Help with service calculation

crtemp

Senior Member
Location
Wa state
I have a customer that has a house that is 2,680 square footage and has electric everything in it (furnace, water heater, ect).

They want to add another 10k electrical furnace and 30 amp heat pump downstairs in the basement. I want to do a load calc and make sure that the 200 amp service is adequate for what they want to add. I'm a little confused on the general lighting load part of the calc. Do I only include the sq footage, small appliance circuits, and laundry loads before I do the first 3000 va at 100% and then the remaining va at 35%. Then do I add the double oven, dryer, water heater after?
 

augie47

Moderator
Staff member
Location
Tennessee
The examples in Annex D should prove to be helpful.
(If you know the actual loads, it has been my experience that the optional method may be a bit more lenient)
 

crtemp

Senior Member
Location
Wa state
The examples in Annex D should prove to be helpful.
(If you know the actual loads, it has been my experience that the optional method may be a bit more lenient)
Here is what I'm getting from Annex D example D2(a) optional calc heating larger than a/c
Does this look correct?


GENERAL LOAD
2680 SQ FT @ 3 VA = 8,040 VA
(2) SMALL APPLIANCE = 3,000 VA
LAUNDRY = 1,500 VA
DOUBLE OVEN = 9,600 VA
WATER HEATER = 2,500 VA
DRYER = 5,000 VA
DISHWASHER = 1,200 VA
GARBAGE DISPOSAL = 600 VA
TOTAL = 31,440 VA

DEMAND FACTOR
FIRST 10 KVA @100% = 10,000 VA
REMAINDER @ 40% = 8,576 VA
TOTAL = 18,576

25 KVA OF HEATING LOAD @ 40% = 10,000 VA

TOTAL = 28,576 VA
28,576 / 240 = 119 AMPS
 

crtemp

Senior Member
Location
Wa state
I had a couple of items that were left out of the first calculation. Here is what I have after I fixed it.

GENERAL LOAD
2680 SQ FT @ 3 VA = 8,040 VA
(2) SMALL APPLIANCE = 3,000 VA
LAUNDRY = 1,500 VA
DOUBLE OVEN = 9,600 VA
WATER HEATER = 4,500 VA
DRYER = 5,000 VA
DISHWASHER = 1,200 VA
GARBAGE DISPOSAL = 600 VA
WELL = 2,400 VA
COOKTOP = 4,500 VA
2 VACUUM SYSTEMS = 9,000 VA
TOTAL = 49,340 VA

DEMAND FACTOR
FIRST 10 KVA @100% = 10,000 VA
REMAINDER @ 40% = 15,736 VA
TOTAL = 25,736 VA

25 KVA OF HEATING LOAD @ 65% = 16,250 VA

TOTAL = 41,986 VA
41,986 / 240 = 175 AMPS
 

kwired

Electron manager
Location
NE Nebraska
I have a customer that has a house that is 2,680 square footage and has electric everything in it (furnace, water heater, ect).

They want to add another 10k electrical furnace and 30 amp heat pump downstairs in the basement. I want to do a load calc and make sure that the 200 amp service is adequate for what they want to add. I'm a little confused on the general lighting load part of the calc. Do I only include the sq footage, small appliance circuits, and laundry loads before I do the first 3000 va at 100% and then the remaining va at 35%. Then do I add the double oven, dryer, water heater after?
I did not look too hard at your actual calculations you presented but want to point out a thing or two -

does the heat pump and electric heat run at same time or are they interlocked by controls so they can not run at the same time? You only need the larger load included if they can not run simultaneously. If they only run simultaneously when in defrost mode - technically you need to use both, but I personally have little issue with disregarding the lighter load for no longer than this defrost cycle will run.

Is your heat pump 30 amps or is the breaker feeding it 30 amps? You need the actual rated load and not the overcurrent device rating for load calculations. 30 amps is a pretty big heat pump for most residential applications, and if 10kW is the back up heat for this it probably will not be enough back up heat so my dollar is on it having a 30 amp breaker with a rated load of maybe only 15 amps max.

Then comes the reality of when heating mode that heat pump probably really only draws 6 or 8 amps max, and maybe 10 to 15 when in cooling mode, but that is reality and not what NEC says.:angel:
 

crtemp

Senior Member
Location
Wa state
I did not look too hard at your actual calculations you presented but want to point out a thing or two -

does the heat pump and electric heat run at same time or are they interlocked by controls so they can not run at the same time? You only need the larger load included if they can not run simultaneously. If they only run simultaneously when in defrost mode - technically you need to use both, but I personally have little issue with disregarding the lighter load for no longer than this defrost cycle will run.

Is your heat pump 30 amps or is the breaker feeding it 30 amps? You need the actual rated load and not the overcurrent device rating for load calculations. 30 amps is a pretty big heat pump for most residential applications, and if 10kW is the back up heat for this it probably will not be enough back up heat so my dollar is on it having a 30 amp breaker with a rated load of maybe only 15 amps max.

Then comes the reality of when heating mode that heat pump probably really only draws 6 or 8 amps max, and maybe 10 to 15 when in cooling mode, but that is reality and not what NEC says.:angel:
I am 99% sure they don't run at the same time. I did not factor in the heat pumps only the heat strips. The current furnace they have now is a 15k but they are adding another one that will be 10k. I just did not want to wire the 2nd one only to have an inspector want the service upgraded to a 320. I far as I can tell from my load calc I should still be safe with a 200 amp service.
 

augie47

Moderator
Staff member
Location
Tennessee
From my experience locally, I lean toward kwired's concern that the heat-pump compressor can run simultaneously with the strip heat. I would suggest checking that possibility as it may put your over the 200 amps.
 

nizak

Senior Member
I would surely think that the chances of ever reaching the 200A capacity would be slim to none. If $$$ is an issue with the owner you may want to show them that they are getting close to exceeding the maximum load that their existing service will handle with the addition of the electric heat.If it's something you have to keep asking yourself over and over again(will it be large enough?) your better off proposing an upgrade to the 320. Would it be possible to set the new 320 meter base and just add a 100A panel to handle the heat loads? I had a similar situation awhile back where a new pool (12KW heater,2HP pump,pool house) put the load near 200A, I changed the meter and set a new panel and fed the equipment from it.Only change to the existing was the 2/0 conductors that fed the 200A service, they needed to be shortened a bit because the new can was larger. Just a thought.
 

kwired

Electron manager
Location
NE Nebraska
Not much you can do if an inspector is making you do the calculations. My experiences are even though the calculations may require more than a 200 amp supply, quite often if you put same load on a 200 amp breaker it never sees enough load to make it trip. But then move a different family into same house and it does trip.
 

crtemp

Senior Member
Location
Wa state
I would surely think that the chances of ever reaching the 200A capacity would be slim to none. If $$$ is an issue with the owner you may want to show them that they are getting close to exceeding the maximum load that their existing service will handle with the addition of the electric heat.If it's something you have to keep asking yourself over and over again(will it be large enough?) your better off proposing an upgrade to the 320. Would it be possible to set the new 320 meter base and just add a 100A panel to handle the heat loads? I had a similar situation awhile back where a new pool (12KW heater,2HP pump,pool house) put the load near 200A, I changed the meter and set a new panel and fed the equipment from it.Only change to the existing was the 2/0 conductors that fed the 200A service, they needed to be shortened a bit because the new can was larger. Just a thought.
I don't think money is an issue for the homeowners. I'm not too knowledgeable on the HVAC side. I'm not sure how I can figure out for sure if the heat strips and the heat pumps could run at the same time. Maybe I will just do the calculation as if they would.
 
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