Residential Load Calculations and 25% of Largest Motor

[winterovereverything]

Hey Everyone,

Maybe I'm a bit dense (as usual) here but what's the score for using AC units in a residential load calculation when factoring in 25% of the largest motor? I know some people say that you should always factor it in but others argue that so long as the furnace is used in the load calculation, we drop air conditioning all together and rely on loads like garbage disposals or trash compactors for our largest motors. Maybe I'm missing something, but I'd like to hear what you guys think.

-WOVE

 

[wwhitney]

If doing the optional calculation 220.82, there is no mention of every applying 125% yourself. It only references the "nameplate rating", so if that nameplate rating incorporates a 125% factor, then it will end up in your result; if it doesn't, it won't.

If doing the standard calculation, then my interpretation of 220.60 on non-coincident loads is that you calculate each of the use cases separately, using a 125% factor on the largest motor in use in each case. That's the only rational way to do it.

Cheers, Wayne

 

[winterovereverything]

If doing the optional calculation 220.82, there is no mention of every applying 125% yourself. It only references the "nameplate rating", so if that nameplate rating incorporates a 125% factor, then it will end up in your result; if it doesn't, it won't.

If doing the standard calculation, then my interpretation of 220.60 on non-coincident loads is that you calculate each of the use cases separately, using a 125% factor on the largest motor in use in each case. That's the only rational way to do it.

Cheers, Wayne

The question was referring to the standard method, I should've mentioned that (my apologies). So let's say we have furnace beating out A/C for non-coincident loads and thus A/C is omitted. You would still go ahead and factor in the AC demand for the 25% of the greatest motor, correct? There seems to be this idea that as soon as A/C is omitted, you drop it from a standard load calculation completely and not even bring it in for 25% of the greatest motor, which to me makes no sense as the code does not specify such.

Once again, maybe I'm being quite foolish and if such is the case please do let me know.

 

[wwhitney]

The question was referring to the standard method, I should've mentioned that (my apologies). So let's say we have furnace beating out A/C for non-coincident loads and thus A/C is omitted. You would still go ahead and factor in the AC demand for the 25% of the greatest motor, correct?

No, I would not. In making the determination that the furnace "beats out" the A/C, I would calculate the total load with only an A/C present and compare that to the total load with only a furnace present. The largest motor in each of those two cases may differ, but whichever total load is biggest will be the final load.

There seems to be this idea that as soon as A/C is omitted, you drop it from a standard load calculation completely and not even bring it in for 25% of the greatest motor, which to me makes no sense as the code does not specify such.

220.60 does have language that addresses this topic, but it has been changing every code cycle and isn't the clearest. I would just advocate doing what is logical: when the furnace use case is a larger load than the A/C use case, then obviously the A/C isn't running, so it can't be the largest motor. In other words, we don't have to worry about the A/C starting surge happening while the furnace is running if they are non-coincident.

Cheers, Wayne

 

[winterovereverything]

No, I would not. In making the determination that the furnace "beats out" the A/C, I would calculate the total load with only an A/C present and compare that to the total load with only a furnace present. The largest motor in each of those two cases may differ, but whichever total load is biggest will be the final load.


220.60 does have language that addresses this topic, but it has been changing every code cycle and isn't the clearest. I would just advocate doing what is logical: when the furnace use case is a larger load than the A/C use case, then obviously the A/C isn't running, so it can't be the largest motor. In other words, we don't have to worry about the A/C starting surge happening while the furnace is running if they are non-coincident.

Cheers, Wayne

Wayne, I don't know how I missed that. Need to go get my head checked. Thank you for all that you do!