16 heater circuit load cal

[rongudlin]

I'm new to calculating loads.I,trying to find the load 16 -1200va heaters.Do I use a demand factor of 75% of the total va,then divide by 240 to get a total load in amperes.Using cord mounted truck heaters at night.What would I look for in section 200..????thanks

 

[coulter]

What are you ttrying to do? A calc to satisfy the AHJ? A system design that is safe, reliable, and does the intended purpose?

The reason I ask is I live where it is imperative to plug in the trucks and equipment or it won't start in the morning. Letting equipment get cold-soaked to -40 is a real bear.

If you are planning on having all 16 in use all night, I'd say the code really doesn't matter. Recommend sizing feeders, xfm for 125% X 16 X 1200va.

If it's an NEC calc to satisfy the AHJ, I wouldn't know - my knowledge is sorely deficient there.

carl

 

[rongudlin]

At 125% I get 24,000va divided by 240v would give me a cal load of 100amperes.Is that a safe way to calculate a load to size a feeder ampacity for 16 circuits????

 

[bsh]

I agree with Carl. You might have some heaters off while others are on when the temp is warm but when it gets cold they may all be on all the time so use the worst case.

 

[coulter]

At 125% I get 24,000va divided by 240v would give me a cal load of 100amperes.Is that a safe way to calculate a load to size a feeder ampacity for 16 circuits????

Sounds good from my side of the keyboard. Couple of other sugggestions:
1. Bullrails (plug-in racks) are usually located way out. Watch the voltage drop. If you can, you may wish to locate the xfm and panel out at the rail.

2. If energy use is an issue, look at adding thermostats, timer, and a contactor feeding a dedicated panel. For example:
Turn on at 30F, 1/2 hour on - 1/2 hour off
Turn on solid at -20F

3. If you do use a contactor and a dedicated panel out at the rail, consider adding a second small panel for area lights and a few un-switched receptacles. An amazing phenomenon of nature: Areas that get cold also get dark:wink:

3. GFIs: I don't think gfis are required (but I don't know that). However, a lot commercial users have them. Some use receptacles, others use CBs. If you end up with gfcis, look for a way to know the receptacles are energized. If nothing else, use lighted cords. Amazing as it sounds, that pesky equipment has a way of occasionally knocking out a gfi.

carl

 

[LarryFine]

If the question is really whether to size for 80a or 100a, the answer is clear: 100a. To me, the cost difference is not worth the performance hit.

 

[LarryFine]

Is that a safe way to calculate a load to size a feeder ampacity for 16 circuits????

The quantity of circuits is not relevant to a feeder calc. If the heaters are 240v units, at 5a each, you could put three on a 20a circuit, giving you 5-1/3 (okay, six) circuits. If they are 120v units, that's one per circuit, unless you could use two per 30a circuit (if the receptacle/plug thing isn't an issue.)