I follow your math but I'm somewhat confused by the fact that the manufacturer tells you the ampere rating and the kw rating. The kw rating is all you need to size the service or the circuit. The ampere rating is going to vary depending on the voltage. So size the circuit using the kw rating and Article 422 and size the Service based on the kw rating of the water heater.
Re: Load Calc for 18kV W/H #91530 01/21/0509:22 AM01/21/0509:22 AM
The reason is that when a three phase circuit is described as carrying X amps, this means X amps per leg, and when the voltage of a three phase circuit is described, this is the line to line voltage. If you convert things to 'line to neutral' voltage, and consider 'aggregate amps' (the total current down all of the wires) then the above equations make more sense, because now VA is simply volts times amps.
In a three phase 'wye' circuit, the line to neutral voltage is the line to line voltage/1.732
P.S. I think that the manufacturer needs to supply the amp rating because with _4_ heating elements, this is probably an unbalanced load.
Re: Load Calc for 18kV W/H #91531 01/21/0510:09 AM01/21/0510:09 AM
all you need is the KW rating or FLA, if it is a storage type water heater, 120 gallons or less, 422.13 says the branch circuit supplying the water must have a rating of not less than 125% of the nameplate rating.
Re: Load Calc for 18kV W/H #91533 01/21/0506:18 PM01/21/0506:18 PM
BEWARE that a W/H with 4 elements will NOT be balanced across three phases. Each element is actually a single phase load. Assuming they are 208 volt elements, each 4500 watt element will pull 21.65 amps. But those amps will not be distributed evenly across the phases.
Assuming 1 element is connected A-B, and 1 element is connected B-C, and 2 elements are connected C-A, you will have phase B carrying the sum of 21.65 amps at 0 degrees and 21.65 amps at 120 degrees. That adds up to 37.5 amps. But phases A and C have 21.65 amps at 0 degrees and 43.3 amps at 120 degrees. Adding that up gives 57.28 amps. So the manufacturer rating of 57A is about right (but a tad low). It's just phases A and C that will carry the 57.28 amps while phase B carries only 37.5 amps.
If the W/H had 3 elements of 6000w, then each would draw 28.82 amps (at 208 volts) and when balanced across three phases that would add up to a nice round 50.0 amps. But since there are 4 elements, it's not balanced, and the load is lopsided.
If you happen to have multiple water heaters of this type, then you'd want to rotate some of the phases to better distribute the total load. But for just one, you'll need to use the full 57.28 amps. And 125% of that is 71.6 amps.
But you'll have to figure how that fits into the code (e.g. if you need to do the 125% circuit rating in that situation). I'm not a licensed EC; I just do computer room designs and outsource the electrical work to ECs after I've done all my calculations.
[This message has been edited by pdh (edited 01-21-2005).]
Re: Load Calc for 18kV W/H #91534 01/21/0506:24 PM01/21/0506:24 PM
While doing a little shopping for water heaters a couple months ago, to get some specifications an options for planning a house build way in the future, I downloaded technical details on many water heaters from several manufacturers, including commercial models because they had more of the point-of-use types I was interested in. In these technical details were wiring instructions and diagrams. I looked at the 3 phase models, too, because I was curious. Some can even be rewired between 1 phase and 3 phase.
The larger commercial models did have a multiple of 3 for the number of elements. But some smaller models did have 2 or 4 elements. What caught my attention was that the replacement element model numbers for 1 phase and 3 phase versions of the same model line were the same. That's not easy to do unless the element really has 6 coils, and that would be an expensive way to manufacture them.
I did look at a 2 element model for 3 phase rather closely. It did indeed place one element on A-B and the other element on B-C. So the B phase would be drawing more current than A or C. My guess is this was done to minimize the overall costs and use the same line of elements between 1 phase and 3 phase, and make it possible to field re-wire the phase configuration.
In smaller models, 2 elements, or maybe 4 elements, makes more sense due to having upper and lower heating elements. 6 elements would make sense, too, but the cost is getting high there, and that would not be viable for smaller models. If you need 9000 watts, 2 elements of 4500 watts is much less costly than 6 elements of 1500 watts.
Equally balanced, 18000 watts would be 50 amps on 208/120. But the stated 57 amps didn't come close to the 125% figure of 62.5 amps you get from the well balanced case. So I assumed that wasn't indicating the necessary circuit size. So I ran the numbers for the current based on balancing 4 elements the best that can be done on 3 phase, and it came up with 57.282196187 amps. Close enough to 57. Thus I have to conclude that this is a water heater with 4 elements wired as such, because I can see no other reason for listing 57 amps on the nameplate.
Anyway, I do know for a fact that water heaters are made this way with that number of elements. With the load numbers matching, what else can it be?