Sizing THHN Conductors per PEC Table 3.10.1 — A Worked Example

Conductor sizing is where most electrical estimates go wrong — either the wire is undersized and fails inspection, or it is oversized and the bid loses. This guide works a single 230 V branch circuit end to end.

The design current

Start from the connected load and convert it to amperes. Our example is an 8 kW continuous single-phase load (a run of package air-conditioners on a dedicated sub-feed) supplied at 230 V.

The PEC requires the conductor and overcurrent device for a continuous load (running 3 hours or more) to be rated at not less than 125% of the continuous current.

So we need a conductor with an allowable ampacity of at least 43.5 A and an overcurrent device (OCPD) of the next standard size at or above it — a 50 A breaker.

Reading the ampacity table

PEC 2017 Table 3.10.1.16 gives allowable ampacities for not more than three current-carrying conductors in a raceway at 30 °C ambient. THHN is a 90 °C insulation, so you may use the 90 °C column for correction math only — the final selection can never exceed the 60 °C or 75 °C termination rating.

Representative of common PEC practice at 30 °C ambient; confirm against the code edition enforced locally.

Applying correction factors

• Ambient temperature. If the raceway runs through a 40 °C ceiling void, the 90 °C-column correction factor is about 0.91.
• Bundling. More than three current-carrying conductors in one raceway triggers an adjustment (0.80 for 4–6, 0.70 for 7–9). Our circuit has two, so none applies.

8.0 mm² THHN has a 90 °C ampacity of 55 A (clears 47.8 A) and a 75 °C termination ampacity of 50 A (clears the 43.5 A design current). Selection: 8.0 mm² THHN copper.

The voltage-drop check

The PEC recommends a maximum 3% drop on a branch circuit, 5% total. For a one-way run of 45 m using copper resistivity of about 0.0172 Ω·mm²/m:

Inside the limit but with no margin. On a run this long it is worth upsizing to 14 mm², which drops the loss to about 1.7% and leaves headroom for load growth.

Finishing the circuit

• OCPD: 50 A.
• Equipment grounding conductor: ~5.5 mm² copper for a 50 A device (PEC Table 2.50.1.22).
• Raceway: 3 × 8.0 mm² (or 14 mm²) THHN plus a 5.5 mm² ground fits 20 mm (¾") conduit below 40% fill.