125% on the 75°C column
What you'll learn
Size a motor conductor with 430.22: the 25% of running headroom, why the terminals pick the ampacity column, and what 208 V does to the same horsepower.
The wire feeding a motor gets sized by one rule with two parts: 430.22 — take the table FLA, add 25%, and find that ampacity in the 75°C column of Table 310.16.
Why 125%
A motor is a continuous load with a thermal personality: it can sit at full-load current for hours, and its own heat is the enemy of the insulation around it. The extra 25% is running headroom — the same philosophy as loading a breaker to 80%, seen from the other side.
Why the 75°C column
Table 310.16 gives three ampacities per wire size (60°C, 75°C, 90°C). You get to use the column your terminations are rated for — and motor-circuit gear (breakers, starters, disconnects) is built with 75°C terminals. THHN insulation is 90°C, but the lugs set the limit: 75°C is the honest column for this work.
Work the conveyor
The packline conveyor is 5 HP on a 208 V system:
| step | math | result |
|---|---|---|
| table FLA | Table 430.250, 208 V | 16.7 A |
| conductor floor | 16.7 × 1.25 | 20.9 A |
| 75°C column | first size ≥ 20.9 A | 12 AWG (25 A) |
Now glance at the breaker on the same one-line: 16.7 × 2.5 = 41.75 → 45 A. A 45 A breaker on 12 AWG wire — lesson two, live. The overload at 20.9 A is who's really watching that copper.
Notice too what 208 V is doing: the same 5 HP at 480 V would draw just 7.6 A. Low voltage makes the wire work harder for the same shaft power — hold that thought for the last lesson.
Run it
Tap the conductors chip in the designer and read the card: floor, column, and a voltage-drop line we haven't talked about yet. On this 90 ft run it's quiet. Stretch the run to 400 ft and watch the wire grow anyway — that's the next lesson.
Check your understanding
Question 1 of 2
A 5 HP motor on 208 V draws 16.7 A from the table. The minimum conductor ampacity is: