220.83(A) - Existing Service Large Enough?

[JDBrown]

I'm trying to improve my understanding of the residential side of the NEC, and I recently came across 220.83, which can be used to determine whether or not an existing service is large enough to serve additional loads. My confusion is with part (A), which doesn't seem to include HVAC loads.

220.83 Existing Dwelling Unit.
...
(A) Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is Not to Be Installed. The following formula shall be used for existing and additional new loads.

____________Load (kVA)_______Percent of Load___________
First 8 kVA of load at...................................100
Remainder of load at.....................................40

Load calculations shall include the following:
(1) General lighting and general-use receptacles at 33 volt-amperes/m² or 3 volt-amperes/ft² as determined by 220.12
(2) 1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit covered in 210.11(C)(1) and (C)(2)
(3) The nameplate rating of the following:
.....a. All appliances that are fastened in place, permanently connected, or located to be on a specific circuit
.....b. Ranges, wall-mounted ovens, counter-mounted cooking units
.....c. Clothes dryers that are not connected to the laundry branch circuit specified in item (2)
.....d. Water heaters

In this section I see general lighting/receptacle loads, SABCs, the required laundry circuit, and specific appliances. What I don't see is any mention of HVAC loads. Obviously, I'm missing something here, because there's no way we don't count existing heating and cooling loads just because no new ones are being installed.

Am I to understand that, in the case of 220.83(A), HVAC units are to be counted as appliances? If that's the case, then ALL loads, including the new loads and the existing HVAC, are totaled up and counted at 100% for the first 8 kVA and at 40% for the remainder. Is this correct?

 

[cadpoint]

Everything looks fine except your understanding and the use of 220.83(B) when trying to use your previous accounting methods.

It's either one or the other thats the greatest but never both. An example if both where the same then you only use one!

 

[JDBrown]

Everything looks fine except your understanding and the use of 220.83(B) when trying to use your previous accounting methods.

It's either one or the other thats the greatest but never both. An example if both where the same then you only use one!

Sorry, I'm not quite sure what you mean. Are you saying that you only have to count heating or A/C, but not both? If that's the case, thanks for the reminder.

I'm not sure where 220.83(B) comes into it, as that section is only for when additional (new) HVAC loads are to be installed. My question is about 220.83(A), which is for existing dwellings where new loads will be added, but not new HVAC loads.

 

[cadpoint]

I can't give you every possible situation! It's part of the "Whole" Article. I'm talking about the
second sentence of (B) in the article.

If you only read the first article (A) and not read all of what's in this one subject matter, are you not at some disavantage?

Your working Existing Calc';s it's a subject matter that might be needed.

 

[david]

Am I to understand that, in the case of 220.83(A), HVAC units are to be counted as appliances? If that's the case, then ALL loads, including the new loads and the existing HVAC, are totaled up and counted at 100% for the first 8 kVA and at 40% for the remainder. Is this correct?

I agree, you do not get to consider load diversity between AC and heating loads in 220.83 (A)
First 8kva at 100 % remainder at 40%
I also agree with you must include ALL existing heating plus AC loads in that calculation

 

[JDBrown]

I can't give you every possible situation! It's part of the "Whole" Article. I'm talking about the
second sentence of (B) in the article.

If you only read the first article (A) and not read all of what's in this one subject matter, are you not at some disavantage?

Your working Existing Calc';s it's a subject matter that might be needed.

Okay, I think I see what you're saying now. I have read part (B), and you make a good point.

Since I'm just trying to make sure I understand this and it's not for an actual job I'm working on at the moment, I'll use my own house as an example (when I was trying to figure this out the other day, I walked around the house writing down nameplate amps from my appliances, and I'll guess at the ones I couldn't find).

220.83(A)(1) -> 1,377 ft² x 3 VA/ft².....................4,131 VA
220.83(A)(2) -> 3 x 1,500 VA...............................4,500 VA
220.83(A)(3) -> Microwave....................................1,650 VA
........................Dryer...........................................5,000 VA
........................Garage Door Opener......................1,176 VA
........................Dishwasher...................................1,320 VA
........................Exhaust Fans (2 @ 144 VA ea.).........288 VA
........................Garbage Disposal...........................1,176 VA
........................Freezer.........................................1,500 VA
........................Air Conditioner_________________4,344 VA
........................Total...........................................25,085 VA

The water heater, range and furnace are gas, so I'll omit them, but if they were electric they would be included under 220.83(A)(3). Thanks, David, for pointing out that 220.83(A) doesn't allow you to account for load diversity between heating and A/C -- I hadn't noticed that.

Now, according to 220.83(A), I can count the first 8 kVA of my total load at 100% and the remainder at 40%:
25,085 VA - 8,000 VA = 17,085 VA
17,085 VA x 0.40 = 6,834 VA
6,834 VA + 8,000 VA = 14,834 VA, or 62 Amps @ 240V, single phase.

This seems odd to me, though, since it seems that everywhere else the HVAC load is not allowed to be included in the general loads that are calculated at a reduced percentage (40% in this case). For example, 220.83(B) requires HVAC loads to be calculated at 100%, and the 40% only applies to other (non-HVAC) loads. Same thing with 220.82.

If I were to follow the same method outlined in those other sections, I would total up everything except the air conditioning (and electric heat, if any), apply the demand factor, and then add in the air conditioning/heating at 100%. To use the example above, the total load not including air conditioning is 20,741 VA.
20,741 VA - 8,000 VA = 12,741 VA
12,741 VA x 0.40 = 5,096 VA
5,096 VA + 8,000 VA + 4,344 VA = 17,440 VA, or 73 Amps @ 240V, single phase.

Since I only have a 100 Amp service, I would be able to add 30 amps of load in the first example, but not in the second. And I'm not 100% certain which method is correct. I think the first one is, but I'd like some confirmation.

So, when adding new load to an existing dwelling, as long as none of the new load is heating or air conditioning equipment, am I allowed to lump the existing heating/air conditioning loads in with all of the other loads and apply the 40% demand factor? Hopefully my example calculations above will clarify a bit what I am trying to determine.

 

[GoldDigger]

The water heater, range and furnace are gas, so I'll omit them, but if they were electric they would be included under 220.83(A)(3). Thanks, David, for pointing out that 220.83(A) doesn't allow you to account for load diversity between heating and A/C -- I hadn't noticed that.

One could argue that since (A)(3) deals with and references existing loads, it implicitly allows the same use of A/C versus heating diversity that was allowable in the original calculation of the existing load. I would prefer this to be clearer though.

 

[cadpoint]

Sorry, got into playing in the leaves!

Didn't look nor do the math!

You can check yourself at a free MH downloadable *.xls HERE

Residental Load Calculations

Now it says based on 2014 it also says "Greater of Heat @ 100% vs. A/C @ 100%"
on the right.

Hope this helps!

 

[JDBrown]

Sorry, got into playing in the leaves!

Didn't look nor do the math!

You can check yourself at a free MH downloadable *.xls HERE

Residental Load Calculations

Now it says based on 2014 it also says "Greater of Heat @ 100% vs. A/C @ 100%"
on the right.

Hope this helps!

Thanks for pointing me to the spreadsheet, cadpoint. Unfortunately, it's geared toward new construction, and I'm trying to learn about the optional calculation for determining if new loads can be added to an existing residential service (NEC 220.83). Still, that's a spiffy spreadsheet, so thanks.