwelling unit not two dwelling units.
The way I read it for two dwelling units on one service you use 220.85 'two dwelling units', unless its a farm.
Or do a general calc.
 
Follow Up and Proposed ADU Feeder Calculation

NEC 220.83 is being used because there will only be the single Service. The ADU will be supplied with a feeder to a 125 amp subpanel.

With the existing house replacing all electrical appliances with gas and the proposed ADU having only electricity and no gas, the final calculation combining both for the Service load came to 192 amps.

Now that it is determined that the existing 200 amp Service can accommodate the additional load, can I use 220.82 to determine the load of the ADU individually, and size the feeder and subpanel based on it which calculates to 125 amps?

Thanks
 
Will Wire said:
Follow Up and Proposed ADU Feeder Calculation

NEC 220.83 is being used because there will only be the single Service. The ADU will be supplied with a feeder to a 125 amp subpanel.
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Regarding the service sizing 220.83 is for a singular 'Existing dwelling unit' it implies the new loads would be for that dwelling unit not another dwelling. I doubt the numbers work out much differently as the calcs are similar either way I'd be currious to see.
For an Additional Dwelling Unit (ADU) and a existing Dwelling unit on one service the technically correct method (which is similar almost the same) is to use 220.85 'Two dwelling units' , as you now have two separate dwelling units on one service (set of service conductors / panel load-center etc)
What are all the items from 220.84 (C) for both units?
  1. How many square feet is the ADU and how many the existing dwelling?
  2. How many small appliance and laundry circuits total 6?
  3. Whats the nameplate of all the following in both units
    • All appliances fastened in place, permanently connected, or located on a specific circuit
    • Ranges, wall mounted ovens, counter mounted cooking units.
    • Clothes dryers that are not connected to the laundry branch circuit .
    • Water heaters
  4. Nameplate amps or kVA of all other motors ( pump's etc)
  5. The larger of the AC or heating load.
Add all that up then multiply by 67.5%

What is the size of the existing service entrance conductors? 3/0 CU? 250 kCMIL AL?
 
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curt swartz said:
Are you calculating both dwellings separately then adding them together? If so that will give you and inflated load. You need to do the service calculating combining all loads of both units.
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That doesn't sound quite right. If there are two dwelling units, then doing separate 220.83 calculations and adding them together is an option. So is using 220 Part III or 220.84 (per 220.85). Any of those might come out lowest.
 
jaggedben said:
That doesn't sound quite right. If there are two dwelling units, then doing separate 220.83 calculations and adding them together is an option.
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Here I can only use 220.83 if the entire load on the service is one single existing dwelling.
Will Wire said:
the final calculation combining both for the Service load came to 192 amps.
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with 192A you'll need to make sure you have service conductors with that ampacity per 310.16.
 
AC\DC said:
220.87 covers what you want I use it all the time.hopefully you have smart meters
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I've looked at hourly smart meter interval data to do due diligence per 220.87, but I've never actually relied on it with an AHJ. I've multiplied kWh consumption by a factor of four to get a very conservative limit on max 15min demand. When you have 15min interval data that is much closer to 'demand' data, and should be acceptable per the exception.
 
jaggedben said:
I've looked at hourly smart meter interval data to do due diligence per 220.87, but I've never actually relied on it with an AHJ. I've multiplied kWh consumption by a factor of four to get a very conservative limit on max 15min demand. When you have 15min interval data that is much closer to 'demand' data, and should be acceptable per the exception.
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I asked ours and I think it is 15 minute or 10 minute intervals I can’t remember what they said but the AJ is fine with it.

I got one of those ridiculously priced amp power loggers if worse comes to worse I think it does like every second
 
jaggedben said:
I've looked at hourly smart meter interval data to do due diligence per 220.87, but I've never actually relied on it with an AHJ. I've multiplied kWh consumption by a factor of four to get a very conservative limit on max 15min demand. When you have 15min interval data that is much closer to 'demand' data, and should be acceptable per the exception.
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FWIW, I submitted a 2026 PI to rearrange 220.87 to indicate that the "15-minute" specification would apply both to the base language (1 year of data) as well as the exception (30 days of data). It was rejected with language that indicated that if you are using 1 year of data, you don't need 15-minute interval data, a longer interval is acceptable.

Cheers, Wayne
 
wwhitney said:
FWIW, I submitted a 2026 PI to rearrange 220.87 to indicate that the "15-minute" specification would apply both to the base language (1 year of data) as well as the exception (30 days of data). It was rejected with language that indicated that if you are using 1 year of data, you don't need 15-minute interval data, a longer interval is acceptable.

Cheers, Wayne
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Lol. Good to know!
 
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