Electrician · Conduit Fill
Conduit Fill Calculator Practice Problems
Free, unlimited conduit fill practice for the Journeyman & Master Electrician exam. Every click generates a new set of THHN conductors in EMT, PVC Schedule 40, or Rigid Metal Conduit — find the minimum trade size.
How to calculate it
Conduit fill, worked by hand
The same three lookups every time: conductor area, fill percentage for the conductor count, and the smallest trade size whose allowable area clears your total.
Problem: You are pulling 5 conductors of 12 AWG THHN into EMT conduit. What is the minimum trade size allowed?
1. Look up conductor area: 12 AWG THHN = each (NEC Ch. 9 Table 5)
2. Total conductor area:
3. Fill limit for 5 (3+) conductors = (NEC Ch. 9 Table 1)
4. Required conduit area:
5. Check EMT trade sizes (Ch. 9 Table 4, "over 2 wires" column): 1/2" allows only — too small. 3/4" allows — clears it.
Minimum trade size = 3/4" EMT
FAQ
Conduit fill — frequently asked questions
Why does 2 conductors get a lower fill allowance (31%) than 1 (53%) or 3+ (40%)?
Per NEC Chapter 9, Table 1: a single conductor can lie anywhere in the raceway, so it gets the highest allowance (53%). Two conductors have to lie side by side, which is the tightest, hardest-to-pull configuration, so it gets the lowest allowance (31%). Three or more conductors distribute more evenly around the raceway, so the allowance goes back up to 40%.
Does conduit fill use the bare copper area or the insulated conductor area?
The insulated area — the actual outside diameter of the conductor including its insulation (THHN, THWN, etc.), from NEC Chapter 9, Table 5. Bare conductor area is never used for fill calculations; this is why the calculator loads the THHN/THWN/THWN-2 column specifically.
What if I'm pulling a mix of different wire gauges?
Look up and total each conductor's own area individually, then compare that sum to the raceway's allowable area for '3 or more conductors' (40%, since a mixed pull is almost always 3+ conductors). This calculator drills same-gauge sets for clarity, but the real exam does test mixed-gauge pulls using the same add-up-the-areas method.
Does the conduit type (EMT, PVC, RMC) change the 40%/31%/53% fill rule?
No — those percentages come from Chapter 9, Table 1 and apply to every raceway type. What changes by conduit type is the internal area available at each trade size (Chapter 9, Table 4), which is why the same set of conductors can need a different minimum trade size in EMT versus PVC versus RMC.
Do equipment grounding conductors count toward the fill calculation?
Yes. Every conductor in the raceway — including equipment grounding and bonding conductors — counts toward total fill, per the notes to NEC Chapter 9, Table 1.