Duplex Optional Method Calc?

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Psychlo

Psychlo

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Melissa, TX
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Master Electrician
Why isn't an optional calc permitted for a duplex? It's permitted for all other dwelling types. Does anybody know? I realize 220.85 gives us an option to test a standard calc against a triplex (using optional). But I'm wondering what's special about a duplex, and why they made a special provision for it. It just doesn't seem very intuitive to me so I was hoping for some insight. If calculating it like a multifamily is not considered adequate (which is the implication), it seems they could have just required each of the two units to be calculated using the optional and then added together. Does anyone else wonder about stuff like this or do I just need a vacation? 🥴
 
I am curious where you see that a duplex is not allowed. If it is the statement in 220.82 that says "a dwelling unit" then I think that is not the intent. As far as I know duplex units can use the optional method.
 
Dennis Alwon said:
I am curious where you see that a duplex is not allowed. If it is the statement in 220.82 that says "a dwelling unit" then I think that is not the intent. As far as I know duplex units can use the optional method.
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I guess I'm having trouble finding where it is allowed. Where are we told we can use the optional method for a duplex? Maybe I'm just missing it.

220.84(A) ...three or more dwelling units...
letgomywago said:
There's also the way of doing an optional triplex if it's lower than a duplex with the same loads
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Precisely. There seems to be no provision for a duplex under the optional method. Why else would 220.85 even exist?
 
Psychlo said:
Precisely. There seems to be no provision for a duplex under the optional method. Why else would 220.85 even exist?
Click to expand...

Here is the trick. You calculate it as a standard calculation then calculate the load for 3 units and take the lesser of the two.

220.85 Two Dwelling Units. Where two dwelling units are
supplied by a single feeder and the calculated load under
Part III of this article exceeds that for three identical units
calculated under 220.84, the lesser of the two loads shall be
permitted to be used.
 
It's when you take the demand factors that in certain situations a triplex using the optional will come out lower than a duplex doing the standard. On the WA admin test you can count on one of these being on there because of the rule not being used much as a Gotcha question.
 
Maybe I'm not being literal enough, but looking at 220.82:

The first sentence tells you "This section applies to a dwelling unit having . . ." and then specifies certain conditions that have to be met. So any dwelling unit that meets those conditions, you can use 220.82 to calculate the load for that dwelling unit.

The second sentence then says "It shall be permissible to calculate the feeder and service loads in accordance with this section instead of the method specified in Part III of this article." To me, if you calculate the individual loads of two dwelling units, each of which meets the limitations in the first sentence, then calculating the load on a feeder or service supplying them both as the sum of those two individual calculations is "in accordance with this section."

I.e. the grant of permission in the second sentence is fairly broad, and does not have any language limiting the permission to deviate from Part III to the case of a single dwelling unit, such as saying "permissible to calculate the feeder and service loads of a single dwelling unit."

Cheers, Wayne
 
Dennis Alwon said:
Here is the trick. You calculate it as a standard calculation then calculate the load for 3 units and take the lesser of the two.

220.85 Two Dwelling Units. Where two dwelling units are
supplied by a single feeder and the calculated load under
Part III of this article exceeds that for three identical units
calculated under 220.84, the lesser of the two loads shall be
permitted to be used.
Click to expand...
Dennis Alwon said:
So you are correct there is no optional calculation per se for a duplex however there is the option mentioned above
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Okay, good, so I'm not crazy. Or at least we're all crazy together. 🤪

Yup, there would would be no need for 220.85 if a duplex was allowed to be calculated under the optional method outright. At least, that's my take.
 
wwhitney said:
Maybe I'm not being literal enough, but looking at 220.82:

The first sentence tells you "This section applies to a dwelling unit having . . ." and then specifies certain conditions that have to be met. So any dwelling unit that meets those conditions, you can use 220.82 to calculate the load for that dwelling unit.

The second sentence then says "It shall be permissible to calculate the feeder and service loads in accordance with this section instead of the method specified in Part III of this article." To me, if you calculate the individual loads of two dwelling units, each of which meets the limitations in the first sentence, then calculating the load on a feeder or service supplying them both as the sum of those two individual calculations is "in accordance with this section."

I.e. the grant of permission in the second sentence is fairly broad, and does not have any language limiting the permission to deviate from Part III to the case of a single dwelling unit, such as saying "permissible to calculate the feeder and service loads of a single dwelling unit."

