Motor cable and Overload Sizing
Motor Sizing Reference & Calculator
Full load current (FLC) lookup, wire gauge, contactor sizing, overload relay, fuse and breaker selection for single-phase and three-phase AC motors. Based on NEC Article 430 and CEC Section 28. Use as a starting point — always confirm against the motor nameplate and applicable code.
FLC vs FLA
Use NEC table FLC for conductor and breaker sizing. Use nameplate FLA for overload relay setting. They are different — mixing them up causes improper protection.
125% conductor rule
Branch circuit conductors must be sized at ?125% of table FLC per NEC 430.22. This accounts for the heat generated by continuous motor operation.
Contactor sizing
NEMA contactors are sized by HP and voltage. The contactor handles inrush; the overload relay protects the motor. Both must be correctly selected.
Overload relay
Set at 115–125% of nameplate FLA (NEC 430.32). A thermal overload set too high is useless — it will let the motor burn before tripping.
Inrush / starting
Motors draw 6–8× FLC on start. Branch circuit breakers and fuses must handle this. Dual-element fuses ?175% FLC; inverse-time breakers ?250% FLC.
Critical: FLC (Table) vs FLA (Nameplate) — Know the Difference
Table FLC (NEC 430.248 / 430.250): Used for sizing branch circuit conductors (NEC 430.22), branch circuit protection devices (NEC 430.52), and disconnect switches (NEC 430.110). This calculator uses table FLC.
Nameplate FLA: Used for sizing overload relay protection only (NEC 430.32). The overload relay setting must be based on the actual motor nameplate current, NOT the table value. Enter your nameplate FLA in the calculator below to get the correct overload relay range.
All FLC values, wire gauge, contactor, overload relay, fuse, and breaker suggestions on this page are typical calculated figures for general planning purposes only. FLC values are from NEC Tables 430.248 and 430.250 for standard squirrel-cage induction motors at normal speeds and torques. These values do not apply to low-speed motors (<1200 RPM), high-torque motors, multispeed motors, wound-rotor motors, synchronous motors, or motors operated with VFDs — which require nameplate-based sizing. Wire gauge is based on NEC Table 310.16 (THWN-2, 75°C, single conductor in conduit at 30°C ambient) without conduit fill derating. Contactor and overload sizing is based on NEMA ICS 2 standard full-voltage-start, AC-3 duty ratings and may differ for jogging, plugging, reversing, or high-cycle applications. Always use the motor nameplate FLA for overload relay settings per NEC 430.32.
All electrical installations must comply with the applicable NEC, Canadian Electrical Code (CEC Section 28), and standards enforced by the Authority Having Jurisdiction (AHJ). This reference does not constitute engineering advice. Always engage a licensed professional for final design, sizing, and installation decisions. The publisher assumes no liability for errors, omissions, or outcomes resulting from use of this information.
Enter motor parameters
Select HP, voltage, and phase to calculate — enter nameplate FLA for overload relay sizing.
| Item | Selection / Size | Basis | Code reference |
|---|
Wire: NEC Table 310.16, THWN-2 75°C, single conductor in conduit, 30°C ambient. Contactor: NEMA ICS 2, full-voltage start, AC-3 duty. Fuse: dual-element time-delay. Breaker: inverse-time. Confirm all selections with nameplate, manufacturer data, and AHJ.
Why use table FLC, not nameplate FLA?
NEC 430.6(A)(1) requires table values for conductor and protection sizing — not nameplate data. This standardises sizing across all motors of the same HP/voltage regardless of efficiency differences between manufacturers.
Overload relay — service factor
SF ? 1.15: set overload at ?115% nameplate FLA. SF < 1.15 or 40°C rise: set at ?125%. Trip class 10 is standard; use class 20 or 30 for high-inertia loads like fans and compressors.
NEMA contactor sizing
Select by HP and voltage from NEMA ICS 2 tables. Contactor handles inrush and breaking duty. For jogging, plugging, or reversing, derate to next size up — these duties are much harder on contacts.
VFD applications
Variable frequency drives change motor current profiles significantly. Conductor sizing and overload protection with VFDs follow NEC 430.122 — contact the VFD manufacturer for specific sizing guidance.
Branch circuit protection
Dual-element (time-delay) fuses: max 175% FLC. Inverse-time breakers: max 250% FLC. If the motor won't start, these limits may be increased up to 225% (fuse) or 400% (breaker) per NEC 430.52 Exception.
Disconnect sizing
Motor disconnect must be rated ?115% of motor FLC (NEC 430.110). For combination starters, the disconnect is built in. Standalone disconnects are typically sized 1–2 current ratings above the motor FLC.
Source: NEC Table 430.250. Values for squirrel-cage induction motors at normal speeds and torques. Use table FLC (not nameplate FLA) for conductor sizing per NEC 430.22 and branch circuit protection per NEC 430.52.
Source: NEC Table 430.248. Values for single-phase motors at normal speeds and torques. 115V and 230V are the most common single-phase motor voltages in North America.
