Article 220 Examples/Options for 1-3 Dwelling Units

[wwhitney]

To improve my understanding, and explore the options in Article 220 beyond the standard calculations in Part III, I came up with the following example for a fairly modest but realistic all-electric dwelling unit: 1200 sq ft, no extra SABCs/laundry, and equipment nameplate data as follows:

Range: 12 kW
Dryer: 24A @ 240V = 5760 VA
Heat Pump: 240V, Compressor RLA 12A, Fans 1A, MCA 16A

Dishwasher: 1200W
Water Heater: 4500W
ERV: 2.5A @ 120V = 300VA
Bath fan: 0.27A @ 120V = 32VA
Range Hood: 2.4A @ 120V = 288 VA
Disposal: 6A @ 120V = 720VA
(The ERV and disposal are marked with HP ratings as well, but 430.6(A)(1) Exception 3 tells us to use the FLA when a listed motor driven appliance is marked with both FLA and HP on the nameplate).

To start with the standard calculation in Part II, some preliminaries:

- The general lighting and receptacle load is 3VA * 1200 sqft + 3 specified circuits (SABC and Laundry) * 1500 VA/circuit = 8100VA. This is subject to first 3 kVA @ 100%, rest @ 35%.
- Range load is 8 kVA for one dwelling unit. Table 220.55 tells us to add 3 kVA each for a second or third range.
- Dryer nameplate exceeds 5000VA, so we use the nameplate.
- Heat pump load @ 100% is 13A * 240V = 3120 VA, with 25% of the largest motor as 25%*12*240V = 720 VA.
- The last 6 items count as appliances and total 7040 VA. They can get a 75% factor per 220.53.

In the calculation for 1 dwelling unit, I'm going to separate out the "base" part (which doesn't increase with more units) from the "scalable part" (which does increase). The upshot for 1-3 dwelling units:

- On the general lighting and receptacle load, there's 65% * 3000VA = 1950VA base and 35% * 8100 VA = 2835 VA scalable.
- For the range, it's 5 kVA base and 3 kVA scalable
- Dryer is 5760 VA scalable (no reduction allowed for 1-4 dryers in Table 220.54)
- The heat pump load is 720VA base (25% of largest motor), 3120VA scalable.
- The appliances are 75% * 7040 VA = 5280 VA scalable.

Adding these up, 1 dwelling unit is 7670 VA base plus 19,995 VA scalable. That totals 27,665 VA, or 115A.

Then for 2 dwelling units, it's 7670 VA + 2 * 19,995 VA = 47,660 VA, or 199A. And for 3 dwelling units it's 7670 VA + 3 * 19,995VA = 67,655 VA, or 282A.

Any errors above? If that's all correct, I'll continue with the Part IV options in the next post.

Cheers, Wayne

 

[Dennis Alwon]

Wayne I have never seen calcs as you have done. lol

I got the same answer so what you have is correct IMO.

 

[wwhitney]

Wayne I have never seen calcs as you have done. lol

Yeah, I knew that I wanted to do the same calculation for 3 cases, i.e. 1, 2, and 3 dwelling units. So rather than just repeat everything 3 times, when doing it the first time for 1 dwelling unit I just kept track of what parts would change with the number of dwelling units, and what parts would not. Makes the first calculation a bit harder, but makes the next two easy.

I got the same answer so what you have is correct IMO.

Thanks! I'll proceed to posting on the optional methods.

It was pointed out to me that for the 2020 NEC, 220.14(J) specifies that small (less than 1/8 HP) motors on lighting circuits in dwelling units "shall be considered part of the minimum lighting load." So my computation is correct only if the 3 fans (ERV, bath fan, range hood) are each either bigger than 1/8 HP or not on a lighting circuit. And oddly, the final result goes up if all 3 fans are less than 1/8 HP and on lighting circuits, as then we have only 3 appliances, and so we lose the 75% factor for the large appliances.

Cheers, Wayne

 

[wwhitney]

OK, on to 220.82, the optional method for a dwelling unit. We are assuming a 120/240V 3-wire service or feeder.

As to the extra 25% of the largest motor, which the standard calculation in Part III requires via the 220.50 reference to 430.24, there is no such reference in 220.82. And the 3 parts of Example D3 in Annex D do not include it. So the heat pump load is 13A @ 240V = 3120VA, and per 220.82(C)(2) we include that at 100%.

