Motor Calculator

Method

Sizes the branch circuit for one motor of 1000 V and less by the 2023 NEC: the short-circuit and ground-fault protective device, conductors, equipment grounding conductor and raceway, with the overload protection, controller and disconnecting means, and the maximum circuit length for voltage drop. Each result in the calculator opens to show its steps and the section that requires it.

Motor

Full-load current

Table 430.248 (single-phase) or Table 430.250 (three-phase) at the rated motor voltage for the system: 115 V on 120 V, 200 or 208 V on 208 V, 230 V on 240 V, 460 V on 480 V, 575 V on 600 V. The table value, not the nameplate, sizes the conductors, the short-circuit and ground-fault protection, the controller and the disconnect (430.6(A)(1)). Synchronous motors use the synchronous columns of Table 430.250 (25 to 200 hp), times 1.1 or 1.25 for 0.9 or 0.8 power factor.

Kilowatts or amperes

A kilowatt rating is converted at 0.746 kW per hp. A motor marked in amperes but not horsepower takes the horsepower of that current in the table (430.6(A)(1)). Between table rows the value is interpolated linearly.

Nameplate current

Sets the overload protection (430.6(A)(2)) and the duty-cycle conductor size (Table 430.22(E)). It replaces the table value for sizing where the 430.6(A)(1) exceptions apply: shaded-pole or permanent-split capacitor fan and blower motors, listed motor-operated appliances, and low-speed or high-torque motors (table notes). 277 V single-phase motors have no table value and always use the nameplate.

Locked-rotor current

Table 430.251(B) (Design B, C and D maximum) or Table 430.251(A) (single-phase), or from the code letter at the top of its Table 430.7(B) range: I = kVA/hp × hp × 1000 ÷ (√3 V). An entered value is used first. Wound-rotor motors are estimated at 2 × full-load current, others without a table value at 6 ×.

Short-circuit and ground-fault protection

Maximum rating

Table 430.52(C)(1) percentage of the full-load current: inverse time breaker 250 %, dual-element fuse 175 %, nontime-delay fuse 300 % (also time-delay Class CC, Note 1), instantaneous setting 800 % (1100 % for Design B energy-efficient and premium efficiency); wound-rotor motors 150 %. Where that is not a standard rating, the next higher standard rating of 240.6(A) is permitted (430.52(C)(1)(a)).

Starting current

Where the motor will not start: up to 400 % for nontime-delay fuses of 600 A or less and time-delay Class CC, 225 % for dual-element fuses, 400 % for inverse time breakers (300 % above 100 A full-load current) and 300 % for fuses of the 601–6000 A class, without the next size up (430.52(C)(1)(b)). Instantaneous settings up to 1300 % (1700 % for Design B energy-efficient and premium efficiency), above 800 % or 1100 % only where an engineering evaluation shows the need (430.52(C)(3)).

Conductor protection

The overload device protects the conductors from overload, so the short-circuit device may exceed the conductor ampacity and 240.4(D) does not apply (240.4(G)).

Conductors and grounding

Ampacity

125 % of the full-load current for continuous duty (430.22); for short-time, intermittent, periodic or varying duty, the Table 430.22(E) percentage of the nameplate current. Table 310.16 at the insulation rating, times the ambient correction of Table 310.15(B)(1)(1) (33 °C added on rooftops, except for XHHW-2) and the adjustment of Table 310.15(C)(1), and not more than the terminal-temperature column. By default 75 °C for motors marked Design B, C or D (110.14(C)(1)(a)(4)); otherwise 60 °C for circuits of 100 A or less and 75 °C above. Not smaller than 14 AWG (430.22(G)). Sets of 1/0 AWG and larger in parallel where one conductor up to the chosen size is not enough (310.10(G)).

Equipment grounding conductor

Table 250.122 with the rating of the short-circuit and ground-fault device (250.122(D)(1)); for an instantaneous-trip breaker, with the largest dual-element fuse permitted by 430.52(C)(1)(a) (250.122(D)(2)). Increased by the ratio of circular mils where the circuit conductors are larger than the minimum (250.122(B)); not larger than the circuit conductors (250.122(A)); one in each parallel raceway (250.122(F)(1)).

Raceway

Conductor areas from Chapter 9 Table 5 (Table 8 for a bare EGC), the EGC included (Note 3); 53, 31 or 40 % of the Table 4 area for one, two or more conductors (Table 1). A jam ratio of 2.8 to 3.2 is flagged.

Voltage drop and maximum length

Running

Ze = R cos θ + X sin θ from Chapter 9 Table 9 at 75 °C (PVC, aluminum or steel conduit); VD = √3 I Ze L / 1000 three-phase, 2 I Ze L / 1000 single-phase, at the full-load current and the running power factor, as a percentage of the system voltage. The informational note suggests 3 % for a branch circuit and 5 % with the feeder (210.19(A) Informational Note No. 3). Sizes can be increased to meet the limit.

Starting

The same formula at the locked-rotor current and the starting power factor. Typical starting power factors are used unless entered: 0.50 to 5 hp, 0.40 to 25 hp, 0.30 to 100 hp and 0.25 above. This is the branch circuit only; the feeder, transformer and source add to the dip.

Maximum length

L = VD% × V × 1000 ÷ (k × I × Ze) for the running and the starting limit; the shorter governs. The table gives the lengths for the next larger sizes, with the grounding conductor and raceway each needs.

Motor control

Overload protection

Separate device responsive to motor current, not more than 125 % of the nameplate current for a service factor of 1.15 or more or a marked temperature rise of 40 °C or less, 115 % otherwise (430.32(A)(1), (B)(1)); up to 140 % or 130 % where needed to start or carry the load (430.32(C)). Three units for three-phase motors (Table 430.37). A thermal protector in the motor trips at not more than 170 %, 156 % or 140 % of the table current (430.32(A)(2)).

Controller

Horsepower rating at the application voltage not less than the motor (430.83(A)(1)). The NEMA ICS 2 full-voltage starter size is shown for reference.

Disconnecting means

Ampere rating at least 115 % of the full-load current (430.110(A)), horsepower rated (430.109(A)); a fused switch also holds the fuses. Standard switch ratings 30, 60, 100, 200, 400, 600, 800 and 1200 A. Location within sight of the controller and the motor (430.102).

Checks

Each case is solved live by this calculator and compared with a published worked example.

Limitations

• One motor on one branch circuit. Several motors on a branch circuit or feeder (430.24, 430.53, 430.62) are not covered.
• Full-voltage starting. Wye-delta and part-winding motors (430.22(C), (D), 430.4), multispeed motors and adjustable-speed drives (Part X) are not covered; the wound-rotor secondary circuit (430.23) is not sized.
• Hermetic refrigerant motor-compressors (Article 440), fire pumps (Article 695), torque motors (430.6(B)), DC motors and motors over 1000 V are outside this calculator.
• Voltage drop covers the branch circuit only. The source, transformer and feeder, the available fault current and the controller short-circuit current rating should be checked for the actual equipment.
• GFCI and other occupancy rules (210.8, Article 680) are not evaluated.