Cheers, Wayne
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I agree that would be a sensible approach, but since I didn't see it explicitly stated as being permitted, I wanted to poll the audience and see what smarter folks thought. This is one of those times where the intent needs to be divined from the text. And I lost my Crack Jack E.S.P. ring years ago. I appreciate the input.
 
Buuuuut....heh, heh.... that doesn't answer my original question....

Does anyone know why a duplex isn't specifically allowed to be calculated under the optional method? There must be some reason they define "multifamily" as three or more units. Why not two or more units?
 
On Mike Holt spreadsheet he has a tab for duplex and in that sheet he has a standard calculation and Optional Calculation with 3 units not 2
 
Psychlo said:
Buuuuut....heh, heh.... that doesn't answer my original question....

Does anyone know why a duplex isn't specifically allowed to be calculated under the optional method? There must be some reason they define "multifamily" as three or more units. Why not two or more units?
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The only thing I can think of is that a duplex calculated under the optional would be too low so they just said to use 3 units for that calculation or use the standard method. The optional is almost always much lower than the standard method.
 
letgomywago said:
On the WA admin test you can count on one of these being on there because of the rule not being used much as a Gotcha question.
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They must be getting trickier. I took my WA admin in '04 and it wasn't too bad. Pretty straight forward. It was Oregon who had all the trick questions. They gave us blank paper and told us to write notes on any questions that we had "issues" with. It turns out, some questions had more than one correct answer, and some had no correct answer. So it was up to us to pick one or more answers and then explain our reasons on the blank paper. That was a nerve-wracking exam!😳
 
Dennis Alwon said:
The only thing I can think of is that a duplex calculated under the optional would be too low so they just said to use 3 units for that calculation or use the standard method. The optional is almost always much lower than the standard method.
Click to expand...
That's my guess as well.
 
Psychlo said:
Why isn't an optional calc permitted for a duplex? It's permitted for all other dwelling types. Does anybody know? I realize 220.85 gives us an option to test a standard calc against a triplex (using optional). 🥴
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I don't know but I have had to do a ton of duplex calcs lately for large homes with a Accessory Dwelling Unit (ADU) being added and I tried Wanes argument with the inspectors / plans reviwevers here and was rejected.
Also there is some debate in my mind as to how to do the 220.85 method for two dwelling units of unequal size.
I make a 'ghost unit' by using half of each unit (for the 3rd unit). But some prefer to use the largest unit, which makes 220.40 win every time.
EDIT I should check Mikes sheet and see what he does.
 
Psychlo said:
They must be getting trickier. I took my WA admin in '04 and it wasn't too bad. Pretty straight forward. It was Oregon who had all the trick questions. They gave us blank paper and told us to write notes on any questions that we had "issues" with. It turns out, some questions had more than one correct answer, and some had no correct answer. So it was up to us to pick one or more answers and then explain our reasons on the blank paper. That was a nerve-wracking exam!😳
Click to expand...
I took my admin 2 years ago right after they went 2020 for questions. The changes to non dwelling load calcs almost threw me every question it applied to since what was considered a continuous load that already had the 125% demand factor included. From what I've heard when they went to PSI they started taking the 3 most common wrong answers and making them the other choices.
 
tortuga said:
I don't know but I have had to do a ton of duplex calcs lately for large homes with a Accessory Dwelling Unit (ADU) being added and I tried Wanes argument with the inspectors / plans reviwevers here and was rejected.
Also there is some debate in my mind as to how to do the 220.85 method for two dwelling units of unequal size.
I make a 'ghost unit' by using half of each unit (for the 3rd unit). But some prefer to use the largest unit, which makes 220.40 win every time.
EDIT I should check Mikes sheet and see what he does.
Click to expand...
Are you allowed to do separate optional calcs for the main dwelling and the ADU, and then combine them? Are these attached ADUs? If so, do you treat detached ADUs any differently (when fed from main dwelling)?

I've never treated an ADU as a duplex. I have always treated it as a feeder load on the main service.
 
Psychlo said:
Are you allowed to do separate optional calcs for the main dwelling and the ADU, and then combine them? Are these attached ADUs? If so, do you treat detached ADUs any differently (when fed from main dwelling)?

I've never treated an ADU as a duplex. I have always treated it as a feeder load on the main service.
Click to expand...
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.
 
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