Dwelling Service Calculations

[Rick 0920]

When doing dwelling service calculations, one of the last steps has always been to increase your largest motor by 25% to accommodate for motor inrush current on the service conductors. With the change made to 220.60 where if the A/C is the smaller of the noncoincident loads, we then increase it by 25% if it is the largest motor. (which it usually is in a dwelling). If this step is done, is it still necessary to take your largest motor and increase it? It seems like we are doing it twice. The example in Annex D1(b) shows this step in the 2020 NEC even though the change is there in 220.60 for noncoincident loads.

 

[wwhitney]

When doing dwelling service calculations, one of the last steps has always been to increase your largest motor by 25% to accommodate for motor inrush current on the service conductors.

You must be talking about doing a standard calculation, as the optional calculation in 220.82 has no such requirement.

With the change made to 220.60 where if the A/C is the smaller of the noncoincident loads, we then increase it by 25% if it is the largest motor. (which it usually is in a dwelling). If this step is done, is it still necessary to take your largest motor and increase it? It seems like we are doing it twice. The example in Annex D1(b) shows this step in the 2020 NEC even though the change is there in 220.60 for noncoincident loads.

Regardless of whatever often unclear wording the operative version of the NEC has in 220.60, the proper way to deal with two noncoincident loads, such as A/C and electric heat which are properly interlocked, is the following: First delete one load from your list of loads and determine the total load. Then delete the other load from your list of loads and determine the total load. Take the larger of these two calculations.

So if one of the noncoincident loads is your largest motor when it is included, then for the other case when it is excluded, something else is the largest motor and it will get the 125% factor instead. Either of the two cases could end up as your final answer, so the final answer may reflect a 125% factor on the largest motor actually installed, or it may reflect a 125% factor on the second largest motor.

Cheers, Wayne

 

[infinity]

Regardless of whatever often unclear wording the operative version of the NEC has in 220.60, the proper way to deal with two noncoincident loads, such as A/C and electric heat which are properly interlocked,

I don't see where that section requires an interlock. If it is unlikely that the heat and AC will be running at the same time you can omit one without any mechanical interlock.

 

[wwhitney]

I don't see where that section requires an interlock. If it is unlikely that the heat and AC will be running at the same time you can omit one without any mechanical interlock.

OK, maybe that wasn't the best wording, I just wanted to exclude the case that the HVAC is in fact designed without any mechanism to prevent simultaneous heat and A/C.

Cheers, Wayne

 

[suemarkp]

As a followup, what value do you use for the AC compressor if that is your largest load? If you use MCA, it already has a 25% factor added for the largest motor. Do you need to add another 25%? It seems that getting the HVAC load without any 125% factors is kind of difficult in some cases. Looking at the nameplate for MCA is easy. If the HVAC unit isn't your largest load, or if you have multiples of them, backing out those 25% factors built into the MCA can also be difficult.

 

[going_commando]

I don't see where that section requires an interlock. If it is unlikely that the heat and AC will be running at the same time you can omit one without any mechanical interlock.

If it's a ducted mini split system with resistive strip aux heating, that's not necessarily true, the heat pump and electric strips can run at the same time to provide heat. It's also not true in variable speed heat pump systems like the Carrier Green speed that have reheat dehumidification, where when running on AC it will fire the electric heat strips to warm up the air slightly to dry it out.

 

[infinity]

If it's a ducted mini split system with resistive strip aux heating, that's not necessarily true, the heat pump and electric strips can run at the same time to provide heat. It's also not true in variable speed heat pump systems like the Carrier Green speed that have reheat dehumidification, where when running on AC it will fire the electric heat strips to warm up the air slightly to dry it out.

I said "if it's unlikely" which is the wording used in 220.60. It wasn't a blanket statement for every possible scenario.