Conduit Fill Calculator
Check conduit fill against NEC Chapter 9, Table 1 — any mix of conductor sizes and insulation types, in EMT, IMC, RMC, PVC or flex. Built for Phoenix work, where a local amendment adds a grounding conductor to every run.
How it’s calculated. NEC Chapter 9, Table 1 maximum fill: 53% for one conductor, 31% for two, 40% for three or more — or 60% for a nipple ≤24″ (Note 4), selectable above. Conductor areas are from Table 5 for THHN/THWN-2 and XHHW-2 (XHHW-2 runs noticeably fatter on small sizes — a #12 is ~36% bigger than THHN, so the type matters); conduit internal areas are from Table 4 (EMT, IMC, Rigid, PVC Sch 40 & 80). The “smallest that fits” rows show the smallest trade size within the allowed fill — for the selected conduit and for every type at once. When every conductor is the same size, Chapter 9, Table 1, Note 7 lets a calculated count with a decimal of 0.8 or more round up to the next whole conductor, so a run can pass at slightly over 40% by area (26 × #12 THHN in 1″ EMT, the count Annex C tabulates, is 40.0%); the calculator applies that rounding and says so in the verdict.
More than three current-carrying conductors in one raceway also requires an ampacity adjustment per NEC 310.15(C)(1) — the calculator flags this, and the Wire Ampacity calculator applies it. Verify equipment-grounding and derating requirements against the full design.
Worked example. A 200 A feeder in four 3/0 AWG THHN copper (0.2679 in² each, Chapter 9 Table 5) with a 6 AWG THHN equipment ground (0.0507 in²) totals 1.1223 in² across five conductors, so the 40% column of Table 1 applies. A 1½″ EMT holds 2.036 × 0.40 = 0.814 in² — too small — while 2″ EMT (3.356 in²) allows 1.342 in², so 2″ is the smallest EMT that fits, at 33.4% fill. The same pull lands in 2″ for IMC, rigid, and Schedule 40 PVC as well; 2″ Schedule 80, with its thicker wall (2.874 in²), still fits but at 39.1%, with almost nothing to spare. Insulation type matters most on small conductors: nine 12 AWG THHN (0.1197 in²) fit a ½″ EMT at 39.4%, but the same nine in XHHW-2 (0.1629 in²) need ¾″. And nine current-carrying conductors in one raceway also pull every one of them down to 70% of its table ampacity under 310.15(C)(1).
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.
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Read the field guide
Fill is one input to a pull. The guides cover the rest — pulling tension and lubricant, bending EMT so the run you calculated still fits, and sizing the pull box where the run turns.
One conductor most calculators forget in Phoenix
In Phoenix, in Glendale and in unincorporated Maricopa County, EMT on its own is not an acceptable equipment grounding conductor. Those jurisdictions amend NEC 250.118(4) to require a wire-type EGC in the raceway. That is one more conductor in every EMT fill calculation on a commercial job there, and it is the single most common reason a set drawn to the national code book comes back with a fill correction from a Valley reviewer.
Run the fill with the grounding conductor included from the start. On a feeder that is already close to the 40 percent limit, the added EGC is often what pushes the raceway up a trade size — which is a material cost, a labour cost and a lead time, and better discovered at the desk than in the field.
Which code edition applies
Phoenix, Mesa, Chandler, Glendale and Buckeye are on the 2023 NEC; Scottsdale is on the 2020; unincorporated Maricopa County is still on the 2017. For Gilbert, Peoria and Tempe we confirm the adopted edition with the jurisdiction before design starts. Chapter 9 Table 1 fill limits are stable across those editions; the local amendments are what change the count.
Phoenix and Glendale also amend 334.10 to keep NM cable out of commercial buildings, so raceway is not optional on that work. Pricing a commercial job in the Valley? Commercial electrical covers rough-in through trim, or use the form at the bottom of that page.
Frequently asked questions
Do I need to add a ground wire to my conduit fill in Phoenix?
Yes, on commercial work in Phoenix, Glendale and unincorporated Maricopa County. Those jurisdictions amend NEC 250.118(4) so that EMT alone does not serve as the equipment grounding conductor; a wire-type EGC must be pulled and it counts toward fill. Include it before you check the 40 percent limit.
Can I use NM cable on a commercial job in Phoenix?
No. Phoenix and Glendale amend NEC 334.10 to keep NM cable out of commercial buildings, so commercial branch circuits there run in a raceway or an approved cable type, and conduit fill applies.
Which NEC edition does the calculator use?
Chapter 9 Table 1 fill limits are the same across the editions in use around the Valley, so the arithmetic does not change. What changes city to city is the local amendment set, which is why the Phoenix grounding-conductor rule is called out above.
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