NEMA starter sizes are selected by motor HP and voltage for full-voltage (across-the-line) starting. Confirm overload relay is set to 115–125% of nameplate FLA per NEC 430.32, regardless of starter size chosen.
| NEMA Size | Max continuous A | Max HP @ 200V | Max HP @ 230V | Max HP @ 460/480V | Max HP @ 575/600V | Typical application |
|---|---|---|---|---|---|---|
| 00 | 9 A | 1.5 HP | 1.5 HP | 2 HP | 2 HP | Small fans, fractional HP, light conveyor |
| 0 | 18 A | 3 HP | 3 HP | 5 HP | 5 HP | Small pumps, HVAC, general purpose |
| 1 | 27 A | 7.5 HP | 7.5 HP | 10 HP | 10 HP | Small industrial motors, conveyors, compressors |
| 2 | 45 A | 10 HP | 15 HP | 25 HP | 25 HP | Medium pumps/fans, material handling |
| 3 | 90 A | 25 HP | 35 HP | 50 HP | 50 HP | Heavier conveyors, process pumps, compressors |
| 4 | 135 A | 40 HP | 50 HP | 100 HP | 100 HP | Large process loads, high-inertia equipment |
| 5 | 270 A | 75 HP | 100 HP | 200 HP | 200 HP | Large industrial motors, MCCs |
| 6 | 540 A | 150 HP | 200 HP | 400 HP | 400 HP | Heavy industrial, large process equipment |
| 7 | 810 A | — | 300 HP | 600 HP | 600 HP | Very large industrial applications |
| 8 | 1215 A | — | 450 HP | 900 HP | 900 HP | Large MCC / power distribution |
| 9 | 2250 A | — | 800 HP | 1600 HP | 1600 HP | Utility-scale industrial motors |
Source: NEMA ICS 2, full-voltage start, AC-3 duty. For jogging, plugging, reversing, or high-cycle duty, select the next size up. IEC contactors are sized differently using AC-3/AC-4 utilization categories — consult manufacturer selection guides.
FLC vs FLA — the critical distinction
FLC from NEC tables is used for conductor and breaker sizing (430.22, 430.52). Nameplate FLA is used for overload relay setting (430.32). Using the wrong value for each leads to either undersized protection or nuisance tripping.
Branch circuit conductor (NEC 430.22)
Single motor: conductors must have ampacity ? 125% of table FLC. For multiple motors on one feeder, see NEC 430.24 — largest motor at 125%, all others at 100%.
Branch circuit protection (NEC 430.52)
Dual-element time-delay fuse: max 175% FLC. Inverse-time breaker: max 250% FLC. If motor won't start, can increase to 225% (fuse) or 400% (breaker) per NEC 430.52 Exception No. 1.
Overload relay — NEC 430.32
Set at ?115% nameplate FLA for motors with SF ? 1.15 or 40°C temperature rise marked. Set at ?125% for all other motors. Trip Class 10 is standard; Class 20/30 for high-inertia loads.
Contactor selection — NEMA
Select NEMA size from HP and voltage table. For jogging, plugging, or reversing duty, derate one size up. Verify SCCR (short circuit current rating) matches available fault current at the installation point.
IEC vs NEMA contactors
NEMA: rugged, oversized for the duty, easy selection by HP. IEC: compact, precisely sized by AC-3/AC-4 utilization category. IEC requires more careful duty-cycle matching but is smaller and lower cost.
Disconnect sizing (NEC 430.110)
Motor disconnect must be rated ?115% of motor FLC. Horsepower-rated disconnects must be rated for the motor HP and voltage. Fusible disconnects provide combined disconnect and short-circuit protection.
Conduit and conduit fill
More than 3 current-carrying conductors requires ampacity derating per NEC 310.15(C). For motor circuits with ground wire, ensure conduit is sized for all conductors including the EGC per NEC 430.23.
VFD motor circuits (NEC 430.122)
VFD branch circuit conductors must be sized at ?125% of VFD input FLC rating. Motor lead wire from VFD to motor should be VFD-rated cable. Overload protection is typically built into the VFD.
Motor starting methods
Across-the-line: 6–8× FLC inrush. Star-delta: reduces inrush to ~33%. Soft starter: adjustable ramp, reduces inrush to ~2–4×. VFD: lowest inrush, best control. Starting method affects contactor and cable sizing.
CEC Section 28 (Canada)
Canadian motor circuit rules are in CEC Section 28. Branch circuit protection rules differ slightly from NEC 430.52. Confirm with the CEC edition adopted by your province and the local AHJ.
Motor circuit application guide
Pump/fan loads: standard duty, Class 10 overload. Compressors: high inertia, Class 20. Conveyor/crusher: high torque, may need Class 30. Jogging/reversing: derate contactor and use appropriate relay.
Low-speed & special motors
Motors <1200 RPM, high-torque motors, wound-rotor, synchronous, and multispeed motors: NEC 430.6(A)(2) requires using nameplate FLA (not table FLC) for conductor and protection sizing.
Always consult the AHJ
Local amendments to NEC or CEC may impose additional requirements. The Authority Having Jurisdiction has final say on code interpretation. Consult a licensed engineer or inspector when in doubt.
