220.82 Dwelling Unit Optional Calc (Multiple Panels)

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Npstewart

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When using the Optional Calculation for dwelling units per 220.82, would it be permissible to use this same calculation when you have more than one panel? For example, if you had a main "MDP" Panel outside with (3) indoor panels serving various loads (ie. Panel A,B,C), can the calculation be used on each individual panel A,B,C to size their respective circuit breakers and feeders (or) would all the loads be required to be taken at 100% on panels A,B,C and then 220.82 would only apply to the main MDP for service feeder rating?

This is a friendly debate with a plans examiner whom is also a member of the forum.

Thanks!
 
My reading of that article tells me it applies only to the service or feeder conductors that serve the entire building. So you can't calculate the loads on any of the sub panels using this method.
 
But why not? For instance, take Jamaica, where multi story homes require panels on each floor, and you must do the calculations for each floor... then you combine those, plus any apartments that may be on that service, before deciding on the main panel...

any extra apartments not calculated in must have their own power calculations turned in at the same time.

This is all for the poco to know the service ordered is correct for the building... because so many people run apartments in their homes...
 
charlie b said:
My reading of that article tells me it applies only to the service or feeder conductors that serve the entire building. So you can't calculate the loads on any of the sub panels using this method.
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I think the confusing part for me is that it doesn't say "Service Feeder", nor "Entire Building" but I do understand this stance.

Lets say for instance you have a meter/main. Out of the meter main you are feeding some receptacles, lights, pumps etc., and then you feed a panel inside the dwelling unit which feeds all the air conditioning etc, receptacles, lights etc. So what you're saying is that the first panel (meter/main) would be sized per 220.82 (Optional), but then the interior panel would be sized at 100%? Including HVAC, general loads etc? The problem here is that the panel inside would likely end up larger than the actual service.

I understand this is different then my initial example but the problem is that using this logic does not seem to work in practice for every scenario, whereas applying 220.82 for each panel individually always seems to work.

Thanks everyone!
 
Npstewart said:
I think the confusing part for me is that it doesn't say "Service Feeder", nor "Entire Building" . . . .
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Well, yes it does. Look at the first sentence of 220.82(A). It says that the article applies to a dwelling unit having the total connected load supplied by a single service or feeder. It goes on to say how to calculate the load on that service or feeder. It does not say that the process can be used to calculate the load on sub panels.

 
Npstewart said:
OK - So the 40% After the 1st 10kW + 65% HVAC etc. would apply to the "Sub-Panel" per the above quote?
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Not at all. Those apply in circumstances covered by the optional calculation method. That method only applies to the load powered by conductors that serve the entire building. So if you add up all the loads in the entire dwelling unit, and apply the factors allowed by the optional method, you can use the results to size the service (or feeder) to the entire unit. But to calculate the loads on a sub panel, you would use the Part III rules. That includes, for example, a lighting load demand of 100% of the first 3 KVA and 35% of the next 117 KVA. It would also allow a 80% demand factor for 4 units of kitchen equipment.

 
charlie b said:
Well, yes it does. Look at the first sentence of 220.82(A). It says that the article applies to a dwelling unit having the total connected load supplied by a single service or feeder. It goes on to say how to calculate the load on that service or feeder. It does not say that the process can be used to calculate the load on sub panels.
Click to expand...

We all agree that the 14 NEC does allow the optional calculation to be used on feeder and service. Unfortunately the wording does not directly state if 1 or more panels can be calculated using the optional method. If you have a meter feeding a 400 amp main feeding an MDP feeding 3- 200 amp panels your house is still fed by 1 service and 1 feeder that carry 100% of the load. It is not stated that all feeders need to carry 100% of the total load. If you had a SFR with a 200 amp meter, main and the main was feeding a 200 amp interior panel and also a small pool equipment panel the optional calc could not be used for the feeder? If the standard calc is used you may need more than 200 amps feeding the interior panel.
 
220.82(A) also states that if applies to a dwelling unit having the total connected load served by a single 120/240V or 208Y/120V set of 3-wire service or feeder conductors with an ampacity of 100 or greater. A 3 phase 208Y/120V service or feeder is a 4-wire set of conductors, correct? So is this language meant to mean it's applicable only to a single phase service or feeder?
 
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