4.5 Motor Circuits: FLA, PF Correction & Current Imbalance

Motor Full-Load Amp (FLA) Tables (NEC 430)

Quick Answer: NEC Table 430.250 lists full-load current (FLC) for three-phase induction motors. At 460 V, the values are 14 A for 10 hp, 21 A for 15 hp, 34 A for 25 hp, 40 A for 30 hp, 52 A for 40 hp, 65 A for 50 hp, and 124 A for 100 hp.

These FLC values are for induction-type squirrel-cage and wound-rotor motors running at usual speeds with normal torque. They are permitted for system voltage ranges of 440 to 480 V (the 460 V column) and 550 to 600 V (the 575 V column). Synchronous motor FLC is in separate columns; for 0.9 PF and 0.8 PF synchronous motors, multiply the unity-PF value by 1.1 and 1.25 respectively.

FLC Versus Nameplate FLA — Which One to Use

NEC 430.6(A)(1) tells you which number to use for which calculation:

• Use the Table 430.250 FLC for branch-circuit conductor sizing (430.22, FLC x 1.25 for continuous-load motors) and for short-circuit/ground-fault protective device sizing (430.52).
• Use the motor nameplate FLA for overload (thermal) protection (430.32).

The reasoning: the table FLC is conservative across motor manufacturers and ensures the conductor and short-circuit device can handle any motor of that horsepower and voltage, while the overload must match the specific motor so it trips before the actual motor insulation is damaged.

Worked FLA Example (10 hp, 460 V)

• Table FLC = 14 A
• Branch-circuit conductor ampacity (430.22) = 14 x 1.25 = 17.5 A minimum -> #12 AWG copper or larger (subject to 430.22 and 240.4(D) small-conductor rules)
• Typical inverse-time breaker max (430.52, 250% FLC) = 14 x 2.5 = 35 A -> next standard size 35 A
• Overload (430.32) set from the motor nameplate FLA, typically 115% to 125% of nameplate

Power Factor Correction for Motors

A motor draws magnetizing (lagging) reactive current that does no real work but loads the source and the line. Power factor correction capacitors supply that reactive power locally so the source sees a higher power factor and lower line current. The benefits are reduced line losses, lower voltage drop, and avoiding utility PF penalties.

The capacitor size to raise PF from PF1 to PF2 for a motor drawing real power P is:

Qc = P x (tan(acos PF1) - tan(acos PF2))

Worked PF Correction Example (100 hp, 460 V motor)

• FLC (Table 430.250) = 124 A; assume measured PF = 0.85
• Real power P = sqrt(3) x 460 x 124 x 0.85 = 83.8 kW
• acos(0.85) = 31.8 deg, tan = 0.620
• Target PF2 = 0.95 -> acos(0.95) = 18.2 deg, tan = 0.329
• Qc = 83.8 x (0.620 - 0.329) = 83.8 x 0.291 = 24.4 kVAR -> round to a 25 kVAR unit

After correction the source sees about 0.95 PF and the line current drops from 124 A to roughly 111 A.

The Leading-PF / Self-Excitation Trap

When the motor is disconnected from the line, the capacitor can keep the motor magnetized and the motor may generate a voltage higher than rated ("self-excitation"). That overvoltage can damage the motor and any connected loads, and the leading power factor is undesirable. The rule of thumb is to keep capacitor kVAR under about 30% of the motor kW rating. Here 25 kVAR / 83.8 kW = 0.30 — right at the limit. For unload problems, reduce the capacitor size or switch the capacitor off with the motor contactor (a common and safe arrangement).

Motor Current Imbalance

A three-phase induction motor should draw roughly equal phase currents. Imbalance is calculated as:

Imbalance % = (max deviation from average current) / average x 100

A key rule: a small voltage imbalance produces a much larger current imbalance — a 1% voltage imbalance can produce 6 to 10% current imbalance because the negative-sequence impedance of an induction motor is low. NEMA MG-1 recommends derating a motor when voltage imbalance exceeds 1%.

Worked Imbalance Example

A 480 V motor draws 60 A, 60 A, 50 A on phases A, B, C:

• Average = (60 + 60 + 50) / 3 = 56.7 A
• Max deviation = 60 - 56.7 = 6.7 A
• Imbalance % = 6.7 / 56.7 x 100 = 11.8%

Anything above about 10% is a red flag. Causes include supply voltage imbalance, a high-resistance connection or failing contactor pole on one phase, winding faults, or an unbalanced single-phase load on the same transformer. Field practice is to measure phase voltages, scan the connection points with a thermal imager, and verify contactor pole resistance with a Ductor before condemning the motor.

Parallel Resistance and Load Sharing

When two resistors (or two parallel load paths) are in parallel, the equivalent resistance is:

1/Rt = 1/R1 + 1/R2 -> Rt = (R1 x R2) / (R1 + R2)

Worked Parallel Example

Two 100 ohm resistors in parallel: 1/Rt = 1/100 + 1/100 = 2/100, so Rt = 50 ohm. Three 100 ohm resistors in parallel give 33.3 ohm. The current divides inversely to the resistance — the lower branch carries more current. In motor circuits this principle governs parallel cable load sharing: if the two parallel cables have different resistances (different lengths, different terminations), the lower-resistance cable carries more than its share and may overheat.

Ohm's Law and Contact Voltage Drop

A Ductor (micro-ohmmeter) measures the resistance of a bolted joint, breaker contact, or bus connection in micro-ohms or milliohms. The test passes a high DC current (commonly 100 A) through the joint and reads the voltage drop, then calculates R = V / I. The same Ohm's law lets you convert a measured resistance to a voltage drop at any operating current:

V = I x R

Worked Contact-Drop Example

A bolted connection measures 0.5 ohm (an unrealistically high value used to illustrate the math — real Ductor readings on a sound joint are tens to hundreds of micro-ohms). At 100 A test current, the voltage drop is:

V = 100 A x 0.5 ohm = 50 V

That 50 V drop at full load would be unacceptable and indicates a bad joint (loose bolt, oxidized surface, wrong washer stack). A more realistic acceptance reading for a 1000 A breaker main contact might be 25 micro-ohm; at 1000 A that is V = 1000 x 25 x 10^-6 = 0.025 V, with the heat dissipated at the contact P = I^2 x R = 1000^2 x 25 x 10^-6 = 25 W. ANSI/NETA comparison values are typically supplied by the manufacturer or by the ATS table; a reading more than about 50% above the comparison value, or more than 20% pole-to-pole variation on the same breaker, warrants investigation.