25 amp breaker for # 10 wire

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DaveBowden

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St Petersburg FL
We were called to a job today for one of our regular customers who manages several apartment buildings.
One of their tennants had called POCO to have them install an energy management device on the water heater in their apartment. POCO's electrical subcontractor had refused to connect the EMD because of what he claimed was a code violation.
The building has a 3 phase Wye service ( 120/208 Volts ), the water heater nameplate said 4500 Watts @ 240 Volts, the heater was wired with # 10 TW ( only 2 conductors in 1/2 inch EMT), and on a 30 Amp breaker. The POCO sub said it had to be changed to a 25 Amp breaker to be code compliant.
I don't know why and wondered if anyone here could explain it to me.
If I'm using Ohm's Law right, I come up with a 3380 Watt 16.25 Amp at 208 Volt load.
 
DaveBowden said:
We were called to a job today for one of our regular customers who manages several apartment buildings.
One of their tennants had called POCO to have them install an energy management device on the water heater in their apartment. POCO's electrical subcontractor had refused to connect the EMD because of what he claimed was a code violation.
The building has a 3 phase Wye service ( 120/208 Volts ), the water heater nameplate said 4500 Watts @ 240 Volts, the heater was wired with # 10 TW ( only 2 conductors in 1/2 inch EMT), and on a 30 Amp breaker. The POCO sub said it had to be changed to a 25 Amp breaker to be code compliant.
I don't know why and wondered if anyone here could explain it to me.
If I'm using Ohm's Law right, I come up with a 3380 Watt 16.25 Amp at 208 Volt load.
Click to expand...

I think it's fine. The wattage will not change. 4500 / 208= 21.6 amps x 1.25 (continuous) and it comes to 27 amps. next size up is 30.

Only thing I would question is if manufacture alows it to operate @ 208
 
Yes it is copper wire.
I don't know which sub it was so I couldn't ask them. Probably the same one that came to my home and removed the energy management equipment with out installing a knock out seal in the hole.
Kind of a moot point now since the apartment manager authorized us to install the 25 amp breaker and will be getting invoiced for it, but I ccouldn't explain to her why it was needed.
 
The POCO person was wrong. Start at 422.10(A). The branch circuit rating cannot be less than the marked rating of the appliance. In your case, the marked rating is 4500 VA at 240 volts, which is equivalent to 18.75 amps. So the breaker has to be at least 18.75 amps, but this article does not give a maximum breaker rating.

Next, go to 422.11(A). If the water heater had been marked with a maximum breaker size, then you would have to meet that requirement. But it is not so marked. So here again the code does not give a maximum breaker rating.

Next, go to 422.13. You didn't give us the water volume of the heater, but I think it is a good bet that we have to consider it a continuous load. That makes the minimum breaker rating 18.75 times 1.25, or 23.4 amps. So a 25 amp breaker will satisfy the code. But here again this is a minimum rating. Nothing prohibits us from using a higher rated breaker, so long at the conductor can handle the current that the breaker will be able to pass along.

Finally, go to 240.4(D)(7). Presuming your answer to hardworkingstiff's question is "yes," then tor a #10 copper, the maximum breaker rating is 30 amps.
 
The breaker is fine at 30 amps. But 4500 watts is the load at 240 not at 208. Only violation i see is the water heater did not say 240/208. That will not cause a problem and i would let it slide.
 
Okay let's assume that the heater is listed for 208 volts-- not sure why it wouldn't be....

As the OP states the wattage, using ohm's law, at 208 volts=3380. Divide by 208 and we get 16.25 amps.

Now go to 422.11(E) -- the ocpd cannot be larger than 150%-- 15.25 equal 24.4 amps. Thus, IMO a 25 amp breaker is req as (E)(3) allows you to go to the next standard size from 24.4-- that is a 25 amp breaker.
 
DaveBowden said:
I would have sworn the wattage would change. I know the 10kw heat strips we used to wire up in condos were only around 7.5kw at 208 Volts.
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Your figures are correct on the amps and wattage for the W H, the wattage and amps both drop with the decrease voltage . Most of the time the wattage for 208 and 240 are both shown on the end of the element. I would like to know where he got his figures for the 25 amp breaker, That would be the max for aluminum #10 TW , are you sure it is not copper clad alum ? It has the same 25 amp rating , unless you can read on the wire you have to cut into it to see if it is aluminum inside or try to scrape it off.. #12 copper TW has a 25 amp rating but still can have only a 20 a breaker .(unless a motor load, but we wont go there at all , unless the WH has a big pump inside lol )
 
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Dennis Alwon said:
Now go to 422.11(E) -- the ocpd cannot be larger than 150%-- 16.25 equal 24.4 amps. Thus, IMO a 25 amp breaker is req as (E)(3) allows you to go to the next standard size from 24.4-- that is a 25 amp breaker.
Click to expand...

I think Dennis wins the prize on this one. Give the man a cigar. :grin:
 
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