220.85 two non-identical units, 1 service

[tortuga]

Greeting group here is one to chew on
Can 220.85 be used for two non identical dwelling units on the same service / feeder ?
Say for example a service that supplies a house and a guest cottage (guest suite / ADU), each are permitted as 'single family dwelling units' and have separate address etc but are to be fed from one service on the property, the units are not identical in any way.

My take is you can

• Sum up the loads per 220.84(C) 1-5
• Multiply that sum by 50% to create 'three identical units'
• Apply the 45% demand from T220.84

Opinions or thoughts?
Thank you

 

[wwhitney]

Multiply that sum by 50% to create 'three identical units'

That's not going to make three identical units, as your two starting units aren't identical.

But you could plausibly take the larger of the 2 units and multiply it by 3.

Cheers, Wayne

 

[tortuga]

That's not going to make three identical units, as your two starting units aren't identical.

But you could plausibly take the larger of the 2 units and multiply it by 3.

Cheers, Wayne

Ok but It does not say the two units your starting with have to be identical though.
Just that you have to create 3 identical units out of the two.

 

[wwhitney]

Ok but It does not say the two units your starting with have to be identical though.
Just that you have to create 3 identical units out of the two.

Right, but unless your two starting units are identical, multiplying by 1.5 isn't going to give you 3 identical units.

Cheers, Wayne

 

[tortuga]

I think the intent is you end up with 3 identical dummy units, so by increasing the two by 50% you get the sum of 3 dummy units.

The proposal (NO. 209) to add this in the 1978 code says
"The load on two apartments will never be greater than three units of
the same type, therefore this proposal will provide a method of com-
plying with the Code by calculating a 'dummy apartment unit' and
still taking advantage of the diversity allowed in 220-32."

 

[wwhitney]

I think the intent is you end up with 3 identical dummy units, so by increasing the two by 50% you get the sum of 3 dummy units.

Yes, but each actual unit should have load no more than the dummy unit. So you can't take 150% of the sum of the larger and smaller units, you'd need to take 300% of the larger unit. [Since you said the two real units aren't identical, one will have a larger load.]

Cheers, Wayne

 

[tortuga]

Yes, but each actual unit should have load no more than the dummy unit. So you can't take 150% of the sum of the larger and smaller units, you'd need to take 300% of the larger unit. [Since you said the two real units aren't identical, one will have a larger load.]

Cheers, Wayne

Alright thank you, that's definitely the most conservative route so I'll go with it.

 

[wwhitney]

Alright thank you, that's definitely the most conservative route so I'll go with it.

Right.

And to demonstrate the problem with the idea in the OP, say the two units came out to 100A and 30A (extreme case). Then 1.5*130A = 195A; with a 45% diversity factor, that's 88A. Smaller than the larger unit's 100A. So obviously unreasonable.

Cheers, Wayne

 

[gene6]

Yes, but each actual unit should have load no more than the dummy unit. So you can't take 150% of the sum of the larger and smaller units

No i disagree, that is not what the code says,
for a two family you do same as multi put all the loads together on one sheet
sum em up
then 1.5
then .45
is the way i was taught.
It would explicitly say "or 2X the largest unit if unequal' if that is what is intended.

Right.

And to demonstrate the problem with the idea in the OP, say the two units came out to 100A and 30A (extreme case).

You could have the same result in the multifamily calc, as you could have 2 small apartments and one large house (3 dwelling units).

 

[wwhitney]

No i disagree, that is not what the code says,
for a two family you do same as multi put all the loads together on one sheet
sum em up
then 1.5

That's not going to give you the load for the "3 identical units" that 220.85 requires. For example, if one unit has a dishwasher, and the second unit doesn't, your total at this point will include 1.5 dishwashers. If that's 3 identical units, each would have 0.5 dishwashers. But you can't install half a dishwasher in a unit.

So you need to pick the load for one unit, then multiply that by 3 for your "3 identical units", and then multiply by 0.45. Which unit do you pick? Obviously the one with the larger calculated load.

Cheers, Wayne

 

[wwhitney]

BTW, I see that I had a misapprehension about Part IV of Article 220, I thought that 220.84 and 220.85 were somehow linked to 220.82, just because of proximity. But they are completely separate, correct? So my comment in post #8 isn't quite right, as I haven't accounted in the comparison for the different base calculation methods in the different approaches.

Thus to step back a bit and look at the big picture, In Article 220 you have:

Part III of 220, the standard calculation, can be used for any service or feeder.

220.82 applies to any 120/240V or 120/208V 3 wire feeder or service that supplies the total connected load of a dwelling unit, and gives you another way to calculate that load. To paraphrase it's "100% of the first 10 kVA of non-HVAC, plus 40% of the rest of the non-HVAC, plus the HVAC which is usually at 100%."

220.83 is weird in that for existing dwelling units you have the option to do the same computation as 220.82 but subtract 1.2 kVA. That's because the 100% only applies to the first 8 kVA, and (100% - 40%) * (10 kVA - 8 kVA) = 1.2 kVA less.

220.84 applies to any feeder or service (no system voltage/configuration limitation) that supplies 3 or more dwelling units, as long as each dwelling units includes certain loads, or at least an allowance for those loads. And in contrast to 220.82, it basically reads "take 100% of everything in the units, apply the Table 220.84 demand factor based on the number of units, and add the Part III house load."

Then 220.85 says for two dwelling units, you can use the lesser of (a) the Part III calculated load or (b) the 220.84 calculated load for 3 identical units.

Now, I see nothing in 220.82 that restricts it to services or feeders that supply only one dwelling unit. So if your two dwelling units are supplied by a 120/240V or 120/208V 3 wire feeder or service, you also have the option to (c) calculate each dwelling unit per 220.82 and add them together. So between 220.82 and 220.85, you can use the lesser of (a), (b), or (c).

