That is what the NEC says to use for the circuit to the motor. They probably said 16 ".5" so you wouldn't use 14 ga wire and a 40a brealer
That is usually a question on the inspector test. FLA=16 with internal O/L protection, what is the min wire size & max breaker. Answer #14cu, 40a. Now days breakers are usually HACR so you don't need one that big to handle the locked rotor on start up but the code hasn't changed.
The 14ga = 15a, 12ga = 20a 10ga = 30a we all know is really 240.4(D) and is aimed at the circuits that are likely to have receptacles, where the installer has no control over what gets plugged in so the
80% safety factor is built into the breaker size limit. Folks will keep plugging things in till the breaker trips, then unplug the clock if that will let it hold.
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...
For a 30 A load i'd use a 30A CB with 10AWG.... [on the safe side]
Induced currents from motor startup could easily overshoot the 19.*A calculated continuos draw value.
As far as useing a bigger breaker than the wires rating It Behooves you all that believe so to reconsider your Logistics., without much math (already given) Underrating a wire to a breaker would have no protection on a circuit that is optimum to the breaker but oblivious to the conductor ~ which may carry the full load of the breaker with forced electrical stress not suitable for such conductor of lower ampacity...
At the least, if yo're lucky stars are shining over you; Brown Out comes to mind.
Roy Q.T. Urban Technician [I don't make em, I just fix em]
Roy Sizing motor conductors to the size of the Circuit Over Current Protective Device that is needed to permit them to start is just a waste of materials. The overload protection of the motor provides the overload protection to the conductors that are sized for the running current of the motor. The circuit OCPD is only providing fault protection for the circuit so there is no reason to size it to provide overload protection to the conductors.
There is a reason they state that you require a 10AWG wire for a 30 amp circuit and that is because it is rated for 30A. #12AWG is rated for 20A continuous. However keep in mind that breakers, unless otherwise stated, are rated to run continuous at only 80% of their rating and over that the clock starts ticking.
Using a 30A breaker on #12AWG wiring is asking for trouble, you will be overstressing and overheating the wire which can lead to fire - good bye house. So it is not a waste of material, it is piece of mind. Code also calls for sizing the breaker and conductor such that you are not going to over load the conductor. Motors will typically have an inrush of 6 times FLA. If you motor has a FLA of 30 then you are going to need a larger feed.
No, his point is that a dedicated circuit, that has motor overload protection for protecting the motor from drawing more than 16 amps continuous, also protects the supply cable from having more than 16 amps continuous draw. So the #12 is protected from continuous currents above 16 amps, just like the motor itself.
A motor connected to a 30 amp breaker better not have a FLA of 30. The 30 amp breaker is based on the requirements for a dedicated motor circuit whose motor has internal protection and draws quite a bit less than 30 amps.
According to the ampacity tables the ampacity of 12 AWG THHN at 75 degrees Centigrade is 25 Amperes. At 90 C it is 30 amperes. Since the breakers terminals are rated for 75 degrees C 20 amperes is 80% of the wires ampacity. Any motor with a running current of twenty amperes or less will have overload protection suitable for protecting the circuit from overload. The Circuit Overcurrent Protective Device is sized to permit the motor to start without nuisance tripping of the breaker or fuses. The US NEC specifically permits this. Saying that is an unsafe practice just demonstrates that your understanding of motor circuit protection is incomplete. The applicable sections of the code read:
240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). (D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or
15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied. (G) Overcurrent Protection for Specific Conductor Applications. Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).
Table 240.4(G) Specific Conductor Applications Conductor Article Motor and motor-control circuit conductors 430, Parts III, IV, V, VI, VII As you can see Motor conductors are specifically provided as an exception to the over current limitations on small conductors because their ampacity is covered in section 430.
You need to study NEC article 430. On motor circuits, the wiring is protected by the motor overload, since there are no other loads (nobody will be plugging in a space heater, for example). The breaker feeding the circuit is sized higher than the wire ampacity, to allow the motor to start. It provides ground fault and short-circuit protection only. This is perfectly safe.
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