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).]