The rest of the loads just get added up and then factored per 220.82(B): 8,1000 (lighting/receptacles) + 12,000 (range) + 5760 (dryer) + 7040 (appliances) = 32,900 VA. The diversity factor gets applied as 60% * 10,000 + 40% * 32,900 = 19,160 VA. Plus the heat pump at 3120VA makes 22,280 VA total. That gives 93A @ 240V, and we need to size the service at 100A per 230.42(B)/230.79(C).

Now what about a feeder or service that supplies 2 or 3 dwelling units? With a distribution panelboard that separates that supply into 2 or 3 separate feeders, each of which individually supplies the entire load associated with exactly one dwelling unit?

Obviously 220.82 applies to each individual feeder. And while not explicit anywhere, I see no reason that an upstream feeder's load can't be taken to be the sum of the loads of the feeders supplied. Certainly it makes no sense to think that the load on a combined feeder would be greater than the loads of the individual feeders.

That would give us 44,560 VA = 186A for 2 identical dwelling units, and 66,840 VA = 279A for 3 identical units. For this example, those values are slightly less than the Part III values. (But if we were to consider 4 dwelling units, then the Part III calculation would come out slightly ahead.)

Comments or corrections? Next will be 220.84 and 220.85.

Cheers, Wayne

 

[Dennis Alwon]

Yeah I would never have used a bath fan of any size.... lol. I just used the numbers you gave me. Range hoods were never used either but they really should be as some have large motors-- In reality they, in most cases, don't make much of a difference anyway.

 

[tortuga]

Obviously 220.82 applies to each individual feeder. And while not explicit anywhere, I see no reason that an upstream feeder's load can't be taken to be the sum of the loads of the feeders supplied. Certainly it makes no sense to think that the load on a combined feeder would be greater than the loads of the individual feeders.

In my area inspectors reject a 'double optional' 220.82 calc for a service that supplies more than a single dwelling unit.

 

[wwhitney]

In my area inspectors reject a 'double optional' 220.82 calc for a service that supplies more than a single dwelling unit.

Well, that's a good lead in to take a look at 220.84/220.85.

For 3 dwelling units, we can use 220.84. It permits a 45% diversity factor applied to the simple sum of the loads.

For our example in the OP, the simple sum of the loads in one dwelling unit is 3120 VA (heat pump) + 8,1000 (lighting/receptacles) + 12,000 (range) + 5760 (dryer) + 7040 (appliances) = 36,020 VA. Multiply by 3 * 45% and we get 48,627 VA, or 203A.

That means for 3 units of this example, 220.84 easily beats Part III (282A) or the possible "triple 220.82" method (279A).

[For huge dwelling unit loads, the "triple 220.82" method would eventually beat 220.84 for 3 dwelling units, just because 40% < 45%. Assuming you add lots of load to our example without changing the heat pump load, the break even point would be a dwelling unit simple sum of loads of 157,440 VA, or 656A. So I don't see that happening in practice.]

Finally on to 220.85. It gives the option for 2 dwelling units of using the 220.84 3 identical dwelling unit answer. So that would again be 203A, which is just over the 199A of the Part III calculation, not useful in our example. But the 220.85 answer would be lower than the Part III answer if the dwelling unit had some more specific loads, as the additional loads would be factored at 75% (if appliances) or 100% (if not) under Part III, while they would only be factored at 45% under 220.85. Conversely, adding more receptacle/lighting load would swing the advantage slightly back towards Part III, as the additional load gets factored at 35% instead of 40%

Of course, the "double 220.82" calculation, if allowed, would be less, at 186A.

P.S. To the inspectors in your area and others arguing against the "double/triple 220.82" calculation, I would say two things. (A) There's no clear language in 220.82 restricting the load calculation to feeders or services that supply only one dwelling unit. And (B) disallowing the simple double or triple 220.82 method means taking the position that the load on a feeder is greater that the sum of the loads of the feeders it supplies. Since that can't be true for the currents on the feeders, the only justification would be to say that the diversity factor(s) need to increase when going from 1 to 2 or to 3 dwelling units. Which is similarly non-sensical, and I see nothing in Article 220 to support that.

Cheers, Wayne