Duplex Optional Method Calc?

[letgomywago]

2 100 amp dwelling unit feeders aren't a 200 amp service.

 

[wwhitney]

You can do that for sizing the feeder to each dwelling. Sizing the service for all total loads is more complicated and would be smaller than just adding the feeder calc together.

The whole point of this thread is that under the interpretation that 220.82 can only be used for a feeder/service that supplies exactly one dwelling unit, for a feeder/service that supplies two dwelling units the result from the standard method or fictitious triplex method may very well be more than the sum of the 220.82 loads of the two dwelling units. Which makes zero sense, you calculate that dwelling A requires a 125A feeder, and dwelling B requires a 100A feeder, but together they require a 250A feeder.

See also:

Article 220 Examples/Options for 1-3 Dwelling Units

To improve my understanding, and explore the options in Article 220 beyond the standard calculations in Part III, I came up with the following example for a fairly modest but realistic all-electric dwelling unit: 1200 sq ft, no extra SABCs/laundry, and equipment nameplate data as follows...

forums.mikeholt.com

PI: 220.82(A)

And the second PI for today. Cheers, Wayne (A) Feeder and Service Load. This section applies to a any 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. It shall be...

forums.mikeholt.com

Cheers, Wayne

 

[Psychlo]

You can do that for sizing the feeder to each dwelling. Sizing the service for all total loads is more complicated and would be smaller than just adding the feeder calc together. This is where the options of the triplex optional can be useful or even the standard.

Right. It all depends on the sizes and situation. I just didn't know if they allowed both options in your area. Like you said, when you're dealing with dissimilar sized units, using 220.85 doesn't apply - according to the wording.

 

[tortuga]

using 220.85 doesn't apply - according to the wording.

In my case the ADU's are permitted as a 'dwelling unit' and I am feeding that dwelling unit from the service panel of an existing main house (a dwelling unit). No separate meter. My options in Oregon are 220.40 (standard) and 220.85
I am not allowed to size the service conductors and main panel (busbar feeders) etc using 220.82. 220.82 is for one "Dwelling Unit". (A) Feeder and Service Load. This section applies to a dwelling unit. Nothing more, all the statements past that are contingent on one dwelling unit.

220.85 is not for two identical dwelling units, it states "Where two dwelling units are supplied by a single feeder"
Not "Where two identical dwelling units are supplied by a single feeder"

You need to combine your two units to create "three identical units" .
Then run 220.84, and 220.40 the lesser of the two loads shall be permitted to be used.

 

[Psychlo]

220.85 is not for two identical dwelling units, it states "Where two dwelling units are supplied by a single feeder"
Not "Where two identical dwelling units are supplied by a single feeder"

You need to combine your two units to create "three identical units" .
Then run 220.84, and 220.40 the lesser of the two loads shall be permitted to be used.

I agree that it isn't explicitly stated, but doesn't it seem to be heavily implied that they are identical? Why else would they restrict it to units being supplied by a single feeder? It seems that 220.85 is trying to make it clear that this is to be treated as a duplex and not like two single family dwellings.

If I understand you correctly, you are calculating each unit as a single family dwelling and then adding them together --- as if they each had their own service or feeder. But they don't. So why would 220.85 make the distinction about a single feeder if it didn't matter? But maybe I misunderstand you.

You need to combine your two units to create "three identical units" .
Then run 220.84, and 220.40 the lesser of the two loads shall be permitted to be used.

This is where it takes some presumptions on our part. When they say "three identical units", what are they identical to? Each of the two units? Or both of the two units combined? IMO, the implication (though certainly not explicitly stated) is that the two units are equal size and therefore each of the hypothetical three units is identical in size to each of the actual units. All units involved in the problem are identical.

I'm curious to know how you "combine your two units to create "three identical units". And where do you get the direction to do that. Could you explain your process?

Using a random duplex example, and using my method, I come up with:
Equal Units A & B = 255A (standard as a duplex)
Equal Imaginary Units C, D, E = 224A (optional as a triplex)

Using the same example, but using what I believe to be your method, I come up with:
Equal Units A & B = 292A (standard as two SFRs independently calculated, then combined)
Equal Imaginary Units C, D, E = [I'm not sure where you would get your figures here]

I did that just to show that it makes a substantial difference which way this is calculated. So further input is appreciated.

 

[tortuga]

I calculate loads for both unequal units as I would for any other multi family complex per 220.84(C) 1- 5 then add 50%, that makes three equal units.
Then apply demand factor of 45%
Essentially its applying a demand factor of of 67.5%

Post inputs of your example and we can run thru all the possibilities.

 

[wwhitney]

I calculate loads for both unequal units as I would for any other multi family complex per 220.84(C) 1- 5 then add 50%, that makes three equal units.
Then apply demand factor of 45%
Essentially its applying a demand factor of of 67.5%

Not just essentially, but exactly.

Thus under the "but if that's what they meant, they would have written the NEC more simply" category: namely you could delete 220.85 and just extend 220.84 to two dwelling units with a demand factor of 67.5%. (Actually, that would make a good PI, just to get the CMP's response.)

So to the extent that the existence of 220.85 is an argument against my interpretation of the applicability of 220.82, the existence of 220.85 is also an argument against your interpretation of 220.85. : - )

Cheers, Wayne

 

[tortuga]

Not just essentially, but exactly.

Thus under the "but if that's what they meant, they would have written the NEC more simply" category: namely you could delete 220.85 and just extend 220.84 to two dwelling units with a demand factor of 67.5%. (Actually, that would make a good PI, just to get the CMP's response.)

So to the extent that the existence of 220.85 is an argument against my interpretation of the applicability of 220.82, the existence of 220.85 is also an argument against your interpretation of 220.85. : - )

Cheers, Wayne

I think a 67.5% PI is a good idea.
Would you also submit another one that adds the word 'services' to 220.85?
I think there was a rule that a feeder need not be larger than a service but not the other way around.

 

[wwhitney]

Would you also submit another one that adds the word 'services' to 220.85?

Take a look at the 2023 NEC.

Cheers, Wayne