Quick answer: Motor circuits are sized using table values, not nameplate current (NEC 430.6). You look up the motor’s full-load current (FLC) in NEC Table 430.250 (3-phase) or 430.248 (single-phase), then size the branch-circuit conductors at 125% of FLC (NEC 430.22) and the overload protection from the nameplate FLA at 115–125%. The short-circuit/ground-fault breaker is sized much larger — up to 250% of FLC for an inverse-time breaker (NEC 430.52) — to let the motor start. Size it all with the motor FLA calculator.
Why motor circuits follow special rules
Motors are unlike any other load, so NEC Article 430 gives them their own sizing method. A motor draws a large inrush current at startup — often 6 to 8 times its running current for a few seconds — then settles to its full-load current. If you sized the protection tight to the running current, it would trip every time the motor started. So the code splits motor protection into three separate parts, each sized differently:
- Conductors — sized for continuous running (125% of FLC).
- Overload protection — protects the motor from running too hot (115–125% of nameplate).
- Short-circuit/ground-fault protection — the breaker or fuse, sized large (up to 250%+) to ride through inrush and only trip on a real fault.
Getting these three right — and knowing which uses table FLC and which uses nameplate FLA — is the heart of motor circuit design.
The key distinction: table FLC vs. nameplate FLA
This is the rule that catches people: NEC 430.6(A) requires you to size conductors and short-circuit protection from the FULL-LOAD CURRENT in Tables 430.248–430.250 — not from the motor nameplate. The table values are standardized, slightly conservative currents for a given horsepower and voltage. The nameplate FLA is used only for overload (running) protection (NEC 430.32).
| What you’re sizing | Current to use | NEC |
|---|---|---|
| Branch-circuit conductors | Table FLC × 125% | 430.22 |
| Short-circuit / ground-fault (breaker/fuse) | Table FLC × up to 250% | 430.52 |
| Overload protection | Nameplate FLA × 115–125% | 430.32 |
Step 1 — find the full-load current
For a standard motor you can estimate FLC from horsepower, or read it from NEC Table 430.250. The three-phase estimate:
FLC ≈ (HP × 746) ÷ (√3 × V × efficiency × power factor)
But for code sizing you use the table value, which for common motors is roughly: a 460 V 3-phase motor draws about 1.3–1.5 amps per horsepower (e.g., a 10 HP 460 V motor = 14 A from Table 430.250). The motor FLA calculator returns the table FLC and every downstream size.
Step 2 — size the conductors (125%)
Branch-circuit conductors for a single continuous-duty motor are sized at 125% of the table FLC (NEC 430.22):
Conductor ampacity ≥ FLC × 1.25
For a 10 HP, 460 V motor (FLC 14 A): 14 × 1.25 = 17.5 A, so you need conductors rated at least 17.5 A — #14 AWG copper (75°C) is 20 A, which works, subject to any derating. Verify against NEC 310.16 and the terminal temperature rating.
Step 3 — size the overload (nameplate)
Overload protection guards the motor windings from overheating during sustained overcurrent. It’s sized from the nameplate FLA (not the table), per NEC 430.32:
- 125% of nameplate FLA for motors with a service factor of 1.15 or more, or a temperature rise of 40°C or less.
- 115% for all other motors.
Overloads are the thermal or electronic elements in the starter, and they trip on a slow, sustained overcurrent — the kind that cooks a motor — while ignoring the brief startup inrush.
Step 4 — size the short-circuit/ground-fault protection (up to 250%)
The breaker or fuse ahead of the motor is not there to protect against overload — that’s the overload relay’s job. It exists to clear short circuits and ground faults, and it must be large enough to let the motor start without tripping. NEC Table 430.52 gives the maximum percentages of table FLC:
| Protective device | Max % of FLC |
|---|---|
| Inverse-time (thermal-magnetic) breaker | 250% |
| Non-time-delay (fast) fuse | 300% |
| Dual-element (time-delay) fuse | 175% |
| Instantaneous-trip breaker | 800% (up to 1,300% if needed) |
For our 10 HP motor (FLC 14 A) with an inverse-time breaker: 14 × 2.5 = 35 A → use the next standard size up if needed to start the motor (430.52(C)(1) Exception allows rounding up to the next standard rating). This is why a small motor can legitimately sit behind a breaker that looks “oversized” — the breaker isn’t protecting the wire from overload, the overload relay is.
