What Exactly Is The "Service Rating"?

[NoahsArc]

Wyane I am sorry I have to disagree thats still not how you do a 220.85 Two dwelling calc.
To arrive at your 95A optional take the following:

​

Total tangent, but why no microwave or disposal?

 

[wwhitney]

Wayne I am sorry I have to disagree that's still not how you do a 220.85 Two dwelling calc.

It is, you are mistaken.

You're getting closer to my example, notice that 194A > 2 * 95A, so already there's a disparity, just not quite enough to be an issue once you round up to standard breaker sizes.

But I specified 15A of HVAC load. So change the HVAC to just one heat pump at 15A / 3600 VA. And to get the 220.82 load back to 95A, add another appliance of 3440 VA.

Now when you do the 220.85 calculation, you'll get a 2-unit load of 206A.

Cheers, Wayne

 

[wwhitney]

Total tangent, but why no microwave or disposal?

These are made up examples to illustrate a point, so if it's convenient, we can leave out appliances that while common are not actually required to be in every dwelling unit.

Cheers, Wayne

 

[tortuga]

It is, you are mistaken.

You're getting closer to my example, notice that 194A > 2 * 95A, so already there's a disparity, just not quite enough to be an issue once you round up to standard breaker sizes.

But I specified 15A of HVAC load. So change the HVAC to just one heat pump at 15A / 3600 VA. And to get the 220.82 load back to 95A, add another appliance of 3440 VA.

Now when you do the 220.85 calculation, you'll get a 2-unit load of 206A.

Cheers, Wayne

Post your proof of work I already posted mine.

 

[wwhitney]

I can see this interpretation, but the "For.... X, Y, OR Z" can also be read as "For X, for Y, OR for Z" rather than "For all X that are Y or Z."

But it says "For a X, the Y, or the Z, blah blah". That lack of parallelism in the articles indicates that the first reading you suggest is not correct. As does the context.

Cheers, Wayne

 

[wwhitney]

Post your proof of work I already posted mine.

I already posted sufficient information for you to verify the computations, but since you asked, I'll update your very nice tables to facilitate that.

In the interest of realism, I'll add a Microwave and bump up square footage, but you could get the extra 3440 VA in a variety of ways.

Cheers, Wayne

​	Single dwelling unit feeder 220.82​	​	​	VA​
Line​	Description​	VA​	QTY​	VA​	Neutral​
1​	General Lighting Load​	​	​	​	​
2​	Apartment square Feet​	3​	1747​	5241​	5241​
3​	Small Appliance​	1500​	2​	3000​	3000​
4​	Laundry​	1500​	1​	1500​	1500​
5​	Water heater​	4500​	1​	4500​	0​
6​	Dishwasher​	960​	1​	960​	960​
7​	Electric Clothes Dryer​	5000​	1​	5000​	3500​
8​	Range​	11000​	1​	11000​	7700​
9​	Microwave​	1800​	​	1800​	1800​
10​	​	​	​	​	​
11​	Sub Total​	​	​	33001​	23701​
12​	​	​	​	​	​
13​	First 10 kva​	​	​	10000​	​
14​	Remainder​	​	​	23001​	​
15​	40% of remainder​	​	​	9200​	​
16​	​	​	​	​	​
17​	​	​	​	​	​
18​	Ductless Heatpump​	3600​	1​	3600​	0​
20​	​	​	​	​	​
21​	Total​	​	​	22800​	​
22​	Amps​	​	​	95​	​

 

[wwhitney]

