New construction single family dwelling with sub panels calculation?

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joedreamliner787

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Hi everyone,
Have a large home that will have an attached guest unit (has its own kitchen, washer/dryer etc. Panels A & B will serve the main house and panel C will be the sub-panel in the guest unit area. When performing the optional method calc do I perform the calc on the entire sq ft of the main house including the guest house or do I perform two separate calcs (1 for main house 1 for sub (standard method) and then add them together for the service?
I will add there will be a 400 amp service into the meter - x2 200 AMP Service rated ATS and each of the ATSs will feed 1 200 AMP Panel. Subpanel C will be fed from panel A on a 100 amp breaker.

Thank you in advance
 
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joedreamliner787 said:
When performing the optional method calc do I perform the calc on the entire sq ft of the main house including the guest house or do I perform two separate calcs (1 for main house 1 for sub (standard method) and then add them together for the service?
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Let's call the two dwelling units 1 (main house) and 2 (attached smaller unit). From your description you have a 400A service for 1 and 2; a 200A feeder for panel A, which supplies part of 1 and all of 2; a 200A feeder for panel B, which supplies the other part of 1; and a 100A feeder (from panel A) for panel C, which supplies all of 2.

So you definitely can use the optional method for the 100A feeder supplying panel C. And for the 200A feeder supplying panel B, you definitely can not; you need to calculate the load on panel B with the standard method.

Now there's a school of thought that the language in 210.82 on the optional calculation limits it to a feeder or service supplying exactly one dwelling unit. Under that theory, you'd need to use the standard calc for the 200A feeder for panel A, as well as for the 400A service.

I don't find that convincing. The language at the start of 210.82 tells you when you can use the load calc for the dwelling unit itself; it doesn't explicitly restrict the calculation to feeders or services that serve only that dwelling unit. This is in contrast to the language in 310.12, which is very clear that it applies only to a service or feeder that serves exactly one dwelling unit. [Which means you can only use 310.12 for the 100A feeder supplying dwelling 2, not for any of the other feeders or services.]

So under this school of thought, for the 400A service you can do separate 210.82 calcs for each dwelling unit and size the service to the sum of the load calcs. For the 200A feeder to panel A, you'd need to do the standard load calc for the part of dwelling 1 supplied, and add to it the optional load calc for dwelling 2.

Cheers, Wayne
 
wwhitney said:
Let's call the two dwelling units 1 (main house) and 2 (attached smaller unit). From your description you have a 400A service for 1 and 2; a 200A feeder for panel A, which supplies part of 1 and all of 2; a 200A feeder for panel B, which supplies the other part of 1; and a 100A feeder (from panel A) for panel C, which supplies all of 2.

So you definitely can use the optional method for the 100A feeder supplying panel C. And for the 200A feeder supplying panel B, you definitely can not; you need to calculate the load on panel B with the standard method.

Now there's a school of thought that the language in 210.82 on the optional calculation limits it to a feeder or service supplying exactly one dwelling unit. Under that theory, you'd need to use the standard calc for the 200A feeder for panel A, as well as for the 400A service.

I don't find that convincing. The language at the start of 210.82 tells you when you can use the load calc for the dwelling unit itself; it doesn't explicitly restrict the calculation to feeders or services that serve only that dwelling unit. This is in contrast to the language in 310.12, which is very clear that it applies only to a service or feeder that serves exactly one dwelling unit. [Which means you can only use 310.12 for the 100A feeder supplying dwelling 2, not for any of the other feeders or services.]

So under this school of thought, for the 400A service you can do separate 210.82 calcs for each dwelling unit and size the service to the sum of the load calcs. For the 200A feeder to panel A, you'd need to do the standard load calc for the part of dwelling 1 supplied, and add to it the optional load calc for dwelling 2.

Cheers, Wayne
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Hi Wayne, thank you for the clear and concise explanation. Panel A is loaded power panel and Panel B is a lighting panel all for the main house. I can feed the guest area from either panel A or B as there is enought head room in each. It is interesting because there is no heating and cooling load in the guest area. It's all geo thermal with no zones and the main house is taking care of it but the guest area does have 2 dedicated small appliance circuits. If I am understanding correctly each dwelling area will get the optional method calc and I will add those together but for the panel B (it is a lighting panel) I have to perform the standard method.
 
