About Conduitwise

Conduitwise is a small reference site for tradespeople and careful DIYers. Its first tool is a conduit fill calculator: pick EMT, PVC Schedule 40 or rigid metal conduit, add your THHN or THWN wires by size and count, and see the fill against the NEC limit. The tables it uses are printed on the page.

What it does today

The calculator uses NEC (NFPA 70) Chapter 9: Table 1 for the fill limits (53% for one wire, 31% for two, 40% for more), Table 4 for the conduit areas and Table 5 for the wire areas of THHN, THWN and THWN-2, from 14 AWG to 500 kcmil. It shows the allowed area, your fill and the smallest conduit size that fits, and you can copy or print the result.

How we checked it

We typed the three tables in a second time as a separate check, made 300 random wire mixes with one to four wire sizes in EMT, PVC Schedule 40 and rigid metal conduit, entered each in Chrome and compared the wire count, the limit, the allowed area, the fill and the smallest size that fits: no differences. We also printed the result to PDF: it fits on one Letter page. The table values were cross-checked against published copies of the 2017 edition, and the Table 4 areas were recomputed from the internal diameters.

How Conduitwise checks the calculator: type in the tables separately, make 300 random wire mixes, compare every figure, 0 differences

What Conduitwise does not do

It is a planning tool, not the code. It does not derate wires for heat, cover insulation types other than THHN, THWN and THWN-2, or apply the 60% exception for short nipples. Electrical work can injure or kill. Follow the code your jurisdiction has adopted, comply with local rules and hire a licensed electrician for installations.

What Conduitwise does not do: replace the code or an electrician, derate wires, cover other insulation types or apply the 60 percent nipple exception

Who runs it

Conduitwise is written and kept up by the Conduitwise Team. We do not publish personal names, credentials or a postal address here. Write to us through the contact page; the privacy policy, terms of use and disclaimer explain the rest.

Frequently asked questions

Is Conduitwise free?

Yes. There is no account and no limit on use.

Which NEC edition do the tables come from?

The values were cross-checked against the 2017 edition. Always confirm against the edition your jurisdiction has adopted.

Is it a substitute for an electrician?

No. It is a planning tool.

Try the Conduitwise conduit fill calculator.