I've been asked to install a service to supply 24 1500 watt plug in resistance type electric heaters in a rooming house to supplement the baseboard hot water heat in 24 rooms.I am wondering if I can consider them as appliances and use the demand factors in table 220-42 or would I have to assume they would be likely to be used at the same time. I was going to figure 20000w at 50% and 16000 w at 40% giving me 16400w of calculated service load if I used the demand table. That seems pretty low .I am also insralling 4 tankless electric water heaters 14.5 kw each. 14.5kw x 4=58 kwx75%=43.5kw. total calculated load would be 59900w if I don't consider any of above as continuos load.59900w/240v=250 amps.do you think I've figuring service way too low
service load calc
- Thread starter tommyrice
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JFletcher
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- Williamsburg, VA
Sounds like a test question rather than a real one....
Is the rooming house usually at or near full capacity? How many sq ft/room? Is the wall insulation decent? Baseboard (fixed) heaters are considered continuous load (424.3(b)), seems that portable ones wold be too, tho I cannot find them specifically mentioned in my 08 NEC. Also, if it's cold in one part of the house, then I'd presume it's cold in all of it, so yes, if you had 24 tenants, you could see 24 (or more) portable heaters running at the same time (unless it's Miami - does the NEC have any tables for loads based off of geography or weather?). However, the heaters I've seen have 600W/900W/1500W operation, w/t-stat and I dont see all 24 running at 1500W at the same time. it is auxiliary heat to the boiler baseboard heat, but I wouldnt even consider that system as if it fails or is mothballed, the portables will be used exclusively. Anyway, I'm overthinking the heck out of this part... fortunately, I found this link:
http://ecmweb.com/code-basics/clearing-multi-family-dwelling-unit-calculations
Looks like you're doing step 2, which would consider heating loads (portable or not) at 100% (and not appliances), and at full use for multi-family dwelling. 1500W x 100% x 24 = 36KW. 36,000W/240V = 150A
I dont know about the tankless, other than they arent continuous loads... all 4 running simultaneously would pull 242A on 240V. 242A + 150A = 392A. Considering all of my calc are on the high end, 400A service would do?
Is the rooming house usually at or near full capacity? How many sq ft/room? Is the wall insulation decent? Baseboard (fixed) heaters are considered continuous load (424.3(b)), seems that portable ones wold be too, tho I cannot find them specifically mentioned in my 08 NEC. Also, if it's cold in one part of the house, then I'd presume it's cold in all of it, so yes, if you had 24 tenants, you could see 24 (or more) portable heaters running at the same time (unless it's Miami - does the NEC have any tables for loads based off of geography or weather?). However, the heaters I've seen have 600W/900W/1500W operation, w/t-stat and I dont see all 24 running at 1500W at the same time. it is auxiliary heat to the boiler baseboard heat, but I wouldnt even consider that system as if it fails or is mothballed, the portables will be used exclusively. Anyway, I'm overthinking the heck out of this part... fortunately, I found this link:
http://ecmweb.com/code-basics/clearing-multi-family-dwelling-unit-calculations
Looks like you're doing step 2, which would consider heating loads (portable or not) at 100% (and not appliances), and at full use for multi-family dwelling. 1500W x 100% x 24 = 36KW. 36,000W/240V = 150A
I dont know about the tankless, other than they arent continuous loads... all 4 running simultaneously would pull 242A on 240V. 242A + 150A = 392A. Considering all of my calc are on the high end, 400A service would do?
I would be a bit concerned.
If it were "fixed" heating, 220.51 would require 100% (unless the exception was allowed).
The "safe" route would be Mr Fletchers 400 amp service.\
I personally don't have a problem applying the 75% to the tankless as they cycle so often.
If you figured the heat as fixed and the water heaters at 43kw I think you would end up with a 330 amp service. I might tend to lean toward a 300 amp at least.
If it were "fixed" heating, 220.51 would require 100% (unless the exception was allowed).
The "safe" route would be Mr Fletchers 400 amp service.\
I personally don't have a problem applying the 75% to the tankless as they cycle so often.
If you figured the heat as fixed and the water heaters at 43kw I think you would end up with a 330 amp service. I might tend to lean toward a 300 amp at least.
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