Worked example (putting it together)
10 HP, 460 V, 3-phase motor, nameplate FLA 13 A, service factor 1.15:
- Table FLC (430.250): 14 A
- Conductors (430.22): 14 × 1.25 = 17.5 A → #14 Cu (20 A) minimum
- Overload (430.32): 13 × 1.25 = 16.25 A trip setting
- Breaker, inverse-time (430.52): 14 × 2.5 = 35 A → use a 35 A (or next standard) breaker
Notice the three different reference currents: table 14 A for conductors and breaker, nameplate 13 A for overload. Mixing them up is the classic motor-circuit error.
Feeders for several motors, and VFD-driven motors
Two real-world cases extend the single-motor rules:
Multiple motors on one feeder
When a feeder supplies several motors, NEC 430.24 sizes the feeder conductors at 125% of the largest motor’s full-load current plus 100% of the full-load current of all the other motors. Only the biggest motor gets the 125% continuous bump; the rest are added at their table FLC. The feeder’s short-circuit protection (430.62) is based on the largest branch-circuit protective device plus the FLCs of the other motors. This prevents the feeder from being oversized as if every motor needed its own 125% and 250% margins.
Motors on variable-frequency drives
A VFD changes the picture because the drive, not the motor directly, is on the line. The conductors to the drive are sized on the drive’s rated input current (typically 125% of it), and the drive provides the motor’s overload protection electronically. The output conductors from the drive to the motor are sized on the motor current. NEC Article 430 Part X and the drive listing govern the details, and the drive’s own short-circuit current rating must suit the available fault current. Because a VFD limits inrush, the wild startup surge that drives the 250% breaker rule is tamed — but you still follow the drive manufacturer’s and the NEC’s specific requirements rather than the across-the-line numbers.
For a single across-the-line motor, though, the four-step method above is the everyday case, and the motor FLA calculator handles it directly.
Common motor circuit mistakes
- Sizing conductors from the nameplate. Conductors and breaker use the table FLC; only overload uses nameplate.
- Sizing the breaker like a normal load. A motor breaker is deliberately large (up to 250%+) to allow starting — the overload relay protects the wire.
- Forgetting the 125% on conductors. Continuous motor conductors are 125% of FLC, not 100%.
- Ignoring the “next size up” exception. If the calculated breaker isn’t a standard size or won’t start the motor, 430.52 lets you round up.
- Overlooking multiple-motor feeders. A feeder serving several motors uses 125% of the largest motor’s FLC plus the sum of the rest (430.24).
- Skipping the disconnect and controller ratings. Article 430 also governs the disconnecting means and controller sizing.
Why the code separates the three protections
It is worth understanding why Article 430 splits motor protection three ways, because it makes the whole method intuitive rather than a set of numbers to memorize. Each device answers a different failure mode. The overload relay watches the motor’s thermal life — it trips on a modest, sustained overcurrent (a jammed pump, a failing bearing, a stalled fan) that would slowly cook the windings, and it deliberately ignores the harmless startup surge. The short-circuit/ground-fault device (the breaker or fuse) watches for a catastrophic fault — a bolted short or ground — and must clear it in a cycle or two, so it is set high enough to never confuse startup inrush for a fault. The conductors simply need to carry the running current continuously without overheating, hence the 125% sizing. Because a single device can’t do all three jobs — a breaker sensitive enough to protect the windings would trip on every start, and an overload relay can’t interrupt a 20 kA fault — the code assigns each job to the device suited to it. Once you see that logic, the “oversized” breaker and the two different reference currents stop looking arbitrary and start looking like exactly the right tool for each risk.
