kVA required from a heater

[jaredx9]

Here are the specs on the heater, then my questions will come:
Here is a link to the heater: http://www.cpotanklesswaterheaters.com/powerstar/ae115_spec.html

Voltage: 240/208
Watts: 17.25/13kW
Amps: 2x40A (80A)

We currently have a 120/208Y XFMR rated at 45kVA. I want to add the water heater to the XFMR, but I want to make sure my thoughts are right as I have never done this before (I'm an EE right out of college).

I'll need two 40A breakers, so then I should feed these from two seperate phases so to calculate the kVA on each phase:

208 x 40/1000 = 8.32kVA
So if I have two breakers each at 8.32kVA, my total load on my XFMR will be 8.32 + 8.32 = 16.64kVA, is that correct?

I assume this will create a horrible imbalance unless I have other loads to make up for it on the 3rd phase. Would it be smarter to purchase a single phase XFMR for the heater?

Let me know if anything I said is incorrect.

Thanks.

 

[winnie]

Hmm.

1) A 208V circuit from a 208/120 wye transformer requires a connection between _two_ phases, using a double pole breaker. So each of the two circuits supplying your heater is loading two phases. If you distribute these loads, then two phases will each see one of the heaters, and one of the phases will see both.

2) The heater requires 13KW, and presumably 13KVA at unity power factor. The 40A breaker rating doesn't enter into the KVA load that the heater is placing on the transformer.

3) For most approximate calculations, you would simply say that the heater adds 13 KVA of load to the transformer. If you need to get more precise, you would need to calculate the asymmetric loading.

4) The current on the 'shared' phase is not a simple sum of the two circuit currents; you need to take into account phase angle.

-Jon

 

[rattus]

Wait a ;minute:

Wait a ;minute:

A 13KVA load would draw 62.5A @ 208V. 40A breakers won't be sufficient. Two of the three transformers would provide 6.5KVA each--almost half the 15KVA per each rating of the individual transformers.

 

[charlie b]

Would it be smarter to purchase a single phase XFMR for the heater?

What do you mean, "purchase"? Is this something you are considering buying for your own home, or are you speaking of a project related to your job?

 

[charlie b]

A 13KVA load would draw 62.5A @ 208V. 40A breakers won't be sufficient.

Very true. Part of the problem here is that the manufacturer's spec sheet, as viewed from the link in the original post, has an incorrect, and misleading statement. The bit about 80 amps (2 x 40) is wrong. What they should have said is that a 2-pole, 80 amp breaker is needed.

Here's a bit of advice to the new electrical engineering graduates of this world: Avoid at all costs the phrases "amps per phase" and "total amps." The whole notion of describing current in this manner should be stamped out forever, and those (including manufacturers) who use such phrases should be given 40 lashes with a wet noodle!

 

[qcroanoke]

Very true. Part of the problem here is that the manufacturer's spec sheet, as viewed from the link in the original post, has an incorrect, and misleading statement. The bit about 80 amps (2 x 40) is wrong. What they should have said is that a 2-pole, 80 amp breaker is needed.

Here's a bit of advice to the new electrical engineering graduates of this world: Avoid at all costs the phrases "amps per phase" and "total amps." The whole notion of describing current in this manner should be stamped out forever, and those (including manufacturers) who use such phrases should be given 40 lashes with a wet noodle!

Charlie,
If you click on the manual tab above the spec sheet and look at AE115 wiring on page 9,
10 and 11 they do specify (2) 40 amp circuits with #8 wiring. However in Canada they require you to feed it with an 80 amp circuit and use a terminal block for the connections that the Canadian Code requires. On page 3 the Canadian code also requires GFI protection.
I guess you could feed it with an 80 amp circuit but that isn't what they tell you.

 

[charlie b]

. . . they do specify (2) 40 amp circuits with #8 wiring.

I see that you are right. I also see this as an explicit violation of 240.8. What I don't see is how they can tell us to violate the code. :-?:-?:-?:-?

 

[jaredx9]

What do you mean, "purchase"? Is this something you are considering buying for your own home, or are you speaking of a project related to your job?

This is a project related to my job. The heater will be for a large production facility that on average uses 150MW of power.

A 13KVA load would draw 62.5A @ 208V. 40A breakers won't be sufficient. Two of the three transformers would provide 6.5KVA each--almost half the 15KVA per each rating of the individual transformers.

