CONDUIT FILL
CONDUIT FILL
fill% = (n · A_wire) ÷ A_conduit
RESULT
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FILL IN ABOVE
NEC fill limits: 53% (1 wire), 31% (2 wires), 40% (3+). EMT + THHN/THWN-2 only.
About this calculator
This conduit fill calculator checks whether the wires you plan to pull fit inside an EMT (Electrical Metallic Tubing) run within NEC limits. NEC Chapter 9 caps fill at 53% for one conductor, 31% for two, and 40% for three or more. Wire areas are taken from NEC Chapter 9 Table 5 (THHN/THWN-2) and conduit internal areas from Table 4. Use it on rough-in or feeder runs to confirm the conduit size you spec'd will pass inspection.
Common questions
What is the NEC fill limit for EMT?
NEC Chapter 9, Note 4: 53% maximum fill for one conductor, 31% for two, and 40% for three or more. The lower limit on two-wire pulls is intentional — two stiff conductors jam in a way that one or three+ don't. Going over these numbers is an inspection failure.
Does this calculator account for conductor derating?
No — this is a physical fill check only. NEC 310.15(C)(1) requires ampacity adjustment when more than 3 current-carrying conductors share a raceway: 4–6 conductors derate to 80%, 7–9 to 70%, 10–20 to 50%. Plan ampacity separately if you're bundling many circuits.
Why use THHN/THWN-2 wire areas?
THHN/THWN-2 is the most common building wire in commercial and residential conduit runs. Other insulation types (XHHW, RHW, USE) have slightly different cross-sections — check NEC Chapter 9 Table 5 for exact areas. For mixed conductor sizes, sum each wire's area individually.