Standards and references
| Reference | What it covers |
|---|---|
| NEC Article 430 | Motors, motor circuits & controllers |
| NEC 430.6 / Table 430.250 | Use table FLC (not nameplate) for sizing |
| NEC 430.22 | Branch-circuit conductors at 125% of FLC |
| NEC 430.32 | Overload protection from nameplate FLA |
| NEC Table 430.52 | Short-circuit/ground-fault device max percentages |
The bottom line
Motor circuit sizing splits into three parts with two different reference currents: use the NEC table full-load current to size conductors (125%) and the short-circuit breaker (up to 250%), and the nameplate FLA to size the overload (115–125%). The breaker is intentionally large so the motor can start; the overload relay, not the breaker, protects the motor from running overcurrent. Get every value from horsepower and voltage with the motor FLA calculator, size the feeder with the cable size calculator, and have the design verified by a licensed electrical engineer against the current NEC.
Frequently asked questions
How do you size a motor circuit?
NEC Article 430 splits motor protection into three parts. Size the branch-circuit conductors at 125% of the full-load current from NEC Table 430.250 (430.22). Size the overload protection at 115 to 125% of the motor nameplate FLA (430.32). Size the short-circuit/ground-fault breaker or fuse much larger — up to 250% of table FLC for an inverse-time breaker (430.52) — so the motor can start without tripping. The key is using table FLC for conductors and breaker, nameplate FLA only for overload.
Do you use nameplate or table current to size a motor circuit?
Both, for different parts. NEC 430.6 requires you to size the conductors and the short-circuit/ground-fault protection from the full-load current in NEC Tables 430.248 to 430.250 — not the nameplate. The motor nameplate FLA is used only to size the overload (running) protection under 430.32. Mixing these up, such as sizing conductors from the nameplate, is the classic motor-circuit error.
Why is a motor breaker sized so large?
Because a motor draws a large inrush current at startup — often 6 to 8 times running current for a few seconds. The breaker or fuse ahead of the motor exists to clear short circuits and ground faults, not to protect against overload, so it must be large enough to ride through inrush without tripping. NEC Table 430.52 allows up to 250% of full-load current for an inverse-time breaker. The overload relay, not the breaker, protects the motor from sustained overcurrent.
How do you size motor overload protection?
Motor overload protection is sized from the nameplate full-load amps under NEC 430.32: 125% of nameplate FLA for motors with a service factor of 1.15 or more or a temperature rise of 40 degrees C or less, and 115% for all other motors. The overload elements in the starter trip on slow, sustained overcurrent that would overheat the windings while ignoring the brief startup inrush.
What size conductors does a motor need?
Branch-circuit conductors for a single continuous-duty motor are sized at 125% of the table full-load current from NEC 430.22, then checked against the conductor ampacity table (310.16) at the terminal temperature rating. For example, a 10 HP 460 V motor with a table FLC of 14 amps needs conductors rated at least 14 times 1.25 = 17.5 amps, so #14 AWG copper at 75 degrees C (20 amps) works, subject to any derating for conditions.
What is the difference between motor FLA and FLC?
FLA (full-load amps) is the actual running current on the motor nameplate and is used to size overload protection. FLC (full-load current) is the standardized, slightly conservative value from NEC Tables 430.248 to 430.250 for a given horsepower and voltage, and it is what you use to size conductors and the short-circuit protective device. They are usually close but not identical, and the code deliberately assigns each to different parts of the circuit.
Is a motor FLA calculator accurate for design?
A calculator that returns the NEC table full-load current and applies the 125% conductor, 115 to 125% overload, and up-to-250% breaker rules gives reliable motor-circuit sizes. A complete design also considers the next-standard-size exception for the breaker, feeders serving multiple motors (430.24), the disconnect and controller ratings, and any derating, verified by a licensed electrical engineer against the current NEC and the motor manufacturer's data.