Motor branch circuit (Article 430)

One motor — choose the supply (three-phase, single-phase or DC), the motor’s voltage (the tables use 115, 230, 460 and 575 V for 120, 240, 480 and 600 V systems, with their own 200 and 208 V columns) and horsepower, the motor type, the short-circuit device (inverse-time breaker, dual-element fuse, non-time-delay fuse or instantaneous-trip breaker), the nameplate current and service factor, copper or aluminium, the terminal temperature rating and the code letter.

The cards give the full-load current, the conductor, the breaker or fuse, the overload setting and the disconnect. The summary card below is laid out like the motor block on a one-line drawing — disconnect, short-circuit device, controller and overload, conductors with the grounding conductor, locked-rotor kVA — with the steps and sections underneath. Print prints the card alone.

Starting — the locked-rotor kVA range for the code letter (Table 430.7(B)), the starting draw for across-the-line, wye-delta, autotransformer, soft-starter or VFD starting, and a rough voltage dip on a generator (its transient reactance, typically 0.2–0.3 per unit) or a transformer (its impedance), plus the generator size for a dip you choose. Generator makers size for motor starting with their own data; treat this as a first look.

Several motors — motors on one feeder at one voltage, plus other loads: conductors at 125 % of the largest motor plus the rest (430.24), and the largest feeder breaker allowed (430.62(A)), rounded down.

Wire, fuse or breaker and disconnect start from the table full-load current, not the nameplate (430.6(A)(1)); overloads use the nameplate. Conductors carry 125 % (430.22). The short-circuit and ground-fault device is sized for starting, not for the wire — inverse-time breaker 250 %, dual-element fuse 175 %, non-time-delay fuse 300 %, instantaneous trip 800 % (1100 % Design B premium efficiency), less for wound-rotor and DC motors; next size up if it isn’t a standard size, more under Exception No. 2 if the motor won’t start. Overloads at 125 % of the nameplate (115 % below a 1.15 service factor). Disconnect horsepower-rated and at least 115 % of the full-load current.

Related: shop power planner (workshop machines on their circuits and whether a start trips the breaker), panel schedule, dwelling load, wire size and voltage drop, conduit fill, grounding and bonding, transformers, fault current, energy usage.

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