I am going to explain how to size branch circuit conductors for a motor. For this example I am going to use a motor with the following specs;
5HP
230 Volt AC
FLA = 15A
Single phase
Continuous duty class
Continuous duty cycle
Service factor 1.15
Tempature rise not over 40 degrees C.
First look at NEC 430.6A which states;
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.
What this means is that for sizing the branch circuit conductors, motor disconnect and OCPD, the amperage listed on the nameplate is not used. Instead the tables in NEC 430 are required to be used.
So even though the nameplate states 15 amps the values in Table 430.248 (Single phase AC motor shall be used).
430.22(A) General states;
(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1).
A 5HP single phase 240V motor draws 28 amps. Multiply the 28 by 125%
28A x 125% = 35A
The conductors have to be sized for 35A.
Table 310.16 indicates #8AWG is rate for 40A.
Next we look at sizing the ground fault, short circuit, overcurrent protective device (OCPD).
430.52 states;
430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.
(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)
Table 430.52 states that for an inverse time circuit breaker protecting a single phase motor it is to be sized at 250% of the FLA as stated in Table 430.248.
28A x 250% = 70A
Based on the above calculation a 70A ITCB is correct.
If the motor fails to start and run then 430.52(C) (1) Exception No. 2(c) can be applied which states;
(c) The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.
28A x 400% = 112A
Since we can’t exceed 400% of the motor FLA we round down to a 110A ITCB on a piece of #8 AWG conductor.
There is also an online calculator that will do all the work for you. Please follow the link.
5HP
230 Volt AC
FLA = 15A
Single phase
Continuous duty class
Continuous duty cycle
Service factor 1.15
Tempature rise not over 40 degrees C.
First look at NEC 430.6A which states;
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.
What this means is that for sizing the branch circuit conductors, motor disconnect and OCPD, the amperage listed on the nameplate is not used. Instead the tables in NEC 430 are required to be used.
So even though the nameplate states 15 amps the values in Table 430.248 (Single phase AC motor shall be used).
430.22(A) General states;
(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1).
A 5HP single phase 240V motor draws 28 amps. Multiply the 28 by 125%
28A x 125% = 35A
The conductors have to be sized for 35A.
Table 310.16 indicates #8AWG is rate for 40A.
Next we look at sizing the ground fault, short circuit, overcurrent protective device (OCPD).
430.52 states;
430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.
(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)
Table 430.52 states that for an inverse time circuit breaker protecting a single phase motor it is to be sized at 250% of the FLA as stated in Table 430.248.
28A x 250% = 70A
Based on the above calculation a 70A ITCB is correct.
If the motor fails to start and run then 430.52(C) (1) Exception No. 2(c) can be applied which states;
(c) The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.
28A x 400% = 112A
Since we can’t exceed 400% of the motor FLA we round down to a 110A ITCB on a piece of #8 AWG conductor.
There is also an online calculator that will do all the work for you. Please follow the link.