




#91536  01/21/05 10:39 PM
Re: Load Calc for 18kV W/H

Member
Registered: 01/20/05
Posts: 354

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 pointofuse 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 AB and the other element on BC. 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 rewire 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?

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