instantaneous hwh

bill@usps06492 · Sep 24, 2004

I am installing two instantanous hwh that require 3 individual single phase, 39a 2-pole circuits@208v each(thats 6 wires& 1 ground 7 total). Using Art210.20,215.2(A)(1)422.10A,422.11E and 422.13 I am looking to install the 3 circuits at 39ax150%=58.5a 60a ocpd and the wire 39x125%=48.75 50a #8 wire, but at 60a should the wire be sized up to a #6? At 39ax208v=8112va per circuit, so thats 24,336va for the entire unit. I also need to install a seperate subpanel for each hwh. Do I need to install a 200a subpanels? I assume that the demand factor is 100% since there is no storage tank I will be coming off of a 500a 208 3phase panel.

[ September 24, 2004, 11:17 AM: Message edited by: bill@usps06492 ]

charlie b · Sep 24, 2004

Re: instantaneous hwh

When you are adding up loads, you should do it terms of VA, not amps. This is especially true when you mix single phase (i.e., 2-pole 208V) with three phase. Your total load is calculated as follows:

39 amps times 208 volts equals 8,112 VA

3 loads each being 8,112 VA gives a total of 24,336 VA

On a 3-phase 208V panel, 24,336 divided by (208 times square root of 3) equals 67.6 amps.

Since your total load is 67.6 amps, a 100 amp panel should suffice (unless, of course, you plan to add more loads later).

Now as to your other observations:

I may be wrong on this point, and I?m sure others will tell you if I am wrong, but I believe that the instantaneous water heater is not a ?fixed storage-type water heater.? So 422.13 does not apply.

No water heater is on continuously, so you don?t need the 125% factor that is mentioned in 422.10(A).

The 150% rating for the OCPD described in 422.11(E) is an upper limit. You don?t need to go that high.

I would go with a 50 amp breaker and a #8 wire.

instantaneous hwh

bill@usps06492

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charlie b

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