Rheem water heater 3 phase chart calculation question

[brother]

Looking at the chart from RHEEM water heater installation manual for recommended ocd 3 phase 480 breaker and wire. I do not see how they came up with the calculation for them, as they are alot higher. I am curious how they wired their heating elements in them, and how many heating elements. most have 2 heating elements, but I have seen 1.

One example is the 12000 watt 3 phase 480 volt. They require 30amp ocp and #10 wire. Why do they need a 30amp? 12000/(480x1.73) =14.45 14.45x1.25=18.06
If they have 2 heating elements, and wired most of them in open deltas, I think I can see the difference in calculation. also, the 9000 watt total tank. 9000/(480x1.73)= 10.838 10.838x1.25= 13.547amps. Yet they want a 25amp ocp and # 8 wire. Why the 25amp?

I suppose if they used 2 heating elements at 4500watts each, to get the 9000, then each heating element will most likely be at single phase calculation. 4500watt/480 =9.375amps 9.375x1.73 =16.21875amps for one of the phases if wired in an open delta configuration, then 16.21875 x 1.25 (ocp) =20.273 minimum ocp. Max ocp 1.5 x 16.21875=24.328 next size up 25 amp.

Am I correct in this assumption?

 

[ramsy]

Why the 25amp?

If both WH elements are always interlocked, only one operates at a time.

That chart applies to single phase elements, so their calculation is correct without any Delta or 1.732 factors.

9kW / 480 * 1.25 = 23.44A or 25A fuse

12kW / 480 * 1.25 = 31.25A or 35A fuse

 

[brother]

If both WH elements are always interlocked, only one operates at a time.

That chart applies to single phase elements, so their calculation is correct without any Delta or 1.732 factors.

9kW / 480 * 1.25 = 23.44A or 25A fuse

12kW / 480 * 1.25 = 31.25A or 35A fuse

The 12kw is at 30 amp not 35 amp for 3 phase. so that calculation you have cannot be correct.

 

[ptonsparky]

Do they have inherent appropriate OC for the elements built into there controller?
Doesn't seem to make much sense otherwise.
It appears we are missing some detail.

 

[ramsy]

The 12kw is at 30 amp not 35 amp for 3 phase. so that calculation you have cannot be correct.

For 12kW chart shows 35A fuse.

Where are you getting 30A?

Where does chart indicate 3 phase?

 

[ptonsparky]

For 12kW chart shows 35A fuse.

Where are you getting 30A?

Where does chart indicate 3 phase?

 

[ramsy]

View attachment 2568873

Yes I see, chart shows both 1 & 3 phase @ 35 & 30A respectively.

Just don't land 3rd phase on grounding terminal!

 

[ptonsparky]

....

Just don't land 3rd phase on grounding terminal!

Now you're getting picky.

 

[brother]

Yes I see, chart shows both 1 & 3 phase @ 35 & 30A respectively.

Just don't land 3rd phase on grounding terminal!

Glad someone had you see it. Dont worry, no hot wires on the ground terminal. LOL

But back to my original question, am I correct in this thinking in how they came up with the calculation?

 

[wwhitney]

But back to my original question, am I correct in this thinking in how they came up with the calculation?

I don't know enough about the construction to say you are correct, but I can confirm your computations assuming that the 3 phase version uses (2) 2-wire heating elements that may be energized simultaneously. So I infer you are correct.

Cheers, Wayne