3 phase tankless water heater calc help please

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DeeBee

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Location
NYC, NY
I need to find out if I an existing 600 amp 3 phase 120/208 service switch is adequate for the existing apartment load.

I have a multi-dwelling - 7 apts, (5)- 1542 sqft, (2) - 1144 sqft

each residential apt has 120/208 3 phase 4 wire 175 amp panel

In addition to the light/app/laundry load, each apt has
(1)- 1800 va dishwasher
(1)- 21.84 Kva 208v single phase tankless water heaters
(3) - 4.5 Kva electric room heaters
6.5 Kva cooktop
4.2 Kva oven
no dryer load

I can't find the section for calculating the tankless or 3 phase equivilent load . I was told by the manufacturer that the tankless water heaters are non-continuous. I need to use the optional method for this, but do I need to find the 3 phase equivilant total load, derate them first then add that total to the building load, or what... for the total load?

Same question about 3 phase equivilent load for the range and heaters

I came up with 397696 va @44% / 360v = 486 amps per phase.

Anybody...can you double check this or comment?

I tried to follow along with Annex D5(a) but I can't seem to come up with their numbers to understand how they got them.

oh...being NYC it has to be NEC 2005
 
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Roush

Member
Location
Chicago
In calculating the load for the single phase 208V 21.84KVA water heater, you don't need the three phase equivalent load persay. KVA is KVA...in calculating the load you use a different factor for different phase voltages. In your case for the single phase heater....

21840VA / 208 V = 105A load (non-continuous)

How is this heater being fed 208V single phase from a 120/208 three phase service? Could this heater possibly be a 2 phase? I believe 1 phase will give you 120V and 2 phases can give you 208V.

If it were a 2 phase 4 wire then the load would be....
21840VA / (208V * 2) = 52.5A load (non-continuous)

Maybe somebady can back me up on this or tear me down if i'm off base:smile:?
 
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DeeBee

Member
Location
NYC, NY
The tankless heater load is 3 - 35 amp 2 pole 208v, so they'll be split across the 3 phases.

It looks like I'm going with the optional method 220.82 for the apt's and 220.84 for the building. The inspector seems to be agreeable. It's just I can't find anywhere in the code that covers 3 phase 4 wire service and 3 phase 4 wire panels...everything covered is 120/240, 120/208 single phase, or 3 phase service/single phase panels . This is going to be a common setup with this builder, so I've submitted my calculations to the Advisory Board to clarify that this method is acceptable in the future and not just this case. It is quite common here to have each inspector see it a different way.

220.82 -all loads except room heaters 100 % , then first 10k Kva plus...40% of the rest plus 65% of heater load because 4 or less.

220.84 - 100% of everytrhing then 44% for 7 units

Thanks for the replys. Sometimes just writing it out makes it more clear.
 
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