Dwelling Unit Optional Calculation

[Grouch1980]

Hey guys,
I'm looking at section 220.82 in the 2008 NEC, the optional calculation for a dwelling unit. In order to be able to use this section, the total connected load has to be served by a single 120/208 or 120/240V set of 3 wire service or feeder conductors, with an ampacity of 100 amps or greater.

My 2 questions are:
1. this section is based on total connected load, and not demand load, is that correct?
2. If my demand load is 78 amps, but my connected load is 102 amps... I could use #4 AWG feeder wiring, which is rated for 85 amps, based on demand load. But am I allowed to use this section? Since the total connected load is 102 amps which would require #2 AWG feeder wiring. But i would only be specifying #4 wire based on demand load.

 

[Hv&Lv]

Hey guys,
I'm looking at section 220.82 in the 2008 NEC, the optional calculation for a dwelling unit. In order to be able to use this section, the total connected load has to be served by a single 120/208 or 120/240V set of 3 wire service or feeder conductors, with an ampacity of 100 amps or greater.

My 2 questions are:
1. this section is based on total connected load, and not demand load, is that correct?
2. If my demand load is 78 amps, but my connected load is 102 amps... I could use #4 AWG feeder wiring, which is rated for 85 amps, based on demand load. But am I allowed to use this section? Since the total connected load is 102 amps which would require #2 AWG feeder wiring. But i would only be specifying #4 wire based on demand load.

How are you getting demand? (220.87?)
Or are you referring to demand factors?

 

[jaggedben]

I don't know what 'demand load' means, that's not a code term. But to answer your first question, yes, you use the total connected load for the calculation proscribed in 220.82.

Also if the connected load is 102A then your 220.82 calculated load is going to be well under 100A because of the allowed demand factor above 10kVA. You might want to review 220.82 again.

Regarding your second question, what allows you to use 220.82 is what you descibed accurately in your first paragraph. No other requirement can override that. However, there are other code requirements that come into play when it comes to choosing minimum disconnect and wire size.

 

[Grouch1980]

How are you getting demand?
Or are you referring to demand factors?

Yes, the demand factors that brought down the total connected load from 102 amps to 78 amps.

 

[jaggedben]

If you are talking about demand factors from 220.40 those are not used whatsoever in 220.82. 220.82 applies its own version of a demand factor to the total connected load.

 

[Grouch1980]

aaaah. ok I got it. that's correct, I should not be even addressing the demand factors from part III at all (starting at section 220.40). So if my total connected load is 102 amps, then i'm allowed to use 220.82, assuming the other prerequisites are met?

 

[jaggedben]

You can use 220.82 if the load is less than 100 amps, you just need to install a feeder that's still rated for that much. 225.39 also requires a 100A rated disconnect for a dwelling feeder, btw. So you may be overthinking this if all the 220 options come out to less than 100A.

You'll also probably get to use #4 copper per 310.12 (2020 NEC reference, previously 310.15(B)(7)).

 

[Dennis Alwon]

You can use 220.82 if the load is less than 100 amps, you just need to install a feeder that's still rated for that much. 225.39 also requires a 100A rated disconnect for a dwelling feeder, btw. So you may be overthinking this if all the 220 options come out to less than 100A.

You'll also probably get to use #4 copper per 310.12 (2020 NEC reference, previously 310.15(B)(7)).

Um, I don't think so or am I missing something

(A) Feeder and Service Load. This section applies to a dwelling
unit having the total connected load served by a single
120/240-volt or 208Y/120-volt set of 3-wire service or feeder
conductors with an ampacity of 100 or greater.

 

[Grouch1980]

So the way i'm interpreting 220.82 (the optional calc), is if your total connected load is 102 amps (just as an example), you can use the calculation under 220.82... so if my calculated load is 55 amps after using 220.82, i can specify a 60 amp breaker using #6 wire. is this wrong?

 

[jaggedben]

Um, I don't think so or am I missing something

You seem to be missing that the ampacity of the feeder is not the same as the result of the load calc.

