Motor Branch Circuit Design

Motor branch circuit design includes sizing of motor-circuit feeders and their over current protection. Most electrical equipment is rated in volt-amperes (VA) or watt input. Basic to this understanding is the fact that motors traditionally have been rated in horsepower output. Circuits supplying motors are sized according to the input to the motor (input equals output plus losses of the motor). The losses are not the type of information found on the nameplate of a motor. Tables NEC-430.249 and 430.250 contain accurate industry-wide input ampere ratings for motors.

However, some motors are available with their output ratings expressed in watts and kilowatts. (One horsepower equals approximately 746 watts.) It is important to understand that circuits that supply motors not rated in horsepower still must be sized according to the input of the motor, rated in amperes. Sizing circuits based solely on kilowatt output results in seriously undersized conductors and the improper application of overcurrent devices. 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. This describes the branch-circuit requirements for single motor installations. Generally, the branch circuit that serves a continuous-duty motor must be sized at 125 percent of the motor full-load current or greater. The provision for a conductor with an ampacity of at least 125 percent of the motor full-load current rating does not constitute a conductor derating; rather, it is based on the need to provide for a sustained running current that is greater than the rated full-load current and for protection of the conductors by the motor overload protective device set above the motor full-load current rating. Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group plus the ampacity required for the other loads.

Motor Overload protection

Each motor used in a continuous duty application and rated more than 1 hp shall be protected against overload by one of the following:

(1) Separate Overload Device: A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:

Table: Motor overload protection

Motors with a marked service factor 1.15 or greater	125%
Motors with a marked temperature rise 40°C or less	125%
All other motors	115%

Where the sensing element or setting of the overload relay selected is not sufficient to start the motor or to carry the load, higher size sensing elements or incremental settings shall be permitted to be used, provided the trip current of the overload relay does not exceed the following percentage of motor nameplate full-load current rating:

Table: Motor overload relay protection

Motors with a marked service factor 1.15 or greater	140%
Motors with a marked temperature rise 40°C or less	140%
All other motors	130%

(2) Thermal Protector: A thermal protector integral with the motor, approved for use with the motor it protects on the basis that it will prevent dangerous overheating of the motor due to overload and failure to start. The ultimate trip current of a thermally protected motor shall not exceed the following percentage of motor tabulated NEC full-load current:

Table: Motor thermal protector

Motor full-load current 9 amperes or less	170%
Motor full-load current from 9.1 to, and including, 20 amperes	156%
Motor full-load current greater than 20 amperes	140%

(3) Integral with Motor: A protective device integral with a motor that will protect the motor against damage due to failure to start shall be permitted if the motor is part of an approved assembly that does not normally subject the motor to overloads.

(4) Larger Than 1500 Horsepower: For motors larger than 1500 hp, a protective device having embedded temperature detectors that cause current to the motor to be interrupted when the motor attains a temperature rise greater than marked on the nameplate in an ambient temperature of 40°C.

Motor Branch-Circuit Short-Circuit and Ground-Fault Protection

The NEC requires that the branch circuit protection for the motor circuits must protect the circuit conductors, the control apparatus, and the motor itself against overcurrent due to short circuits or grounds.

The first, and clearly necessary, rule is that the branch circuit protective device for an individual circuit to a motor must be capable of carrying the motor starting current without opening the circuit. Given this condition, the NEC places maximum values on the ratting or setting of protective devices. This setting depends on the type of motor used and the type of its short circuit protection device as shown in . If such values do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices,or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted. Where these values do not permit the motor to start, the device may be rated as follows:

(1) The rating of a non-time-delay fuse not exceeding 600 amperes or a time- delay fuse shall be permitted to be increased but shall in no case exceed 400 percent of the full-load current.

(2) The rating of a time-delay (dual-element) fuse shall be permitted to be increased but shall in no case exceed 225 percent of the full-load current.

(3) 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.

(4) The rating of a fuse of 601–6000 ampere classification shall be permitted to be increased but shall in no case exceed 300 percent of the full-load current. Maximum percentage Rating or Setting of Motor Branch-Circuit Short Circuit and Ground-Fault Protective- Devices (NEC table 430-52) is as shown in the following table:

Table: motor branch circuit short-circuit and ground-fault protection

Type of Motor	Non-time delay fuse	Dual Element (Time-delay)	Instantaneous trip breaker	Inverse time breaker
Single phase all types- no code letter	300	175	800	250
Squirrel cage - other than Design B energy- efficient	300	175	800	250
Design B energy- efficient	300	175	1100	250
Synchronous	300	175	800	250
Wound rotor	150	150	800	150
Direct current (constant voltage)	150	150	250	150