And for 3 dwelling units that are supplied by a 120/240V or 120/208V 3 wire feeder or service (and not, say, a 208Y/120V 4 wire feeder or service), you still have the choice of using 220.82 or 220.84. The 220.84 diversity factor is 45%, while the 220.82 factor is 40%, except the HVAC and the first 10 kVA are counted at 100%. So if "5% of everything" > 3*6 kVA (60% of the first 10 kVA, for each of the 3 units) + 60% of the HVAC, i.e. if there's a lot of non-HVAC load and not much HVAC load, you might have a lower load applying 220.82 than using 220.84.

Is that a fair summary of the big picture, with respect to the different options for load calculation strategy in the various cases?

Cheers, Wayne

 

[gene6]

I'll see if I can find the example from a recent class I took.
But to do the Calc in 220.85 ignore 220.82, and set up your sheet just like a 220.84 calc.
simple list sum up all the loads:
total square footage X3
Number of Small Appliance circuits X 1500
If any have a Laundry circuit those X 1500
Range
Dryer
Heat / AC
All at nameplate
Sum up total.
------------------------------
The code just wants a total load for what would be three identical 'dummy' units.
Load is load it does not care what load is what unit.
Take total X 1.5
Then apply .45 demand.
(Or just use .675)

 

[wwhitney]

The code just wants a total load for what would be three identical 'dummy' units.

Yes, and I'm saying that while not explicitly stated, it is clear that each actual unit load should be no more than that dummy unit load. Your procedure creates a dummy unit whose load is the average of the two units. That's not conservative, nor realistic (no half dishwashers as previously commented).

Cheers, Wayne

 

[tortuga]

Well the plot thickens, now I am curious to see what others have to say, I'll have to compare the standard and the 220.85 calc (with 3X the largest unit) with a few scenarios.

Now, I see nothing in 220.82 that restricts it to services or feeders that supply only one dwelling unit.

It says "this section applies to a dwelling unit" ... How can we apply it to a single service that supplies two dwelling units?

 

[wwhitney]

It says "this section applies to a dwelling unit" ... How can we apply it to a single service that supplies two dwelling units?

Right, it's just about calculating a dwelling unit load whose "total connected load (is) 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." For any such dwelling unit, the section gives you the dwelling unit load.

But any upstream feeder or service conductor that supplies multiple such dwelling units, its load would just be the sum of those dwelling unit loads (plus any other loads supplied). There's nothing in the text there about supplying only one dwelling unit.

Cheers, Wayne

 

[tortuga]

There's nothing in the text there about supplying only one dwelling unit.

Except where it says "this section applies to a dwelling unit" the section only applies to the conductors that supply a single dwelling. As soon as you supply a duplex or multifamily service entrance that section does not apply.

 

[wwhitney]

Except where it says "this section applies to a dwelling unit" the section only applies to the conductors that supply a single dwelling.

I think I see where perhaps we are reading it differently. The start of 220.82(A):

"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. 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."

Now if it said "the load of that service or feeder" in the second sentence, instead of "feeder and service loads," then I would 100% agree with you.

But I am reading "feeder and service loads" more generally, any service or feeder that supplies a dwelling unit that meets the restrictions in the first sentence.

Cheers, Wayne

 

[tortuga]

I think I see where perhaps we are reading it differently. The start of 220.82(A):

"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. 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."

Now if it said "the load of that service or feeder" in the second sentence, instead of "feeder and service loads," then I would 100% agree with you.

But I am reading "feeder and service loads" more generally, any service or feeder that supplies a dwelling unit that meets the restrictions in the first sentence.

Cheers, Wayne

Yeah unfortunately I have to read it the way the AJH's here in Oregon do.
If I submitted a calc like that it would be rejected.

 

[wwhitney]

Yeah unfortunately I have to read it the way the AJH's here in Oregon do.
If I submitted a calc like that it would be rejected.

So just to clarify, the result is that for a service supplying just single family residences, with a single unit you have the choice of calculating via Part III or 220.82. While for 2 units, your only choice is between Part III or 220.85, and for 3 or more units it's Part II or 220.84.

In other words, among the non-Part III options (assuming Part III would be higher, which may not be true and requires checking), for identical dwelling units, 1 can be done via 220.82, 2 can be done via 220.85, and 3 can be done via 220.84. With the outcome that 2 and 3 are the same load.

That obviously makes zero sense, and I'm sorry your AHJs are misinterpreting 220.82 that way. You should be able to use 220.82 for each dwelling unit individually, and then add up the results.

Cheers, Wayne

 

[tortuga]

So just to clarify, the result is that for a service supplying just single family residences, with a single unit you have the choice of calculating via Part III or 220.82. While for 2 units, your only choice is between Part III or 220.85, and for 3 or more units it's Part II or 220.84.

Correct, thats the way its been since I started doing calcs in 04', and confirmed it with AHJ today.
Does not make it correct, but I also find your reasoning there unpersuasive.

In other words, among the non-Part III options (assuming Part III would be higher, which may not be true and requires checking), for identical dwelling units, 1 can be done via 220.82, 2 can be done via 220.85, and 3 can be done via 220.84.

Yes, there are the 'adding load to existing' options, but for a new build house + Accessory Dwelling on one meter your looking at Part III or 220.85.

With the outcome that 2 and 3 are the same load.

How 220.85 is done correctly still seems up for debate, however I agree your way is prudent.
When running actual numbers today if I do the 220.85 'dummy unit' 3X the largest unit ,I find a part III 220.40 calc is always lower.
And the smallest possible unit load you can get is 50 Amps not 30 Amp.
I cannot for the life of me find a singe example of a 220.85 calc with two non-identical dwelling units.