Every motor circuit calculation starts with one number: the Full Load Current (FLC) from the NEC tables. Get that right and the rest of the calculation flows in order — wire size, OCPD, then the equipment grounding conductor.
This article covers standard single-motor branch-circuit calculations using copper conductors, 75°C-rated terminations, and no ambient-temperature correction or conductor-adjustment factors. Always verify the motor nameplate, equipment listings, local amendments, and AHJ requirements.
FLA vs FLC — Know the Difference
Before anything else, this is one of the most common mistakes on motor circuits.
FLA (Full Load Amps) is the nameplate value stamped on the motor.
FLC (Full Load Current) is the value from the NEC tables:
Table 430.248 for single-phase motors Table 430.250 for three-phase motors
For branch-circuit conductor sizing and branch-circuit short-circuit and ground-fault protection sizing, the NEC requires the table value — FLC — to be used under 430.6(A).
The motor nameplate FLA is used when sizing overload protection under 430.32.
Step 1 — Branch-Circuit Conductor Size [NEC 430.22]
Motor branch-circuit conductors must have an ampacity of at least 125% of the motor FLC.
[ Minimum conductor ampacity = FLC × 1.25 ]
Then select a conductor from Table 310.16 that meets or exceeds that required ampacity.
Single-Phase Example — 5 HP, 230 V Motor Table 430.248 FLC = 28 A 28 A × 125% = 35 A minimum conductor ampacity Table 310.16, 75°C column = #10 AWG copper rated 35 A
Result: #10 AWG copper branch-circuit conductors
This section covers a single Design B, E, or C motor operating in continuous duty. Under NEC 430.22(A), branch-circuit conductors must have an ampacity of at least 125% of the motor full-load current determined under 430.6(A)(1). Motors used in other-than-continuous-duty applications are sized under 430.22(E).
Why #10 Copper Is Sufficient
The conductor only needs to meet 125% of motor FLC under 430.22. It does not need to match the OCPD rating.
That is one of the special rules for motor circuits. The branch-circuit short-circuit and ground-fault protective device can be larger than the conductor ampacity because it must allow the motor to start without nuisance opening.
This example assumes the equipment terminations are rated 75°C under 110.14(C). If the circuit is limited to the 60°C column, #10 copper is only rated 30 A and would not meet the required 35 A ampacity. In that case, you would step up to #8 copper.
Three-Phase Example — 10 HP, 460 V Motor
Table 430.250 FLC = 14 A 14 A × 125% = 17.5 A minimum conductor ampacity Table 310.16, 75°C column = #14 AWG copper rated 20 A
Result: #14 AWG copper branch-circuit conductors
Step 2 — OCPD Size [NEC 430.52 and Table 430.52]
Motor branch-circuit OCPDs are sized differently than standard branch circuits.
Motors draw high inrush current when starting. Because of that, the NEC permits a branch-circuit short-circuit and ground-fault protective device that is larger than the conductor ampacity.
This OCPD is not the motor overload protection. The OCPD protects against short circuits and ground faults. The overload relay protects the motor against sustained overload.
| OCPD Type | Maximum Percentage of FLC |
|---|---|
| Inverse-time circuit breaker | 250% |
| Dual-element time-delay fuse | 175% |
| Non-time-delay fuse | 300% |
| Instantaneous-trip circuit breaker / MCP | 800%* |
Instantaneous-trip breakers and motor circuit protectors have additional listing requirements and applicable exceptions. They are not a simple general-purpose calculation for every motor circuit.
Where the calculated maximum OCPD does not match a standard ampere rating, 430.52(C)(1) permits the next higher standard rating listed in 240.6(A).
