We ran into a situation where a contractor installed a 12/2 nm cable to a 4000 watt 240 volt electric heater. He protected it on a 20 amp overcurrent device. Is this breaking the NEC? He is saying that he derate the #12 from the 90 degree column which is 25 amps. If you take 80% of 25 amps it is 20 amps. He says because article 240 protects the wiring on the 20 amp overcurrent device because 20 amps is the largest overcurrent device I can put on a #12 wire.
4000watt heater 240 volts on 20 amp breaker
- Thread starter mstepka
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Is it 4000 watt or 4500 watt. Almost all water heaters I have seen are 4500 watt.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The contractor cannot use 75C rating for #12 if he is using nm cable. NM cable is rated 60C so he is in violation. Also a water heater is a continuous load so you must have a wire with a ampacity of 125% of the load. I am assuming a storage water heater.
- Location
- Logan, Utah
Welcome to the forum.
The contractor is incorrect.
424.3(B) requires the branch circuit for fixed electric space heating equipment to be considered a continuous load. Therefore 210.19(A)(1) requires that branch circuit be sized at 125% of the continuous load and 210.20(A) requires that the overcurrent protective device also be sized at 125% of the continuous load.
Therefore with a 4000 Watt heater at 240 volts the ampacity is 16.666 amperes and if you multiply this by 125% you are required conductors and an overcurrent protective device that are rated at least 20.83 amperes.
Chris
The contractor is incorrect.
424.3(B) requires the branch circuit for fixed electric space heating equipment to be considered a continuous load. Therefore 210.19(A)(1) requires that branch circuit be sized at 125% of the continuous load and 210.20(A) requires that the overcurrent protective device also be sized at 125% of the continuous load.
Therefore with a 4000 Watt heater at 240 volts the ampacity is 16.666 amperes and if you multiply this by 125% you are required conductors and an overcurrent protective device that are rated at least 20.83 amperes.
Chris
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
Sure, probably will.
I'd try and swap guts to a 3500w heater if it's not practical to repull the wire.
Last edited:
Heater
Heater
This is a Electric Baseboard Heater. We were discussing this in class today. What if 1 had 2x4 troffers and installed 17 of them that drew 1 amp a piece and installed it on 12/2 MC and did the same thing on a 20 amp overcurrent device. Because this time it is not NM wiring and it is not a continiuous load.
Heater
This is a Electric Baseboard Heater. We were discussing this in class today. What if 1 had 2x4 troffers and installed 17 of them that drew 1 amp a piece and installed it on 12/2 MC and did the same thing on a 20 amp overcurrent device. Because this time it is not NM wiring and it is not a continiuous load.
- Location
- Logan, Utah
The same sections apply. If the load were not in fact a continuous load the you could have 20 1 amp 2X4 troffers on a 20 amp circuit with #12 MC cable and a 20 amp circuit breaker. Most likely though lighting will be considered a continuous load. Remember that the definition of a continuous load is a load that run for 3 hours or more.
Chris
- Location
- Connecticut
- Occupation
- Engineer
There is no problem with putting seventeen 1 Amp (non-continuous) troffers on a 20A c/b with 12/2 MC.
For that matter, there is no problem with putting seventeen 1 Amp (non-continuous) troffers on a 20A c/b with 12/2 NM.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Chris 210.19 states that the conductor must have an ampacity of the load times 125%. If mc is rated 25 amps then the wire is rated for the load. Does it say the OCPD must also be 125%???
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Dennis I think that you need to read that one again for non-continuous loads.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I read it but it implies you cannot do this for continuous loads. I have never done it but if the wire is rated 25 amps why can't you use 17 amp continuous load if you use a 20 amp breaker.
But i just remembered the breaker would have to be rated 100% for continuous load also.
- Location
- Logan, Utah
Correct, 210.20(A) requires the OCPD to be sized at 125% for a continuous load. That is really the catch all in this situation.
Chris
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