If I were to connect the heater one breaker to A and B, and the other breaker to B and C, wouldn't the kVA be split more like:
A - 3.25kVA
B - 6.5kVA
C - 3.25kVA

Also, why are 40A breakers not sufficent? Your not going to see all 62.5A through ethier breaker, each breaker should see half (31.25A)? Am I wrong?

Please help me understand, as right now I feel almost clueless.

 

[qcroanoke]

I see that you are right. I also see this as an explicit violation of 240.8. What I don't see is how they can tell us to violate the code. :-?:-?:-?:-?

Diagram 2 on page 9 does not appear to show a parallel connection of the unit. It looks as if there are 2 different heaters on 2 different circuits, but not in parallel.

I would hope that it is labeled that the heater may be fed from 2 different breakers though.

 

[jghrist]

The manual does not call for the breakers to be installed in parallel in violation of 240.8. Each 40A circuit serves a separate heater module. You can improve the phase balance by putting one A-B and one B-C. The line current in ?A and ?C will be 6500/208 = 31.25A with a pf of 86.6% (one phase leading, one lagging). The line current in ?B will be sqrt(3)?31.25 = 54.13A with a pf of 100%.

The 54.13A in ?B will be split into the two breakers, each will carry 31.25A. If this is the only load, you will need a 30 kVA transformer because ?B has 6.5 kVA of load.

Complete phase balance could be achieved by buying the Model AE125, which has three heater modules. With this, you would need only a 15 kVA transformer because each phase would carry 4.33 kVA.

 

[charlie b]

Diagram 2 on page 9 does not appear to show a parallel connection of the unit. It looks as if there are 2 different heaters on 2 different circuits, but not in parallel.

It had looked to me as though the circuits were connected in parallel internal to the equipment. But perhaps I was looking at the "install in Canada" version.

So perhaps a pair of 40 amp breakers is the correct way to supply this equipment.

 

[gar]

100210-1425 EST

As I interpret this device it has two 240 V 17.25/2 KW heating elements. Thus, each 240 V heater has a resistance of 6.68 ohms. The current per 240 V heating element is 240/6.68 = 35.9 A. Thus, two separate 2 pole 40 A breakers are required to wire as separate circuits from 2 hot (ungrounded) lines. Otherwise one 2 pole 80 A breaker, if one exists, supplying both heating elements.

.

 

[gar]

100210-1437 EST

This paragraph created confusion.

I'll need two 40A breakers, so then I should feed these from two separate phases so to calculate the kVA on each phase:

Two 40 A breakers sort of implies single pole. Then on each phase further confuses. What was meant by phase? Leading one to think line to neutral. My next paragraph provides a meaning for two phases.

If these heating elements are totally isolated in the water heater, then one could connect one between lines A and B, and the other between B and C or A and C. Somewhat better loading balance.

.

 

[mivey]

100210-1437 EST

This paragraph created confusion.Two 40 A breakers sort of implies single pole. Then on each phase further confuses. What was meant by phase? Leading one to think line to neutral. My next paragraph provides a meaning for two phases.

If these heating elements are totally isolated in the water heater, then one could connect one between lines A and B, and the other between B and C or A and C. Somewhat better loading balance.

.

IMO, it should not have created confusion. With the info the OP provided, it is easy to see what he was talking about. Some of use just did not pay attention and probably just skimmed over the info provided. Not the OP's fault.

Also, why are 40A breakers not sufficent? Your not going to see all 62.5A through ethier breaker, each breaker should see half (31.25A)? Am I wrong?

As you have probably figured out by now, you are correct.

 

[rattus]

This is a project related to my job. The heater will be for a large production facility that on average uses 150MW of power.



If I were to connect the heater one breaker to A and B, and the other breaker to B and C, wouldn't the kVA be split more like:
A - 3.25kVA
B - 6.5kVA
C - 3.25kVA

Also, why are 40A breakers not sufficent? Your not going to see all 62.5A through ethier breaker, each breaker should see half (31.25A)? Am I wrong?

Please help me understand, as right now I feel almost clueless.

Yes, two 40A, double-pole breakers will suffice. Some of us just assumed two single pole breakers in a single 208V branch circuit.

 

[jaredx9]

Thank you everyone for all the replies and discussion on my post, I have a better understanding now of how this heater will affect the total load.

The current 45kVA XFMR I mentioned only has 10kVA of load on it at the moment, so the heater will be added to this XFMR.

Thanks again everyone.