Conduit Fill Calculator

Check mixed THHN conductors against NEC Chapter 9 fill limits and the smallest trade size that passes. Use it after cable size, breaker size, and voltage drop.

Fill inputs

Raceway

Trade size to check

Group 1 quantity

Group 1 THHN size

Group 2 quantity

Group 2 THHN size

Group 3 quantity

Group 3 THHN size

About this calculator

Fill percent is total THHN area divided by the raceway internal area. Limits are 53%, 31%, and 40%. Continue on the protection calculators hub.

Result

Planning estimate. Confirm the raceway area and insulation before you order conduit.

Assumptions: THHN conductor areas and standard EMT, PVC Schedule 40, or RMC internal areas. Jam ratio and the 24-inch nipple exception are not checked. This sizing screen does not use power factor.

Continue Your Calculation

If fill fails, go back to cable size and reduce the conductor count or upsize the raceway.

Resize cable →

Chapter 9 fill limits and THHN count chart

Conductors	Maximum fill
1	53%
2	31%
3 or more	40%

Maximum count of one THHN size, using the same areas as the calculator. Two conductors use the 31% limit, so a size can hold fewer wires than the 40% rule alone suggests. RMC is slightly larger than EMT; use the calculator for RMC and for mixed sizes.

EMT trade size	#14	#12	#10	#8	#6	#4
1/2 in	12	9	5	3	1	1
3/4 in	21	16	10	5	4	2
1 in	35	25	16	9	6	4
1-1/4 in	61	44	28	16	11	7
1-1/2 in	83	61	38	22	16	9
2 in	138	100	63	36	26	16

PVC Schedule 40	#14	#12	#10	#8	#6	#4
1/2 in	11	8	5	3	1	1
3/4 in	20	15	9	5	4	1
1 in	34	25	15	9	6	4
1-1/4 in	59	43	27	15	11	7
1-1/2 in	81	59	37	21	15	9
2 in	135	98	62	35	25	15

Formula

Fill % = (sum of conductor areas ÷ internal raceway area) × 100. Example: three 10 AWG THHN in 3/4 EMT is 3 × 0.0211 / 0.533 × 100 ≈ 11.9%, under the 40% limit. Two #4 THHN in the same 3/4 EMT is 2 × 0.0824 / 0.533 × 100 = 30.9%, under the 31% limit for two conductors.

Worked example

Three 10 AWG THHN in 3/4 in EMT: about 11.9% fill, Pass, and 3/4 in is a passing trade size.

Two #4 THHN in 3/4 in EMT: 30.9% fill, Pass. A third #4 THHN is 46.4% and fails the 40% limit, so the chart allows 2.

FAQ

What NEC fill limits does this use?

Chapter 9 Table 1: 53% for one conductor, 31% for two, and 40% for three or more.

Can I mix wire sizes?

Yes. Enter up to three THHN groups. Fill uses the sum of their areas, which covers a mix of wire sizes.

Does this check jam ratio?

No. Jam ratio and the 24-inch nipple 60% exception are marked Not checked.

How many #4 THHN wires fit in 3/4 EMT?

Two. On this page's areas, two #4 THHN use 30.9% of 3/4 in EMT, just under the 31% limit for two conductors. Three #4 THHN would be 46.4%, over the 40% limit.

How many wires fit in 1 inch EMT?

It depends on the THHN size. With this page's areas and the Chapter 9 limits, 1 in EMT holds 35 of #14, 25 of #12, 16 of #10, 9 of #8, 6 of #6, or 4 of #4 when every wire is the same size. Mixed sizes use the three groups above.

What conduit types and trade sizes does this cover?

EMT, PVC Schedule 40, and RMC, in trade sizes 1/2 through 4 inch. IMC is not on this screen. EMT is thin steel tubing, PVC Schedule 40 is plastic, and RMC is threaded rigid metal. The count chart shows 1/2 through 2 inch; larger sizes stay in the calculator.