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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Commercial mains wiring: 12ga wire for 30 amp load?
Ignoramus26750 wrote: On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester wrote: NEC reference says that 12ga THHN wire can carry 30A max current. Motor load is currently drawing constant 20A (230v 1ph circuit). Breaker is 30A. It is my understanding that wire must be matched to the breaker. So, if the breaker is 30A, the wire should be rated for 30A as well. That means 10 gauge. NO, not according to NEC and wiring protection practices. You size the breaker to be 80% of the rated FLA of the conductor. You are protecting the wire with the breaker, not the motor...If the wiring overheats you got a fire. John i (A bit of history: Previous motor (running from 208v) was drawing less (15A?) current until machine broke. New machine's motor (running from 230v via boost autotransformer) is drawing 20A (measured at breaker). Motor spec plate says 16A.) Is 12ga wire sufficient, or should I bump it up to 10ga? Would the autotransformer account for the increased current draw? FBt |
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Commercial mains wiring: 12ga wire for 30 amp load?
Exactly!
Typically you protect a #12 wire with a 20 amp or less breaker, and a #10 wire with a 30 amp or less breaker. For #14 wire a 15 amp breaker is the maximum, but a 10 amp breaker provides an added margin of safety. The terminal design on many breakers intentionally make it difficult (although not impossible) to attach a wire that is undersized for their rating. Harry C. |
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Commercial mains wiring: 12ga wire for 30 amp load?
scada wrote:
"John Hudak" wrote in message ... Ignoramus26750 wrote: On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester wrote: NEC reference says that 12ga THHN wire can carry 30A max current. Motor load is currently drawing constant 20A (230v 1ph circuit). Breaker is 30A. It is my understanding that wire must be matched to the breaker. So, if the breaker is 30A, the wire should be rated for 30A as well. That means 10 gauge. NO, not according to NEC and wiring protection practices. You size the breaker to be 80% of the rated FLA of the conductor. You are protecting the wire with the breaker, not the motor...If the wiring overheats you got a fire. John True, but why would anyone undersize the wire? If it's a 30A breaker - you best be using at least 10AWG wire! Even larger if youre 50' or more from the source. Why would you Jeopardize burning out the motor from a large voltage drop in the wire? Stall current (starting current to break inertia) is typically 20* running I... i (A bit of history: Previous motor (running from 208v) was drawing less (15A?) current until machine broke. New machine's motor (running from 230v via boost autotransformer) is drawing 20A (measured at breaker). Motor spec plate says 16A.) Is 12ga wire sufficient, or should I bump it up to 10ga? Would the autotransformer account for the increased current draw? FBt scada wrote: "John Hudak" wrote in message ... Ignoramus26750 wrote: On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester wrote: NEC reference says that 12ga THHN wire can carry 30A max current. Motor load is currently drawing constant 20A (230v 1ph circuit). Breaker is 30A. (A bit of history: Previous motor (running from 208v) was drawing less (15A?) current until machine broke. New machine's motor (running from 230v via boost autotransformer) is drawing 20A (measured at breaker). Motor spec plate says 16A.) Is 12ga wire sufficient, or should I bump it up to 10ga? Would the autotransformer account for the increased current draw? FBt It is my understanding that wire must be matched to the breaker. So, if the breaker is 30A, the wire should be rated for 30A as well. That means 10 gage. NO, not according to NEC and wiring protection practices. You size the breaker to be 80% of the rated FLA of the conductor. You are protecting the wire with the breaker, not the motor...If the wiring overheats you got a fire. John True, but why would anyone under size the wire? If it's a 30A breaker - you best be using at least 10AWG wire! Even larger if your 50' or more from the source. Why would you Jeopardize burning out the motor from a large voltage drop in the wire? Stall current (starting current to break inertia) is typically 20* running I... Motor Circuits are unique animals. In general the Over Current Protective Device for a motor branch circuit is sized to permit the motor to start without nuisance tripping of the breaker or fuse in question. The OCPD serves only to provide fault protection to the circuit. Overload protection is provided by the Motor Overload Protective Device. This may be a thermal protector on the motor or it may be heaters sized to the motor running current and installed in the motor starter. The circuit conductors are sized at 1.25 times the full load current of the motor it serves based on the ampacity in the NEC table. The note to the table reads "except as otherwise permitted by this code" and it is that phrase that allows for full current ampacity of conductors to be used in sizing motor branch circuits and feeders. In other words the requirement to protect number twelve copper conductors with an OCPD not greater than twenty amperes does not apply to motor circuits. -- Tom Horne |
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Commercial mains wiring: 12ga wire for 30 amp load?
On Fri, 11 Nov 2005 04:28:09 GMT, "Member, Takoma Park Volunteer Fire
Department" wrote: Motor Circuits are unique animals. In general the Over Current Protective Device for a motor branch circuit is sized to permit the motor to start without nuisance tripping of the breaker or fuse in question. The OCPD serves only to provide fault protection to the circuit. Overload protection is provided by the Motor Overload Protective Device. This may be a thermal protector on the motor or it may be heaters sized to the motor running current and installed in the motor starter. The circuit conductors are sized at 1.25 times the full load current of the motor it serves based on the ampacity in the NEC table. The note to the table reads "except as otherwise permitted by this code" and it is that phrase that allows for full current ampacity of conductors to be used in sizing motor branch circuits and feeders. In other words the requirement to protect number twelve copper conductors with an OCPD not greater than twenty amperes does not apply to motor circuits. There are also other exceptions where the characteristics of an overload protected hard wired load are allowed to protect the conductors instead of the circuit breaker, such as for welding machines with limited duty cycle. And then there are loads which do not require overload protection at all, such as certain emergency fire pumps, where the wiring burning up due to overload is deemed less hazardous than the pump shutting down early in a fire emergency. The NEC is a complex beast, not easily understood in its entirety! |
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Commercial mains wiring: 12ga wire for 30 amp load?
"John Hudak" wrote in message ... Ignoramus26750 wrote: On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester wrote: NEC reference says that 12ga THHN wire can carry 30A max current. Motor load is currently drawing constant 20A (230v 1ph circuit). Breaker is 30A. It is my understanding that wire must be matched to the breaker. So, if the breaker is 30A, the wire should be rated for 30A as well. That means 10 gauge. NO, not according to NEC and wiring protection practices. You size the breaker to be 80% of the rated FLA of the conductor. You are protecting the wire with the breaker, not the motor...If the wiring overheats you got a fire. John True, but why would anyone undersize the wire? If it's a 30A breaker - you best be using at least 10AWG wire! Even larger if youre 50' or more from the source. Why would you Jeopardize burning out the motor from a large voltage drop in the wire? Stall current (starting current to break inertia) is typically 20* running I... i (A bit of history: Previous motor (running from 208v) was drawing less (15A?) current until machine broke. New machine's motor (running from 230v via boost autotransformer) is drawing 20A (measured at breaker). Motor spec plate says 16A.) Is 12ga wire sufficient, or should I bump it up to 10ga? Would the autotransformer account for the increased current draw? FBt |
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