220.82(c) clarification

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crtemp

crtemp

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Wa state
In this example I have an electric furnace with a 15kw heat strip and a heat pump with a MCA of 22. Am I just using 15kw in my calculation and nothing for the heat pump since the heat pump does not run when back up heat is on?
 
JFletcher said:
You do not calculate the heat pump, however you need to add the amperage of the blower motor in to the 15 KW.

is this a test question, or an actual install?
Click to expand...


Actual install. Trying to do up a load calc on this house. I don't have the exact nameplate ratings of all the equipment but I have some.

3435 sq ft
2 dishwashers at 7.1 amps
water heater
hood fan (guessing 300 watts)
microwave (I'm guessing 1800 watts)
2 small appliance circuits
1 laundry circuit
2 dryers (one normal dryer and the other is a washer dryer stack-able unit)
2 ovens (I'm guessing 18 amps each)
1 36'' electric cook top (I'm guessing 9600 watts)
ductless heat pump MCA 14.1
2 wall heaters (1600 watts and 700 watts)
septic tank pump
well (1/2 horsepower)
electric furnace (15kw)
heat pump (MCA 22)

I'm getting 194 amps (optional method) not including the septic pump. I have no idea what it would draw.
 
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Well, the blower for the furnace will not make any real impact on the overall load calculations. I would think the heat pump would be on a 30 amp circuit and the 15kw Heat in two stages, one on a 60 amp and the other on a 30 amp.

The microwaves are probably not 1800 watts, more like 12 to 1300. If this house has two kitchens, and by the looks of it it does, you would need 4 sabc circuits vs 2. There may be two garbage disposals as well.

You will need a 320 / 400-amp service. The electric water heaters, electric dryers, and electric emergency backup heat put you at 180 amps before anything else is considered.

Eta: 1 5kW water heater drops that to 140-ish amps, but I think you're still gonna wind up over 200A.
 
the second kitchen area has no provisions for cooking so it is not a kitchen according to article 100. I don't think it would need the 2 sabc by code. Now I'm getting 196 amps. Here is how. Let me know if I'm missing something.

3435 sq ft = 10,305
4500 water heater
10000 for 2 dryers
972 dishwasher
972 dishwasher
3000 sabc
1500 laundry
9600 cook top
4320 oven
4320 oven
1800 microwave
1488 well

total 52,777
minus the first 10000
42,777 @ 40% =17,111 + 10,000 = 27,111


electric furnace 15000
ductless heat pump 3384
wall heaters (2 wall heaters, 1600+700=2300. 2300@65%=1495

total 19,879

27,111+19,879=46,990

46,990/240=196 amps
 
If the heat pump and heat strips are wired thru the T-stat to both come on- as if often the case in our area- then you need to add both loads. The heat pump can be wired to turn off when the strips come on
 
So, 196 amps, and you still don't know the septic pump, correct? Are there any garbage disposals?

I'm not going to beat around the bush I will get right to it: it seems to me you were trying desperately hard to not put a 320 / 400-amp service in this house. My question is why? If even a heated bathroom fan is added in the future, it will require a service upgrade.
 
JFletcher said:
So, 196 amps, and you still don't know the septic pump, correct? Are there any garbage disposals?

I'm not going to beat around the bush I will get right to it: it seems to me you were trying desperately hard to not put a 320 / 400-amp service in this house. My question is why? If even a heated bathroom fan is added in the future, it will require a service upgrade.
Click to expand...

no disposals. I just got word on the ovens that are being installed though.

http://www.sears.com/whirlpool-mew9...DcFRLvskdZ4t7XCuCke5lq26j0jVBvaYaAgNrEALw_wcB

I have never seen a 40 amp single oven. Looks like he has no choice but to go 320 amp now.
 
Though code doesn't really recognize it, when calling for aux heat is typically not going to be running anyhere near rated current - because is is colder then a witches ___ outside and they just don't draw much when moving very little heat. The other thing is when they go into defrost mode they may run the heat pump at the same time as the resistance heat - but not for very long - chances of it putting you over 200 amps long enough to trip the main is pretty slim - but again NEC doesn't always use common sense.

If you are that close to 200 amps with your calculations do your client a favor and put in the 400 amp service anyway. They will have no room for added load in the future if you don't.
 
There's always the compromise of two 150a panels, but I agree the cost savings compared to two 200a panels is minimal.
 
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