NEC Chapter 9 conduit fill for solar, explained

Cramming too many conductors into a raceway causes two problems: the wires are hard to pull without damaging insulation, and they can't shed heat. NEC Chapter 9 solves both with a simple geometric limit — the conduit fill rules. For solar, where rooftop conduits get hot and carry PV plus EGC conductors, fill and heat go hand in hand.

The 40% rule

Chapter 9, Table 1 sets the maximum percentage of a conduit's internal cross-section that conductors may occupy:

Almost every solar circuit has three or more conductors, so the 40% rule is the one you'll use nearly every time.

The tables you need

Chapter 9 splits the data across a few tables:

• Table 4 — the internal area of each conduit type and trade size (EMT, PVC, RMC each have their own sub-table).
• Table 5 — the cross-sectional area of each conductor, by insulation type and AWG/kcmil.
• Table 1 — the fill percentage (above).

The workflow is: add up conductor areas from Table 5, then confirm the sum is at or below 40% of the conduit's Table 4 area.

A worked example

Say you're running a PV circuit in 3/4" EMT with four #10 THWN-2 conductors (two circuit conductors, plus in this illustration additional conductors) and a #10 THWN-2 EGC:

1. Area of one #10 THWN-2 ≈ 0.0211 in² (Table 5). 2. Five conductors × 0.0211 = 0.1055 in² total. 3. Internal area of 3/4" EMT ≈ 0.533 in² (Table 4). 4. 40% of 0.533 = 0.213 in² allowed. 5. 0.1055 in² used is well under 0.213 in² — the fill passes with room to spare.

Notice the EGC is counted in the fill even though it doesn't carry load current in normal operation. This is a frequent oversight: people count only the current-carrying conductors and forget the ground.

Fill vs derating — two different counts

Conduit fill and ampacity derating both care about "how many conductors," but they count differently:

• Fill (Chapter 9): counts every conductor including the EGC and grounded conductor.
• Ampacity derating (310.15): counts only current-carrying conductors — the EGC is generally excluded.

So a raceway with four current-carrying conductors plus an EGC has five conductors for fill but four for the derating factor. Mixing these up leads to either an over-stuffed conduit or an under-derated conductor.

Why it matters for solar specifically

Rooftop conduit runs are already fighting a high ambient temperature correction. A tightly packed raceway both fails the 40% geometry and worsens the heat that drives the derating in 110.14 termination limits and the 310.16 ampacity tables. Upsizing the conduit one trade size is often cheaper than upsizing every conductor.

A practical tip: when all the conductors in a raceway are the same size and insulation, Chapter 9 Annex C gives you ready-made tables that list the maximum number of conductors allowed in each conduit type directly, so you can skip the area arithmetic entirely. Fall back to the Table 4 and Table 5 method whenever the conductors are mixed sizes — the annex tables only apply to uniform fills. Either way, count the EGC, and remember that nipples 24 inches or shorter are permitted to fill to 60%.

Confirm the enforced NEC edition and any local raceway amendments with your AHJ, and have a licensed electrician or PE review, sign and stamp the design before you file or energize. OneLine Studio is a design aid, not a permitting authority. For the surrounding wiring-method rules, see 690.31 PV wiring methods.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.