Result
Live Utilization Gauges
These gauges update in real time as you adjust inputs on the left.
Motor Branch-Circuit One-Line
Each element updates with your inputs.
Why This Matters
Motor circuits follow different rules because motors behave differently. During startup, a motor draws several times its running current for a few seconds. If the branch OCPD were sized to protect the conductor at 125% — the rule for ordinary loads — every start could be a nuisance trip.
The NEC splits motor protection into separate devices with separate jobs:
Short-circuit & ground-fault only
Sized large (from table FLC) to let the motor start. Protects conductors from faults, not from overload.
Running overload only
Sized close to nameplate FLA. A separate device. Trips on sustained over-current.
Motor Starting Current Simulation
Illustrative startup vs running behavior — why the branch OCPD can't be small.
Illustrative inrush (~6× FLC for a typical Design B motor) decaying over ~1–3 s. Actual inrush depends on the motor code letter, load, and drive. The OCPD must ride through starting without tripping.
Current & Ampacity Comparison
Side-by-side view of the ampere values in the branch calculation.
Time–Current Coordination (illustrative)
Motor starting (purple) vs branch OCPD (red) vs overload (yellow). Schematic only — not a substitute for a coordination study.
Conductor Size Comparison (Copper vs Aluminum)
For your required ampacity, at the selected insulation/termination settings.
Table 430.52(C)(1) Multiplier Matrix
Max branch OCPD as a percent of FLC per motor row / OCPD type. Your selection is highlighted. Instantaneous-trip values are settings.
FLC vs. Horsepower (At Your Voltage)
How table FLC scales across HP at your voltage. Your motor is highlighted.
Step-by-Step Calculation Trace
Every value below is produced by the pure calculation engine (window.MotorCalc).
Assumptions
Exclusions (not evaluated by this tool)
- Feeder conductors and feeder OCPD (430.24 / 430.62).
- Multiple motors / combination loads on one circuit.
- VFD / adjustable-speed-drive branch circuits (Article 430 Part X / 409).
- Disconnecting means (430.109) and controller (430.83) ratings.
- Available fault current, SCCR, and OCPD interrupting rating (110.9 / 110.10).
- Voltage drop, and local / AHJ amendments.
Sizing Results
Interpretation Notes
Overload vs. Short-Circuit / Ground-Fault Protection
Overload Protection
A separate overload device (heaters, electronic, or integral). Sized from motor nameplate FLA — typically 115% or 125%, or up to 140%/130% under 430.32(C).
Short-Circuit & Ground-Fault Protection
The branch OCPD. Sized large from table FLC to ride through starting. Protects conductors from faults.
Conductor vs. OCPD Comparison
◆ Design Challenge Mode
A motor scenario is generated and graded against the same engine used everywhere else in this tool.
Your Challenge History
Track your progress across scenarios.
How Motor Circuits Are Different
For most loads, the NEC sizes the OCPD to protect the conductor and the conductor to carry the load. Motors break this pattern: inrush current is several times running current during startup. Sizing the branch OCPD at 125% of FLC could trip on every start.
The NEC solves this by splitting protection into layers:
- Branch-circuit short-circuit and ground-fault protection (430.52) — sized large from table FLC.
- Motor overload protection (430.32) — a separate device sized from nameplate FLA.
FLC → Conductor & OCPD Sizing Path
What to Notice
Common Mistakes
- Using nameplate FLA for conductor / branch OCPD. Those come from Tables 430.248 / 430.250 (table FLC). Nameplate FLA is for overload — and for non-continuous-duty conductor sizing under 430.22(E).
- Applying Article 240 conductor-protection rules. Motor branch OCPDs can be much larger than conductor ampacity — by design.
- Forgetting the separate overload device. The branch OCPD does not protect against overload.
- Mixing copper and aluminum ampacities. They are not interchangeable per size.
- Ignoring the termination temperature limit. You may derate from the 90°C column, but the final ampacity is capped at the 60°C or 75°C termination column (110.14(C)).
- Not using the next standard size. 430.52(C)(1) Exception 1 permits the next higher standard size (240.6) when the calculated value isn't standard — except instantaneous-trip breakers, which are settings.
◉ Interactive Quiz
Test your understanding. Each question has one correct answer.
NEC Reference Panel
Article and table references. NEC language is paraphrased for education — always consult the actual code text.
Quick Article Index (Article 430)
- 430.6(A) — Use table FLC (not nameplate) for conductor / branch OCPD sizing
- 430.22 — Branch-circuit conductor sizing (single motor)
- 430.22(E) — Duty-cycle conductor sizing (non-continuous)
- 430.32 — Overload protection (nameplate FLA)
- 430.52 — Branch SC / GF protection
- Table 430.248 — Single-phase motor FLC
- Table 430.250 — Three-phase motor FLC
- Table 430.52(C)(1) — Max OCPD percentages
- Table 310.16 / 310.15(B)(1) / 310.15(C)(1) — Ampacity, ambient, adjustment
- 240.6 / 250.122 — Standard sizes / EGC
✓ Built-in Self-Test
Runs the pure calculation engine against known NEC cases plus a DOM smoke test. Also runnable headless via tests/motor-calc.spec.mjs and callable as window.runSelfTests().