Inverters & equipment

Conduit fill calculator

Enter the conductors going into your raceway, up to three different gauges. Get the minimum EMT trade size that meets NEC Chapter 9 fill percentage rules.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Conductor group 1
THWN-2/XHHW-2 insulation area assumed, standard for solar conductors.
Count only current-carrying and grounded conductors, not the bare equipment ground.
3 Conductor group 2 Optional
Leave at 0 if every conductor in the run is the same gauge.
4 Conductor group 3 Optional
Use this row for an equipment grounding conductor of a different gauge, if applicable.

How it works

Three steps to your conduit size

Step 1

Settle on wire gauge first

Run the wire gauge sizing calculator and voltage drop calculator first, since the conductor gauges you land on are the direct inputs here.

Step 2

List every conductor sharing the raceway

Count each current-carrying and grounded conductor, and split them by gauge if a circuit uses more than one size, like a smaller equipment ground alongside larger current-carrying wires.

Step 3

Add headroom if you expect future circuits

The result shown is the code minimum. If you're planning to add strings later, sizing one trade size up now avoids re-pulling the conduit run when that day comes.

Understanding conduit fill

Why fill percentage limits exist

Cramming too many conductors into a conduit makes them harder to pull during installation and traps heat that each wire needs to shed, the same heat-dissipation concern behind the conductor-count adjustment in the wire gauge sizing calculator. NEC Chapter 9 Table 1 caps how much of a conduit's internal area the conductors can occupy, based on how many conductors share the raceway, working alongside the ampacity limits in NEC Table 310.16.

Why the limit tightens at two conductors, then loosens at three or more

This is one of the more counterintuitive parts of the rule. A single conductor is allowed up to 53% fill, since there's no other wire to fight for space or bend around. Two conductors are actually capped tighter, at 31%, because a pair of wires jams against each other in a way that a bundle of three or more, which can settle into a more efficient round cross-section, doesn't. Three or more conductors get 40%, which covers nearly every real solar circuit, since most combiner and inverter runs carry at least a positive, negative, and ground conductor together.

Why mixed gauges in one run are common

A typical DC home run might carry several 10 AWG current-carrying conductors, sized with the parallel strings current calculator, alongside a single smaller-gauge bare or insulated equipment ground. This calculator supports up to three separate gauge groups so you can total the area correctly instead of assuming every conductor is the same size, which would either understate or overstate the true fill. If you haven't finalized your overall string count yet, the string length vs panel count calculator is worth running first, since it determines exactly how many conductors this raceway needs to carry.

How this connects to your ampacity and voltage drop checks

The conductor count you use here should match what you used in the wire gauge sizing calculator, since that's the same raceway. If your final gauge changed after checking the voltage drop calculator, come back and update the gauge here too, since a larger conductor takes up more room in the same conduit. The wiring loss output calculator can show what that gauge choice costs you in ongoing production, separate from the fill question this tool answers.

What this tool doesn't cover

This calculator assumes standard EMT (electrical metallic tubing) and THWN-2/XHHW-2 insulated conductors, the common combination for solar installations. Other conduit types, like PVC schedule 40 or 80, or rigid metal conduit, have different internal area tables, and different insulation types change each conductor's cross-sectional area. Always confirm the final conduit type and size against your local electrical code and the conduit manufacturer's fill tables before purchasing, or start with SEIA's consumer overview of how solar and inverters work if you're new to how these installation details fit into the bigger picture.

Questions

Frequently asked

Four 10 AWG THWN-2 conductors, common for two combined strings, fit comfortably in 1/2 inch EMT at about 28% fill. Add up the cross-sectional area of every conductor going into the raceway, then find the smallest conduit trade size where that total doesn't exceed the NEC's allowed fill percentage.

NEC Chapter 9 Table 1 sets different limits because pulling conductors through a conduit gets harder as more wires compete for space and bend around each other. A single conductor is allowed up to 53% fill, two conductors are capped at 31%, and three or more conductors are capped at 40%, which is the rule that applies to nearly every real solar raceway.

Not directly, but they share an input. The ampacity adjustment in the wire gauge sizing calculator depends on how many current-carrying conductors share a raceway, the same conductor count used here. Running the two calculators together with matching numbers keeps both the wire gauge and the conduit size consistent.

It's common practice to size one trade size larger than the bare minimum if you expect to add conductors later, since re-pulling a conduit run is far more work than installing slightly larger pipe upfront. This calculator shows the code minimum; adding headroom for future circuits is a planning decision on top of that.

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