IOW, say I have a dwelling with a 3-wire feeder. My 220.40 load calculation comes out to 125A but my 220.82 load calculation comes to 60A. I can use the 220.82 option, but I have to install a 100A rated feeder in order to do so.

 

[jaggedben]

So the way i'm interpreting 220.82 (the optional calc), is if your total connected load is 102 amps (just as an example), you can use the calculation under 220.82... so if my calculated load is 55 amps after using 220.82, i can specify a 60 amp breaker using #6 wire. is this wrong?

No, see previous post. Your feeder needs to be rated 100A to use 220.82. Your feeder is rated greater than your 55A calculated load, so that's fine.

Again, the result of the load calc is not a condition for using 220.82. Neither are the inputs. It's just that the section sets a minimum feeder ampacity of 100A.

What you will find for many dwellings is that the 220.40 load calculation is over 100A, but the 220.82 (or other Part IV) calculation is under 100A. So 220.82 often allows you to downsize things to a minimum of 100A.

 

[Dennis Alwon]

But your statement said that to use 220.82 your load had to be under 100 amps.... @jaggedben

 

[Grouch1980]

225.39 also requires a 100A rated disconnect for a dwelling feeder, btw.

I just checked that section... isn't that section under part II of article 225, which is "More than one building or structure". does that section apply? In our case, I'm asking about one apartment, or the feeder to each apartment within the same multi-family dwelling apartment building.

 

[Grouch1980]

I just checked that section... isn't that section under part II of article 225, which is "More than one building or structure". does that section apply? In our case, I'm asking about one apartment, or the feeder to each apartment within the same multi-family dwelling apartment building.

actually I think you mentioned 225.39 just as an example.... in other words, by the way this section exists as well,

 

[Grouch1980]

in all honesty, 220.82 needs to be re-worded. the way its written is not clear at all. I can guarantee there's plenty of engineers out there each interpreting it differently. What's the point of the section saying "total connected load", if it doesn't even matter. even after the calcs of 220.82, bringing your amperage down from the total connected load, your feeder size still can't go below 100 amps.

 

[jaggedben]

But your statement said that to use 220.82 your load had to be under 100 amps.... @jaggedben

No, I didn't, although perhaps I could have worded it better, e.g. 'you can use 220.82 when the load is less than 100A'. But 'if' by itself doesn't mean 'only if'. There's a reason lawyers and logicians use 'only if' or even 'if and only if' and even 'iff' for short.

 

[jaggedben]

in all honesty, 220.82 needs to be re-worded. the way its written is not clear at all. I can guarantee there's plenty of engineers out there each interpreting it differently. What's the point of the section saying "total connected load", if it doesn't even matter. even after the calcs of 220.82, bringing your amperage down from the total connected load, your feeder size still can't go below 100 amps.

The 'total connected load' part limits the application to services and feeders that supply the entire dwelling. You cannot use this section for, say, a subpanel that contains only part of the dwellings loads.

Also although your feeder can't go below 100A, I've done many 220.82 (or 220.83) calculations that allowed a feeder or service 25 or 50 amps less than allowed by 220.40. It often means the difference between needing a 200A service or feeder and a 125 or 100A service or feeder.

 

[Grouch1980]

ok, after rereading it a hundred times i'm getting what you're saying. It's not the way i would've worded it. But i'm not a code guy! Follow up question: so 220.82 says "service or feeder conductors with an ampacity of 100 or greater". so say my calculated load is 55 amps... I understand i have to use #3 wire, so that the feeder ampacity is at least 100 amps. can my breaker though be 60, or does the breaker have to be 100 amps?

 

[Grouch1980]

ok now it's not making sense anymore lol. I do not understand the conditions for this section at all. The way it's written opens the door to mistakes IMO. So my total connected load... what if it's 60 amps, just for argument sake. can i still use 220.82? it sounds like i can as long as my feeder wire is at least #3.

 

[Grouch1980]

So my total connected load... what if it's 60 amps, just for argument sake. can i still use 220.82? it sounds like i can as long as my feeder wire is at least #3.

.... would this be correct, or wrong?