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Minimum size NM cable for water heater

Jpflex

Jpflex

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
The answer in this electrical code calculations book has to be wrong


It ask what the minimum size NM cable to supply a 50 gallon storage type water heater. 240 volt single phase supply, non simultaneous upper and lower heating elements at 4500 watts. Second, it also ask what size 60/70 degree circuit breaker will be required


The book correctly calculated the NM branch circuit conductor at

4500 watt / 240 volt = 18.75 amperes

For continuous load 18.75 i x 1.25 = 23.44 i ampere conductor

Book correctly selects #10 AWG good for 30 amperes at 60 degrees

However, the continuous load is to be protected for 23.44 i amperes would equate to the next standard larger size OCPD (not over the ampacity of wire but over continuous load amperes) to be used at 25 ampere breaker but not a 30 ampere breaker selected by the book

In contradiction NEC does show a 10 AWG conductor being protected by a 30 ampere breaker for SMALL CONDUCTOR CODE, but should this mean that a larger ampacity conductor cannot be protected by a smaller OCPD breaker nor should a load be protected by an OCPD rated higher than the next higher standard size breaker
 

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As you noted a #10 NM would be required and, also as you noted, #10 NM needs to protected at it's 30 amp rating (with exceptions)
What you didn't note was 422.11(E) which al;lows OCP for that water heater to be 150% of the appliance current (next size up allowed) so the 30 amp protection meets the NEC requirement.
 
augie47 said:
As you noted a #10 NM would be required and, also as you noted, #10 NM needs to protected at it's 30 amp rating (with exceptions)
What you didn't note was 422.11(E) which al;lows OCP for that water heater to be 150% of the appliance current (next size up allowed) so the 30 amp protection meets the NEC requirement.
Click to expand...
But I took it as the book asking for the minimum requirements? I guess small conductor ICPD code contradicts/ supersedes 240.6A
 
Jpflex said:
In contradiction NEC does show a 10 AWG conductor being protected by a 30 ampere breaker for SMALL CONDUCTOR CODE, but should this mean that a larger ampacity conductor cannot be protected by a smaller OCPD breaker
Click to expand...
No it does not mean that. You could use larger conductors but the OCPD is still limited to 150% as Augie stated.
 
infinity said:
No it does not mean that. You could use larger conductors but the OCPD is still limited to 150% as Augie stated.
Click to expand...
Yes i see, but if this question were asking for the minimum OCPD and it were THHN branch circuit conductor then we would be selecting a 25 ampere breaker instead of a 30 ampere breaker (excluding NM small conductor code rule)?

Also an OCPD limited to 150% of the water heater amperes would still not be allowed even if the amperes were not listed on the water heater nameplate and it specs over 13.33 amperes for a water heater of 150 gallons or less, if such a calculated breaker ampacity ecxceeded the small conductor OCPD code limits such as using an an OCPD over 30 amperes on 10 AWG NM (150% of water heater amperes)

Using the max OCPD at 150% of water heater amperes depending on calculations results may only work if forced to use a larger size NM cable or run THHN that is not subject to the small conductor OCPD code rule?
 
I guess the answer to my question is that the reason the book did not select a 25 ampere breaker as an optional size OCPD was ONLY because of the OCPD for small conductors code rule for 10 AWG to be protected by 30 ampere OCPD.

However, if you could legally install a breaker less than or equal to 30 amperes on a larger 10 AWG conductor then why wasn’t this an option in the calculations book? Just trying to know which answer to use for journeyman’s testing?

Also correction. THHN i believe is subject to small conductor rule as Romex
 
I fail to understand you premis
The question asks for the minimum size NM which, for a 23 amp load, would be a #10

If you used a conductor larger than #10, 422.11 would restrict the OCPD to 30 amps.
If you used a #10 THHN conductor 240.;4(G) would limit the OCPD to 30 amps.
 
augie47 said:
I fail to understand you premis
The question asks for the minimum size NM which, for a 23 amp load, would be a #10

If you used a conductor larger than #10, 422.11 would restrict the OCPD to 30 amps.
If you used a #10 THHN conductor 240.;4(G) would limit the OCPD to 30 amps.
Click to expand...
But you could use a 25A breaker if you wanted to yes?
 
augie47 said:
I fail to understand you premis
The question asks for the minimum size NM which, for a 23 amp load, would be a #10

If you used a conductor larger than #10, 422.11 would restrict the OCPD to 30 amps.
If you used a #10 THHN conductor 240.;4(G) would limit the OCPD to 30 amps.
Click to expand...
We agree that the smallest size conductor will be #10 AWG

The small conductor code will limit the OCPD to a maximum 30 amperes - again i agree with you

However, if journeymen testing were asking for the minimum OCPD in this situation, i would say 25 i amperes not 30 amperes

A larger ampacity 10 AWG conductor can be protected by a smaller ampacity limiting breaker but not the other way around

This is why i say the answer should have been 25 i but not 30 i if seeking minimum OCPD



Reason:

18.75 i load amperes x 1.25 (continuous) = a branch supply conductor capable of at least = 23.4 i amperes

10 AWG Romex satisfies this with 30 i ampacity at 30 degrees celcius

NEC requires this 10 AWG conductor to be protected by a 30 ampere OCPD MAX but does not disallow a smaller OCPD hence a 25 standard breaker satisfies the 23.4 load and protects the 10 AWG conductor. So i chose 25 ampere breaker as an answer
 
electrofelon said:
But you could use a 25A breaker if you wanted to yes?
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This was the point of entire post. Why couldn’t 25 i OCPD could have been the answer. The book only put 30 ampere OCPD as an optional answer
Dennis Alwon said:
The correct answer would be 25 amps however the test writers failed to see that a 25 amp ocpd is readily available.
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ok thank you!!! The journeyman’s test won’t accept multiple answers so thanks for your guys help
 
Most 50 gallon 4500W resistance waterheater's we see (and they are quite common here) if not all come with instructions that say to use a 25A breaker for 240V, which is why supply houses stock 2 pole 25's but you don't see two pole 35's or 45's stocked. So either 110.3(B) and/or not voiding the warranty they usually end up on a 25A breaker.
Years ago I worked with a guy that tried to argue with an inspector you could use #12 MC on a 25A breaker to a 4500W waterheater, he lost that argument.
Read the test questions carefully and just focus on what its asking.
 
tortuga said:
Most 50 gallon 4500W resistance waterheater's we see (and they are quite common here) if not all come with instructions that say to use a 25A breaker for 240V, which is why supply houses stock 2 pole 25's but you don't see two pole 35's or 45's stocked. So either 110.3(B) and/or not voiding the warranty they usually end up on a 25A breaker.
Years ago I worked with a guy that tried to argue with an inspector you could use #12 MC on a 25A breaker to a 4500W waterheater, he lost that argument.
Read the test questions carefully and just focus on what its asking.
Click to expand...
Only time I have used 35 and 45 amp breakers was for hvac loads either furnace or outdoor units. 35 amp is fairly common and stocked because of this. The 45 is harder to come by.
 
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