Sizing OCPD and Wire for HWH

[tyha]

we have a water heater with upper and lower elements that run do not run simultaneously. The unit is marked 3PH 4500W upper and 4500W lower. Doing the math it comes out to be about 12 1/2 or so amps which would be 3p 20A CB with #12 Thhn. Why am I thinking that is wrong and it needs to be #10. maybe because Im thinking of a single phase unit. Am I missing something or is it right?

 

[480sparky]

Can you show your math?

 

[tyha]

208 x 1.73 = 360

4500/360 = 12.5

the unit states 4500W total

 

[tyha]

im sorry I forgot to say it was 208.

 

[480sparky]

im sorry I forgot to say it was 208.

I assumed that when you said 3-ph.
Is there anything (label or instruction) about max over-current protection size?

 

[augie47]

From waht you state, all is correct.

I must say it would be a first as I have never seen a 3 phase 4500 watt water heater nor have I seen a 4500 watt 208 unit. (most 4500 units I have seen are @240)
I hope you have actually seen the nameplate and not relying on hearsay.

 

[tyha]

nope. this is exactly how it is stated. upper 4500W lower 4500W total 4500W. the plumber marked the correct model and it states inside the wiring diagram that the upper and lower do not run simultaniously and that it is 3ph and there is nothing that states min ckt or max ocpd

 

[480sparky]

I come up with 12.51 amps. 4500/208/1.73.

Either we're suffering from the same dilusion, or we're both right.

 

[hardworkingstiff]

nope. this is exactly how it is stated. upper 4500W lower 4500W total 4500W. the plumber marked the correct model and it states inside the wiring diagram that the upper and lower do not run simultaniously and that it is 3ph and there is nothing that states min ckt or max ocpd

How many elements did the wiring diagram show conneted at the to and then connected at the bottom? It should show 3 elements.

 

[rt66electric]

flip-flop either/or

flip-flop either/or

I hate to admit it but a @#$% PLUMBER had to explain it to me...
It has a upper and a lower element of the same wattage. There is also a small temp controller on the unit that ... Heats the top element first to desired temp them flip/flops to heat the lower element. the process is more efficient and has a faster recovery time, for longer showers.

 

[hardworkingstiff]

I hate to admit it but a @#$% PLUMBER had to explain it to me...
It has a upper and a lower element of the same wattage. There is also a small temp controller on the unit that ... Heats the top element first to desired temp them flip/flops to heat the lower element. the process is more efficient and has a faster recovery time, for longer showers.

Just one upper element? How would you hook up 3 wires to one element?

 

[480sparky]

Just one upper element? How would you hook up 3 wires to one element?

Same way you hook up three wires to a motor.

 

[infinity]

208 x 1.73 = 360

4500/360 = 12.5

the unit states 4500W total

If this were a test question you left one part out:

12.5amps * 125% = 15.625 would be your minimum circuit ampacity since a storage type HWH is considered a continuous load.

 

[hardworkingstiff]

Same way you hook up three wires to a motor.

In a 3-phase motor you have 3 (or is it 6) coils to connect to.

In a 1-phase motor I think you have 4.

Motors were never my strong point.

 

[paul]

These assemblies are usually comprised of three elements, but only one unit. They have three screw terminals that the wires connect to.

 

[480sparky]

In a 3-phase motor you have 3 (or is it 6) coils to connect to.

In a 1-phase motor I think you have 4.

Motors were never my strong point.

Actually, nine in a 3-phase.

But you still hook 3 energized wires to it.

 

[hardworkingstiff]

Actually, nine in a 3-phase.

But you still hook 3 energized wires to it.

Right, in 3 groups of 2?

You can't hook a 3-phase power supply to 1 element. To 1 unit consisting of 3 elements or 3 groups of elements, but not 1 element.

 

[joebell]

would 422.13 apply to this ,not that it would make a difference in the size of the conductors or OCPD, but just in terms of the calculation?

 

[tyha]

it has 3 wires sticking out in a junction box that we connect to. just 3 phase wires and ground and thats it. the wire diagram shows 2 phases going through a coil of some sort and 1 phase going directly from where we hook it up to a timer inside.

 

[LarryFine]

First, the "4500w upper/4500w lower/4500w total" rating is typical for water heaters. Once the upper element is satisfied, it switches to the lower, which switches off when it's satisfied. The lower element gets most use.

Second, I have seen water heaters with diagrams for 1ph or 3ph use. They're wired with two of the three wires connected together for 1ph, and like an open Delta on 3ph, with one conductor shared by both elements.

And, naturally, for a given power level, the current will be lower when the designed voltage rating is higher, because elements of different resistances are used. Having 3ph also lowers line current, and the two advantages add.