I would think it's a single dewelling unit. Unless your adding the attached area. Not two separate units. Your not adding sq ft to an existing dewelling. It's not a separate structure. One permit, one service, one calculation. It's simply another room to the house.
Would not be any different if it was a finished basement with another kitchen.

Your title says:
New construction single family dwelling with sub panels calculation?

If your calculating the attached area for so called second dewelling for panel C that portion is included in you service calc then divided between your two panels.
Then take your loads for that separate area and add to C.
Size your feeder based on those loads.

I do not think you can pick and choose between the two for a single dewelling under a single permit new construction.
 
Tulsa Electrician said:
I would think it's a single dewelling unit.
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That depends a lot on the details. From the description I'm envisioning one roof, and two separate volumes under that roof with no way to move internally between them, each with its own "front door". And of course each providing "permanent provisions for living, sleeping, cooking, and sanitation." That would make it two dwelling units, and my answer was based on that.

Cheers, Wayne
 
wwhitney said:
That depends a lot on the details. From the description I'm envisioning one roof, and two separate volumes under that roof with no way to move internally between them, each with its own "front door". And of course each providing "permanent provisions for living, sleeping, cooking, and sanitation." That would make it two dwelling units, and my answer was based on that.

Cheers, Wayne
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It is one dwelling with a guest area. 1 roof, 1 permit, 1 service. The sub-panel in that area is being installed because it cuts down on a lot of home runs from the main panel. There is an interior door in the hall between section A and section B. I'm to attach the power plan as a separate reply.
 
Hi everyone and I appreciate all the advice. Here is the power plan so we can see the layout to hopefully make more sense of it. The lighting is a separate plan.
 

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  • Power Plan.pdf
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wwhitney said:
That's just one dwelling unit, the "guest wing" has no kitchen.

So for the service, one optional calc; for each feeder, the standard calc.

Cheers, Wayne
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Hi Wayne, thank you.
The Den area has a dishwasher, undercabinet refrigerator and will have an induction cooktop. It's not shown on the plan but the home owner is placing one in. Circuit 10/12 is a dedicated 50 amp range recepticle. Same rule still applies?
 
One method only. It's one or the other, not one and the other! It's one service!
The only thing that can be used in both methods is the neutral load as figured in 220.61.
 
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Little Bill said:
One method only. It's one or the other, not one and the other! It's one service!
The only thing that can be used in both methods is the neutral load as figured in 220.61.
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So I can use the optional calc for the entire home am I correct or do I need to do what Wayne is mentioning and for each 200A feeder do an optional calc and a standard for the subpanel?
 
You can use either method, but you can't go back and forth between them. Usually, the optional method will give you a lower total load.
 
Little Bill said:
You can use either method, but you can't go back and forth between them. Usually, the optional method will give you a lower total load.
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Okay so I can just use the optional method for the entire home 3va per square foot and all the other norms - heating/cooling and small appliabcr circuits etc.
 
Little Bill said:
One method only. It's one or the other, not one and the other! It's one service!
Click to expand...
Sure, for the service you pick one method. For example, if it's all one dwelling unit, you can definitely use the optional method.

But if you then need to calculate the load on a feeder that is carrying only part of the dwelling unit load, the optional method isn't applicable, correct? So you have to use the standard method for that feeder.

Cheers, Wayne
 
wwhitney said:
Sure, for the service you pick one method. For example, if it's all one dwelling unit, you can definitely use the optional method.

But if you then need to calculate the load on a feeder that is carrying only part of the dwelling unit load, the optional method isn't applicable, correct? So you have to use the standard method for that feeder.

Cheers, Wayne
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I will do the optional method 3va per sq foot for the whole home and then do the standard calc for the sub panel and the area it serves to size the sub panel feeder. The reason I got confused was because I thought you had to add those together (whole home & sub that serves the 2nd area) but if I did I would be counting the 2nd area twice.
 
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