Single-Phase Example — 5 HP, 230 V Motor With Dual-Element Time-Delay Fuses
FLC = 28 A 28 A × 175% = 49 A 49 A is not a standard fuse size Next standard size = 50 A
Result: 50 A dual-element time-delay fuse
Three-Phase Example — 10 HP, 460 V Motor With an Inverse-Time Breaker
FLC = 14 A 14 A × 250% = 35 A 35 A is a standard OCPD rating
Result: 35 A inverse-time circuit breaker
Step 3 — Equipment Grounding Conductor Size [NEC 250.122(D)(1), 250.122(A), and Table 250.122(A)]
For motor circuits, start by looking up the equipment grounding conductor from the motor branch-circuit short-circuit and ground-fault protective-device rating or setting permitted by 430.52.
Use that value in Table 250.122(A) to find the starting EGC size.
Then apply 250.122(A): a wire-type equipment grounding conductor is not required to be larger than the circuit conductors supplying the equipment.
Single-Phase Example — 50 A OCPD With #10 Copper Phase Conductors
Table 250.122(A), 50 A OCPD = #10 AWG copper EGC Phase conductors are #10 copper EGC is not required to be larger than the phase conductors
Result: #10 AWG copper EGC
Three-Phase Example — 35 A OCPD With #14 Copper Phase Conductors
Table 250.122(A), 35 A OCPD = #10 AWG copper EGC However, 250.122(A) does not require a wire-type EGC to be larger than the circuit conductors Phase conductors are #14 copper
Result: #14 AWG copper EGC
Worked Examples — Full Summary
| Item | Calculation | Result |
|---|---|---|
| FLC | Table 430.248 | 28 A |
| Conductor size | 28 A × 125% = 35 A minimum | #10 AWG copper |
| OCPD | 28 A × 175% = 49 A, then next standard size | 50 A time-delay fuse |
| EGC | Table 250.122(A), based on 50 A OCPD | #10 AWG copper |
| Item | Calculation | Result |
|---|---|---|
| FLC | Table 430.250 | 14 A |
| Conductor size | 14 A × 125% = 17.5 A minimum | #14 AWG copper |
| OCPD | 14 A × 250% = 35 A | 35 A breaker |
| EGC | Table 250.122(A) gives #10, then apply 250.122(A) | #14 AWG copper |
Common Mistakes
Using Nameplate FLA Instead of Table FLC for Conductor and OCPD Sizing
For the branch-circuit conductor and branch-circuit short-circuit and ground-fault protection calculations shown above, use NEC table FLC.
Use the motor nameplate FLA when sizing overload protection under 430.32.
Confusing Overload Protection With Short-Circuit and Ground-Fault Protection
The OCPD sized under 430.52 does not protect the motor from sustained overload.
That job belongs to the overload relay or other motor overload protective device. Overloads are generally sized from the motor nameplate FLA under 430.32, commonly at 125% or 115% depending on the motor service factor and temperature-rise information.
Both types of protection are required.
Starting the EGC Calculation With the Phase-Conductor Size
Do not start by choosing an EGC solely from the phase-conductor size.
First, use the motor branch-circuit short-circuit and ground-fault protective-device rating or setting permitted by 430.52 to find the starting EGC size in Table 250.122(A).
Then apply 250.122(A): a wire-type EGC is not required to be larger than the circuit conductors supplying the equipment.
Forgetting About 75°C vs. 60°C Terminations
A conductor may appear large enough in the 75°C column but not in the 60°C column.
For example, #10 copper is rated 35 A in the 75°C column but only 30 A in the 60°C column. Always verify the temperature rating of the equipment terminations under 110.14(C).
Final Takeaway
For a standard motor branch circuit, the calculation order is straightforward:
Use NEC table FLC. Size branch-circuit conductors at 125% of FLC. Size the branch-circuit short-circuit and ground-fault protective device under Table 430.52. Use the OCPD rating or permitted setting to begin sizing the EGC under Table 250.122(A). Apply 250.122(A) so the wire-type EGC is not required to be larger than the circuit conductors.
Keep FLC, nameplate FLA, overload protection, branch-circuit OCPD sizing, and EGC sizing separate. That is what makes motor calculations predictable and code-compliant.