Industrial Equipment

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KSue

KSue

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Minnesota
A system was designed with 1500W heaters (each fused 10a) , and the AC in is 120VAc on a 15A circuit, 14awg.

They had intermittent tripping of the internal 15A breaker. More information came to light. With the box, temperatures reach to 51deg C (has a small internal boiler.)

My first thought was the breaker is too small. De-rating the breaker at about 55C, brings it to about 12A, which is 120X12A=1440W. If it was ambient temperature, this would still be marginal.

I suggested a 20A breaker on 12awg. De-rated brings it to 16A, which is 120x15A = 1920W.

The question came back, can we use the 20A breaker with the 14awg, if the conductor is only about 30" long.

They ran a short circuit test on 20A/14awg, and it didn't blow the internal breaker, but did blow the breaker for the receptacle they were using.

The end of the (short) wire welded to the screw.

Tell me what happened here?
 
updated. It is 3 heaters 500W each (total 1500W on circuit)

updated. It is 3 heaters 500W each (total 1500W on circuit)

KSue said:
A system was designed with 1500W (3 heaters 500W each, and each fused at 10a) , and is 120VAc on a 15A circuit breaker, 14awg.

They had intermittent tripping of the internal 15A breaker. More information came to light. With the box, temperatures reach to 51deg C (has a small internal boiler.)

My first thought was the breaker is too small. De-rating the breaker at about 55C, brings it to about 12A, which is 120X12A=1440W. If it was ambient temperature, this would still be marginal.

I suggested a 20A breaker on 12awg. De-rated brings it to 16A, which is 120x15A = 1920W.

The question came back, can we use the 20A breaker with the 14awg, if the conductor is only about 30" long.

They ran a short circuit test on 20A/14awg, and it didn't blow the internal breaker, but did blow the breaker for the receptacle they were using.

The end of the (short) wire welded to the screw.

Tell me what happened here?
Click to expand...

Updated this link.
 
Last edited:
NEC 240.4(D)(3) prohibits the use of a 20A breaker to protect 14 gauge conductors. It does not say why. I have read another source that stated any breaker over 15A cannot provide short circuit protection for a 14 gauge conductor. This could explain the welding.
 
........the weld could be the result of overloading, a loose connection, or a dead short at one time, all tripping the breaker.
 
SG-1 said:
NEC 240.4(D)(3) prohibits the use of a 20A breaker to protect 14 gauge conductors. It does not say why. I have read another source that stated any breaker over 15A cannot provide short circuit protection for a 14 gauge conductor. This could explain the welding.
Click to expand...

Whips on fluoresent fixtures are 16 guage. 20A branch circuits are SOP for commercial wiring. Is this a concern?
 
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