NEC Load Calculations, Explained Without the Headache

Calculations scare apprentices more than they should. The truth is the journeyman exam reuses the same few calculation types over and over — once you know the procedure for each, they become free points. Let's walk the big three.

Conductor ampacity

Start at Table 310.16. Find your conductor size and read across to the correct temperature column. Here's the catch most people miss: even if you're running a 90°C insulation like THHN, terminations usually limit you to the 75°C column under 110.14(C). So a 3/0 AWG copper conductor is good for 200 A at 75°C — not the higher 90°C value.

Then apply correction and adjustment factors if the question gives you ambient temperature or more than three current-carrying conductors.

Box fill

Box fill is pure addition once you know the per-conductor volumes from Table 314.16(B):

• 14 AWG → 2.00 in³
• 12 AWG → 2.25 in³
• 10 AWG → 2.50 in³

Count every conductor, device yoke (counts double), and clamp the table tells you to, multiply, and add. Compare the total to your box's cubic-inch capacity. Done.

Motor full-load current

This one trips people up because of a single rule: for conductor sizing, you use the table full-load current, not the nameplate. Per 430.6(A)(1), a three-phase motor's FLC comes from Table 430.250. Conductors are then sized at 125% of that value for a single continuous-duty motor.

The pattern behind all of it

Every NEC calculation follows the same rhythm: find the right table → read the base value → apply the required multiplier. Master that rhythm and the math stops being scary — it becomes the most reliable points on the whole exam.