​	Two dwelling units NEC 220.85​	​	​	​	​	​	​
​	220.84 Multi family​	​	​	​	​	VA​
Line​	Description​	VA​	QTY​	NEC Section​	​	Phase​	Neutral​
1​	General Lighting Load​	​	​	​	​	​	​
2​	Total square Feet​	3​	3492​	220.84(C)(1)​	​	10482​	10482​
3​	Small Appliance​	1500​	4​	220.84(C)(2)​	​	6000​	6000​
4​	Laundry​	1500​	2​	220.84(C)(2)​	​	3000​	3000​
5​	Water heater​	4500​	2​	220.84(C)(3)​	​	9000​	0​
6​	Dishwasher​	960​	2​	220.84(C)(3)​	​	1920​	1920​
7​	Electric Clothes Dryers​	5000​	2​	220.84(C)(3)​	​	10000​	7000​
8​	Main House Range​	11000​	2​	220.84(C)(3)​	​	22000​	15400​
9​	Microwave​	1800​	2​	220.84(C)(3)​	​	3600​	3600​
10​	Ductless Heatpump​	3600​	2​	220.84(C)(5)​	​	7200​	0​
11​	Sub Total​	​	​	220.84(C) 1-5​	​	73202​	47402​
12​	​	​	​	​	​	​	​
13​	Multiply Line 11 by 150% to make 3 'dummy’ units​	​	1.5​	220.85​	​	109803​	71103​
14​	​	​	​	​	​	​	​
15​	Apply 45% Demand Factor For Total Load​	​	​	T220.84​	​	49411​	31996​
16​	​	​	​	​	​	​	​
17​	Amps​	​	​	​	​	206​	133​

 

[wwhitney]

BTW, this level of detail isn't necessary to recognize the incongruity of 220.85. All you need to consider is the treatment of the marginal VA in 220.82 times two vs 220.85.

What happens in 220.82? 1 extra VA gives you an extra 0.4 VA in your final result (the 40% factor). Do that for two units, and you get an extra 0.8 VA in the final answer.

What happens in 220.85? 1 extra VA in each unit gives you 3 extra VA in your hypothetical triplex, and the 220.84 factor of 45% makes that 1.35 VA in the final answer.

1.35VA > 0.80 VA, which means I can construct an example like the above.

Cheers, Wayne

 

[tortuga]

Ok yeah I see what you mean, I just wanted to make sure you were not using 220.82 as an input to 220.85.
220.85 is basically the same thing as adding a row to table 220.84 for two units at 67.5%.

 

[wwhitney]

Ok yeah I see what you mean, I just wanted to make sure you were not using 220.82 as an input to 220.85.

Maybe next time you'll believe me when I say I'm not doing that.

Anyway, my point is that if you have a duplex as above, and you have a main service panel with two 100A breakers to supply the two dwelling units as the only load (no house loads), then it is obviously OK to supply that service panel with just a 200A service.

In other words, 220.85 is not necessary, you can just add up the results of 220.82. To the extent that the NEC has language suggesting otherwise, it's just being sloppy. Any other conclusion is non-sensical, as the example shows.

Cheers, Wayne

 

[tortuga]

you can just add up the results of 220.82.

You can't use 220.82 for a duplex, its only allowed for one dwelling unit.

 

[wwhitney]

You can't use 220.82 for a duplex, its only allowed for one dwelling unit.

We've had this disagreement before, and my example above shows why that interpretation is non-sensical. That interpretation says a 200A service to a panel containing only two 100A breakers is a violation, that the service would need to be at least 205A.

Anyway, I submitted a PI for the 2026 NEC that would settle this question, so we'll see. One upside of working out this example is that if the CMP rejects my proposal, the example will be a good substantiation for my subsequent PC to show them the error of their ways. So thanks for your help with it.

Cheers, Wayne

 

[tortuga]

We've had this disagreement before,

Its not so much that I disagree your logic is good, its that if I use that on a plan submittal I get red lined and turned down, I have had conversations with AHJ's on it and they are unmoved, so I think its misleading to say people can use it unless your a practicing EE and get it approved on your plans.

 

[wwhitney]

Its not so much that I disagree your logic is good, its that if I use that on a plan submittal I get red lined and turned down,

Is your experience of that happening in Oregon only?

Certainly encountering that interpretation from an AHJ is a hazard, but I would hope the above example would sway any reasonable AHJ.

Cheers, Wayne

 

[tortuga]

Is your experience of that happening in Oregon only?

Yeah I only work Oregon at present, there are a ton of ADU's going in now so it comes up often.
Thinking about this thread topic years ago we got rejected with 'service rating' based a load calc
We had a new SFD that had a 300A calculated load, all electric lots of appliances + electric on demand water heater.
We used a class 320 meter, 350AL in service mast and two 200A panels off the meter.
Inspector turned down the 350AL.
The permit on residential typically does not go through electrical plan review so any calcs are verified in the field post install.
If I opt for plan review when its not required I can get stuff like that passed the electrical inspector, becasue there are stamped plans.
Cheers