220.84(A)(3)

[charlie b]

If you have a personnel housing building (each unit treated as a dwelling unit) that has a central boiler and a central chiller, and if each unit only has a fan coil unit that pushes air past coils through which either hot or cold water is flowing, does that count as the units each having heat or air conditioning? Can we use the optional method for calculating the service load?

I have seen this debated before in a specific jurisdiction. I am in a different jurisdiction for which we are our own AHJ.

 

[LarryFine]

I lived in an apartment building with a system like that. You had heat or off in the winter until they switched the main system AND the T-stat in EVERY UNIT unit for cool or off in the summer. And you were SOL if the weather didn't suit their switchover dates.

I'd say each unit only has the blower as a motor load, and not a heating or cooling system. That's on the house panel.

 

[augie47]

Opinion: For the individual units I would accept 220.82. It does not say heat/AC has to be present just how to treat it when it is, but for the multi-unit 220.84 I would not as it states heat/air is present.

 

[charlie b]

I agree with you both. However, I have seen proposals to the AHJ in which they add a fictional load to each unit, "pretending" it is the unit's air conditioner, and including it in the optional calculation. I don't recall how they picked the value of this fictional load. It may have been the total load of the chiller divided by the number of units. What do you think about this concept?

 

[GoldDigger]

I agree with you both. However, I have seen proposals to the AHJ in which they add a fictional load to each unit, "pretending" it is the unit's air conditioner, and including it in the optional calculation. I don't recall how they picked the value of this fictional load. It may have been the total load of the chiller divided by the number of units. What do you think about this concept?

I find that approach unsupportable, since there will be no diversity in the common heater/chiller load. If you are not going to calculate it separately as part of the house load, you cannot apply a diversity factor to it by spreading it over the individual units. JMO.

 

[Eddie702]

I don't think you have a heating or AC load. Just a motor load that is the same in summer or winter

 

[LarryFine]

I agree with you both. However, I have seen proposals to the AHJ in which they add a fictional load to each unit, "pretending" it is the unit's air conditioner, and including it in the optional calculation. I don't recall how they picked the value of this fictional load. It may have been the total load of the chiller divided by the number of units. What do you think about this concept?

I think it stinks!

Seriously (Hey, it could happen!), as I said, it's a real load on the house panel, and will be calculated as such for the building service, so to divvy it up and apply it to each unit would be redundant.

 

[charlie b]

I am not promoting that approach. I am preparing to defend my position against the designer possibly offering that approach. I am reviewing a design, and the load calc for the individual units is using the optional method. It will have a significant impact on the calculation results, if they can't use the optional method.

But to clarify the approach I mentioned, the chiller and boiler would be calculated as a house load, for the service calculation. The larger of the two would be "shared" with the 30 or so units, so that the units can be calculated using the optional method. Or alternatively, they add a completely nonexistent load (e.g., 5 KVA) to each unit's panelboard, and keep the chiller/boiler on the house panel.

 

[LarryFine]

The larger of the two would be "shared" with the 30 or so units, so that the units can be calculated using the optional method. Or alternatively, they add a completely nonexistent load (e.g., 5 KVA) to each unit's panelboard, and keep the chiller/boiler on the house panel.

I still don't see the logic in either approach. Why must a single load be counted twice?

 

[wwhitney]

I am not promoting that approach. I am preparing to defend my position against the designer possibly offering that approach. I am reviewing a design, and the load calc for the individual units is using the optional method. It will have a significant impact on the calculation results, if they can't use the optional method.

I don't know these calculations very well, but I have made the following inferences based on the discussions here, and if they are correct, I suggest not arguing against the use of the optional method:

(a) The standard method gives a very conservative number.
(b) The optional method gives a conservative number, usually lower than the standard method.
(d) The optional method is only allowed when there's an HVAC load.
(d) The difference between the two results is very often larger than a reasonably sized HVAC load.

The above (if correct) means the sizing procedure in the NEC is illogical and broken. Therefore I find the idea of a phantom HVAC load to allow the optional method completely defensible.

It's just a matter of saying "We have building A. The hypothetical building B with all the same loads, plus some reasonably sized HVAC loads, allows the optional method and gives us an answer X. Since building A has a strict subset of the loads of building B, and the answer X is good enough for B, it's good enough for A."

Cheers, Wayne

 

[LarryFine]

Are you saying that including a phantom HVAC load for each unit can allow for smaller feeders than with no HVAC load at all?

 

[charlie b]

I think the notion is that including a phantom HVAC load for each unit can allow for smaller feeders than the standard method would call for.

 

[wwhitney]

Are you saying that including a phantom HVAC load for each unit can allow for smaller feeders than with no HVAC load at all?

I haven't done the calculations myself, so I am just going on what others have stated, but yes, that's the idea.

Cheers, Wayne

 

[wwhitney]

I think the notion is that including a phantom HVAC load for each unit can allow for smaller feeders than the standard method would call for.

If the idea of a "phantom" load just doesn't sit well with you, I suggest thinking of it as a comparable building with an additional load.

Cheers, Wayne

 

[LarryFine]

Now I'm not sure whether we're discussing individual unit feeder loads or that of the entire building.

 

[wwhitney]

(c) The optional method is only allowed when there's an HVAC load.

OK, I got this part wrong, my apologies, I should have read the section more carefully, and I'd like to revise my comments. My new understanding:

The optional method is allowed for single family dwellings without any such heating/cooling load restriction. For two family dwellings, it's allowed with effectively a 50% penalty (which is more or less the effect of calculating it for 3 identical units instead of 2) (and which seems weird to me).

So now I understand the issue to be that the demand factors for 3 or more dwelling units for the optional method are only allowed when each dwelling unit has electric cooking and also electric heating or cooling. While the standard method does the demand factors on a per appliance type basis, so you'd still get to, for example, use a demand factor for electric cooking when there's no electric heating or cooling.

Further, it's seems that the NEC text doesn't allow this phantom HVAC load approach. Because the requirement on electric cooking, 220.84(A)(2), has an exception allowing the demand factors with a phantom 8kw cooking load. But the requirement on electric heating or cooling, 220.84(A)(3) does not have an exception.

Nonetheless, it makes absolutely no sense not to allow the phantom heating or cooling method. You could design a building with each unit having a small one ton mini-split inverter heat pump drawing 3 kW, supplemental to a central plant HVAC system, and it would satisfy 220.84(A)(3). It makes no sense to say "remove the heat pumps, and now your service size goes up".

Cheers, Wayne

 

[mikeames]

Where in the NEC does it say Heat and AC must be included in the service calc? Unlike SABC that's required, Heat and AC are not. 220.84 is still satisfied because the units will have heat and AC but energy is not on the feeders only on the service. Why not add the fan as a motor load/ appliance for feeder calc.

The entire AC/Heat load going on the house panel/service, and the feeders to the units sized without heat and AC? I agree with the above posts #5,#7, and #9

I get the feeling I am missing something, or I am wrong?

 

[GoldDigger]

...
...
I get the feeling I am missing something, or I am wrong?

What you are missing is that heat/A/C must be included in the individual units to qualify for using the optional method, which will generally give a lower load calculation than the standard method.