NEC 220-60 Noncoincident Loads

[anbm]

My understanding of article NEC 220-60 is... if a building has air handler units (fans + compressors, condensers) to provide AC; And electric fan power boxes to provide electric heat. When we size the main service panel for the building, we only take the larger load among these two load + air handler fans load into our calculation.

My fellow mechnical man said I have to count both data into my calculation since even in winter time, some folks want to turn on the heat in one room and other want to turn on AC at anoother room, but if I count both data my panel size will be huge.

And I beleive there are no standard power factor that we can applies in this calculation to reduce electrical load. Anyone has any solutions? Actually, in the past, we have one project and the plan reviewer has rejected our project because he saw main switchboard calculation is overloaded even only 45A. If we only count electric heat load (larger load) + fans and take our AC load, we would pass. But I need to find a good reason to backup ourself.

Thannks!

 

[chris kennedy]

My fellow mechnical man said I have to count both data into my calculation since even in winter time, some folks want to turn on the heat in one room and other want to turn on AC at anoother room, but if I count both data my panel size will be huge.

Each AHU or FCU has it's own thermostat. How can it be on heat and cool at the same time?

 

[anbm]

Chris,

Read again, I said
- Air handler with fans + compressor, conder to provide AC (cooling air).
- Fan power box to provide electric heat.

These are two separate units. Anyway, air handler can have both heating coil and compressor if we specified that way. Thanks!

 

[chris kennedy]

Chris,

Read again, I said
- Air handler with fans + compressor, conder to provide AC (cooling air).
- Fan power box to provide electric heat.

These are two separate units. Anyway, air handler can have both heating coil and compressor if we specified that way. Thanks!

Yes I'm sorry.

building has air handler units (fans + compressors, condensers) to provide AC; And electric fan power boxes to provide electric heat

220.60 would not apply here. Can't help you on a way around it. Although having the heat and AC on at one time is unlikely.:smile:

220.60 Noncoincident Loads.
Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.

 

[R Bob]

Not necessarilly "unlikely" that the AC and heat would be on at the same time. In fact, it is quite common.

Consider the following:

The solar gain on the southern exposure of the building is increasing the space temperatures above setpoint, thus creating a demand for cooling.
In sharp contrast, the space temperatures on the opposite side of the building, the northern exposure, may be below setpoint creating a demand for heat.

Based on this scenario, it would not be unreasonable to assume that any increase in demand for heat would be countered by a decreased demand for cooling and vice versa. Energy consumption between heating and cooling would be a matter of efficiency.

Many factors come into play here. Is the AC system CAV, VAV or VVT. How is the air volume controlled?
Does the AC refrigeration circuit have compressors that are staged ang/or do they have unloaders to modulate capacity?

The mechanical engineer should be intimately familiar with this information. It might not be a bad idea to sit down with him/her to evaluate the situation so you can validate your decision on the load calc.

 

[LarryFine]

Some heat pumps do not de-energize the compressor 100% of the time the aux/emer. heat runs.

The solar gain on the southern exposure of the building is increasing the space temperatures above setpoint, thus creating a demand for cooling.
In sharp contrast, the space temperatures on the opposite side of the building, the northern exposure, may be below setpoint creating a demand for heat.

I would suggest some forced-air movement of the heat to the cold side.

 

[Doug S.]

Although having the heat and AC on at one time is unlikely.

On the contrary. Not only are they often both run for different space temps, but cooling, then slightly re-heating air is a common means of de-humidifying. I thought it was nuts when I first heard of it, but have found it to be rather common in commercial industrial settings. Another example would be where I'm at right now, the boilers are running to keep the place warm, but the chillers are working away to keep the IT and production environments cool, again it's seems silly but common.

Regards,
Doug S.

 

[anbm]

In this situation, we probaky will ask mechanical man to decrease his electrical heat load or we will increase our panel size...whichever costs less. Thanks for all your comments.