specification of breaker size
- Thread starter theodore
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jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: specification of breaker size
Just what is the load on this circuit? AC or a motor ?
Just what is the load on this circuit? AC or a motor ?
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: specification of breaker size
Let me restate the question, for I don?t know the answer myself. What is the reason for 240.4(D)? Anyone know the history?
By the way, Theodore, that article gives several exceptions to the rule requiring a 15 amp breaker, and is the reason for Jim?s question.
Let me restate the question, for I don?t know the answer myself. What is the reason for 240.4(D)? Anyone know the history?
By the way, Theodore, that article gives several exceptions to the rule requiring a 15 amp breaker, and is the reason for Jim?s question.
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
Re: specification of breaker size
[ September 28, 2004, 12:08 PM: Message edited by: petersonra ]
Realistically there probably is no hazard. OTOH, there might well be a liability. I'd be inclined to replace it with a 15A CB if posisble.
[ September 28, 2004, 12:08 PM: Message edited by: petersonra ]
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: specification of breaker size
With regard to the original question and with respect to Jim's response, some circuits can "overload" by the inherent nature of the load, e.g. a stalled motor; other circuits (especially residential branch circuits) overload because someone just keeps adding loads. All circuits are subject to short-circuits or ground-faults, of course and, for the most part, a #14 is just as well protected by a 20A as a 15A CB.
This is strictly "gut feel." I think it and its predecessor (footnotes formerly in tables 310-16&17) were originally means to automatically derate residential wiring where most circuits weren't "engineered." Those footnotes didn't appear in the "high-temp" Tables 310-18&19. This is one of several "legacy" items in the NEC that I think we can only speculate about.
With regard to the original question and with respect to Jim's response, some circuits can "overload" by the inherent nature of the load, e.g. a stalled motor; other circuits (especially residential branch circuits) overload because someone just keeps adding loads. All circuits are subject to short-circuits or ground-faults, of course and, for the most part, a #14 is just as well protected by a 20A as a 15A CB.
Re: specification of breaker size
One thing that must be addressed is the ability of the conductors to be able to carry enough amperage to trip the over current protection device. I have seen houses burn down and people killed because the 30 amp plug fuse on a 14 guage wire would not open with a dead short at the end of the wiring.
Will a 20 amp circuit breaker trip if there was a short on the end of a long branch circuit?
One thing that must be addressed is the ability of the conductors to be able to carry enough amperage to trip the over current protection device. I have seen houses burn down and people killed because the 30 amp plug fuse on a 14 guage wire would not open with a dead short at the end of the wiring.
Will a 20 amp circuit breaker trip if there was a short on the end of a long branch circuit?
- Location
- Illinois
- Occupation
- retired electrician
Re: specification of breaker size
derwith,
Don
derwith,
The current required to trip a small breaker (100 amps or less) under short circuit conditions is almost the same for all breaker ratings. If the #14 won't carry enough current to trip the 30 amp breaker on a short circuit, it likely won't carry enough current to trip the 15 amp breaker.
Don
WaveGuide
Member
Re: specification of breaker size
The question I would ask is not about a simple short circuit, but rather about the future loading of this circuit by an unsuspecting user.
What if for example, during a cold winter, someone plugs in 19A of light bulbs and electric heat into this circuit? Over the night how much heat will be generated in the #14 wire, and is it run through insulation where the heat will build up or near other heat sources?
The hightest electrical generated temperatures often occur not in the wire but at the connections between wires. It is where the resistance is the highest. Have you ever seen a hot connection, but where the wire is cool?
What if the 20A breaker is a little out of spec? What if it takes 24A to trip it? I have seen older breakers fail to operate at their specified current. There needs to be a little room for error to add up and not create an unsafe condition.
Finally, there's one sure way to devalue otherwise good property: install an electrical system which is non-standard and questionalbe. If a future prospective buyer finds this code violation he or she will question the entire electrical system in the building.
The question I would ask is not about a simple short circuit, but rather about the future loading of this circuit by an unsuspecting user.
What if for example, during a cold winter, someone plugs in 19A of light bulbs and electric heat into this circuit? Over the night how much heat will be generated in the #14 wire, and is it run through insulation where the heat will build up or near other heat sources?
The hightest electrical generated temperatures often occur not in the wire but at the connections between wires. It is where the resistance is the highest. Have you ever seen a hot connection, but where the wire is cool?
What if the 20A breaker is a little out of spec? What if it takes 24A to trip it? I have seen older breakers fail to operate at their specified current. There needs to be a little room for error to add up and not create an unsafe condition.
Finally, there's one sure way to devalue otherwise good property: install an electrical system which is non-standard and questionalbe. If a future prospective buyer finds this code violation he or she will question the entire electrical system in the building.
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