Dwelling Load Calculations - 220.82 vs. 220.83

[steve66]

In the 2020 NEC, 220.82 is an optional load calculation for a dwelling unit (assume the dwelling units meet the criteria in both 220.82 and 83 including a single feeder or service, 100A min, right voltage, etc)

220.83 is a similar calculation but only for an existing unit. It will generally give a lower calculated load, at least for not adding any new HVAC.

Why would the calculation be different for a new unit vs. and existing unit? That makes absolutely no sense to me.

Two of the exact same units on paper, with the exact same appliances., but they get different load calculations. So just for example, I might be able to add a washer and a dryer to an existing unit, but the exact same unit that hasn't been built yet can't be shown with a washer and dryer because the load calc puts it over the service size. But after its been built, then its existing, and now all the sudden someone can add a washer and dryer?

Also doesn't seem to make sense why having existing HVAC is treated differently than having new HVAC?

I'm reluctant to use 220.83. It just seems like cheating.

Can someone please explain why an existing unit is treated differently than a new one? Am I missing something about actually measuring the loads on the existing units, or something else like that?

 

[charlie b]

My only guess is that the difference lies in not knowing for certain what the unit will actually draw when it is finally in use, as opposed to having certain knowledge of the load history after the unit has been occupied for some time.

 

[tortuga]

I'm reluctant to use 220.83. It just seems like cheating.

I felt the same way, but I use 220.83 often, as a EE you go with what your comfortable with.
When I use 220.83 and the numbers are close or its a big or unsual home I make sure to get real nameplate data, its quite interesting what you can find and cant find.
Here in the Pacific North West (PNW) we have long had cheap power so people had these electric furnaces and boilers, I had a electrician tell me a electric boiler name plate was 21kw.
21kw?
I was like are you sure its not 2.1kw?
I had to go see for my self that one.
Sure enough.
I have seen several ranges over 14kW would I would guess 8-12 was standard.
I have seen a 36kw on demand water heater on occasion.
one guy had a home forklift complete with 7kw charger.
Another one had a commercial kitchen in their garage for catering that they forgot to mention.
Often with outdoor heatpumps I find the nameplate is gone, or too faded to read.
You never know what you'll find.

 

[steve66]

My only guess is that the difference lies in not knowing for certain what the unit will actually draw when it is finally in use, as opposed to having certain knowledge of the load history after the unit has been occupied for some time.

But there is nothing in the paragraph that brings the load history of the unit into play?

So assume we have two identical units, one is built, one is not, and they get different load calculations?

I'm at a loss.

 

[steve66]

It appears that Tainted has also asked this question before:

Article 220.83(A) - Air-Conditioning Loads

Let me put it this way: So really what is the purpose of 220.82 if 220.83(A) will always give you a more liberal result? 220.82 can be used for a yet-to-be built dwelling or service or feeder. 220.83 cannot.

forums.mikeholt.com

I also question if the 40% demand can be applied to existing heating and AC equipment. It reads to me like that is allowed.

For example, it clearly says "load calculations shall include the following:" and lists (1), (2), and (3).

(3)a includes "Appliances fastened in place." Well maybe a furnace or condensing units isn't considered an appliance? But the definition in 100 sure seems like it would include HVAC equipment. It even mentions Air conditioning as an example.

But on the other hand, Appliances is article 422, and we have separate articles for HVAC, like 424 and 440. So maybe HVAC is not included in (3)a after all?

Going back to 220.83, it looks like the loads in 1-3 are added together. Then only the first 8KVA is at 100%, and everything else (maybe including HVAC?) gets a very generous 40% demand factor.

 

[tortuga]

I also question if the 40% demand can be applied to existing heating and AC equipment. It reads to me like that is allowed.

That is how it reads to me also.
If you have access to the 2026 NEC you can see they reigned that in a bit.