Recently I was contacted by a local university to 'put wire nuts on some recently replaced wire'. Upon further inquiry, I was told that the university was doing an outside experiment measuring the effects of global warming and had some outside heaters being used to simulate the suns heat, and one of the circuits powering a particular set of heaters had a fire and 'they' replaced the wire, but needed wire nuts put on the splices. So, I said I would 'assess' the situation. This is what I found: 8 heaters ( each one about the size of a shoe box) radiant type...4 each at one location, 4 at the next location, about 10 feet apart. 1000 watts each, fed by 277 volts. The fire ocurred in the junction box of the first location. Circuit consists of #10 thhn, protected by a 35 (yes 35) amp breaker. Now, maybe I am wrong, but as I see it, 1000 watts x 8 = 8000 watts/277 volts= 28.88 amperes load. Given they are heaters, operating for more than 3 hours, multiply 28.88 x 1.25 % = 36.10 amperes. That should mean that the circuit should have #8 thhn as a conductor, not #10. The breaker should be 40 amperes. Am I right?
Wires on fire in junction box for outside heaters
- Thread starter GeorgeW
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hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Seems right to me. Could be the reason for the 1st failure.
mcclary's electrical
Senior Member
- Location
- VA
#10 is sufficient for this load unless you're talking about a huge run (voltage drop) also, #10 is fine on a 35 breaker.
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
I think the more important issue here is why, with a load within the #10 wire's ampacity, something caught fire in the first place. What exactly caught fire in that junction box, was it a splice, wire that was bundled, wire that was in the direct path of the radiant heat?
Despite what the Code says, I see no problem with the 35 amp breaker either. :grin:
Despite what the Code says, I see no problem with the 35 amp breaker either. :grin:
iMuse97
Senior Member
- Location
- Chicagoland
Exactly the question: Why? There's something they haven't said yet. They asked George to "wirenut those terminations" because they had a problem with it the first time, when they did it themselves. The breaker's fine, the ampacity is fine. We have to know the specifics about what happened the first time around. They know but don't want to say.
BAHTAH
Senior Member
- Location
- United States
Heater circuit
Heater circuit
You are correct. 240.4(D) limits the load on #10 to 30amps. 424.3 states fixed electric space heaters shall be considered a LCL. 240.4 states conductors shall be protected based on their ampacies and I see no exception to this 240.4(G) where electric heat is not listed.
210.19(A) requires conductors for LCL to have an ampacity based on 125% of the continuous load. The 40 amp breaker meets 240.4(B) and is the next standard size over your 36.1 amps. What they have wired may work but IMO does not meet code. Someone probably made a bad splice that caused the failure. Make the splices and list the violations on your bill with a note as to what should be corrected.
Heater circuit
You are correct. 240.4(D) limits the load on #10 to 30amps. 424.3 states fixed electric space heaters shall be considered a LCL. 240.4 states conductors shall be protected based on their ampacies and I see no exception to this 240.4(G) where electric heat is not listed.
210.19(A) requires conductors for LCL to have an ampacity based on 125% of the continuous load. The 40 amp breaker meets 240.4(B) and is the next standard size over your 36.1 amps. What they have wired may work but IMO does not meet code. Someone probably made a bad splice that caused the failure. Make the splices and list the violations on your bill with a note as to what should be corrected.
termination connectors
termination connectors
Normally I would not yell fire in a crowded elevator unless it was full of wire nuts. The wiring for the above hookup needs a little more detail about how the wiring was paralleled, how many conductors in each wirenut, size conductor and with wire connectors for UL listed combination types used and approved. I believe the type of connector used will make a difference. Could this data be available? Another question arises as to the replacement of the #10 awg on the branch circuit. If the wiring was fried, why didn't the breaker trip to protect the wiring? rbj
termination connectors
Normally I would not yell fire in a crowded elevator unless it was full of wire nuts. The wiring for the above hookup needs a little more detail about how the wiring was paralleled, how many conductors in each wirenut, size conductor and with wire connectors for UL listed combination types used and approved. I believe the type of connector used will make a difference. Could this data be available? Another question arises as to the replacement of the #10 awg on the branch circuit. If the wiring was fried, why didn't the breaker trip to protect the wiring? rbj
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hardworkingstiff
Senior Member
- Location
- Wilmington, NC
I think this is heading in the right direction. The reason they asked you to make the connections is they probably know it was a poorly made connection that caused the 1st problem along with marginally sized conductors.
Another question I would have is where is this junction box located? Is it above the heater? If so, did the heat from the heater contribute to the failed connection?
Is the connection made up of all stranded or stranded/solid? Either way, those connections are easy to have an undetected loose connection under the wirenut, especially if it was undersized.
Another question I would have is where is this junction box located? Is it above the heater? If so, did the heat from the heater contribute to the failed connection?
Is the connection made up of all stranded or stranded/solid? Either way, those connections are easy to have an undetected loose connection under the wirenut, especially if it was undersized.
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