By Blissom · Updated 2026-11-04
Why 16 wires is not 8 circuits
The fill table counts copper and insulation that fits. Table 310.16 and 310.15(C)(1) decide how hot that bundle may run. With #12 on 20 A breakers you can have 9 current-carrying conductors, not 16.
Eight circuits with their own neutrals would put 16 current-carrying wires in the pipe. At that count the factor is 50%, and #12 THHN drops to 15 A. A 20 A breaker would then protect the wire above its ampacity, which 240.4 does not allow.
Where each wire size stops
| Current-carrying conductors | Factor | #14 on 15 A | #12 on 20 A | #10 on 30 A |
|---|---|---|---|---|
| 1–3 | 100% | 25 A OK | 30 A OK | 40 A OK |
| 4–6 | 80% | 20 A OK | 24 A OK | 32 A OK |
| 7–9 | 70% | 17.5 A OK | 21 A OK | 28 A too low |
| 10–20 | 50% | 12.5 A too low | 15 A too low | 20 A too low |
| 21–30 | 45% | 11.3 A too low | 13.5 A too low | 18 A too low |
So the ceilings are 9 for #14 on 15 A, 9 for #12 on 20 A and 6 for #10 on 30 A. #10 runs out first because its breaker sits so close to its own 90 °C rating.
The derated number may not exceed what the terminations allow, usually the 60 °C column on breakers and devices of 100 A or less (110.14(C)). For these three sizes the 60 °C value equals the breaker, so it never changes the answer.
A rooftop or boiler room adds an ambient correction from 310.15(B) on top of this, which can push every ceiling lower.
Which neutrals count
- Own-neutral 120 V circuit: neutral carries the same current as the hot. Counts.
- 120/240 V multiwire circuit (two hots, one neutral): the neutral carries the difference only. Does not count (310.15(E)).
- Two phases and a neutral from a 208Y/120 V system: that neutral carries roughly full current. Counts.
- Three phases and a neutral feeding mostly electronic, nonlinear load: harmonics load the neutral. Counts.
- Equipment grounding conductor: never counts for heat (310.15(F)). It still takes a fill slot.
| Wire and breaker | Own neutrals | 120/240 V shared neutrals |
|---|---|---|
| #14 THHN, 15 A | 4 | 8 |
| #12 THHN, 20 A | 4 | 8 |
| #10 THHN, 30 A | 3 | 6 |
Shared neutrals double the circuit count in 3/4" because the pipe still has spare room. They need simultaneous disconnecting, by handle tie or two-pole breaker (210.4(B)), and GFCI or AFCI devices have to be chosen to work with a shared neutral.
3/4" EMT fill table
Internal area 0.533 in² (Chapter 9 Table 4). Same-size conductors, ground included.
| Wire | THHN / THWN-2 | XHHW-2 | THW |
|---|---|---|---|
| #14 | 22 | 15 | 10 |
| #12 | 16 | 11 | 8 |
| #10 | 10 | 8 | 6 |
| #8 | 6 | 5 | 4 |
| #6 | 4 | 3 | 3 |
| #4 | 2 | 2 | 1 |
| #3 | 1 | 1 | 1 |
| #2 | 1 | 1 | 1 |
| #1 | 1 | 1 | 1 |
| #1/0 | 1 | 1 | 1 |
| #2/0 | 1 | 1 | 1 |
| #3/0 | 1 | 1 | 1 |
| #4/0 | 1 | 1 | — |
Counts of 1 or 2 come from the 53% and 31% limits of Table 1. THW figures use the larger Table 5 dimensions, as Annex C does. Bare grounds use Table 8 instead.
Three real 3/4" pulls
Three 20 A circuits, own neutrals
| Conductors | Area each (in²) | Subtotal (in²) |
|---|---|---|
| 6 × #12 THHN (3 hots, 3 neutrals) | 0.0133 | 0.0798 |
| 1 × #12 THHN (ground) | 0.0133 | 0.0133 |
| Total, 7 conductors | 0.0931 | |
| Allowed: 40% of 0.533 in² | 0.2132 |
17.5% full. Six current-carrying conductors put you at 80%: 24 A per wire, fine for 20 A. A fourth circuit would make eight, still legal at 21 A.
Six 20 A circuits on three shared neutrals
| Conductors | Area each (in²) | Subtotal (in²) |
|---|---|---|
| 6 × #12 THHN (hots) | 0.0133 | 0.0798 |
| 3 × #12 THHN (shared neutrals) | 0.0133 | 0.0399 |
| 1 × #12 THHN (ground) | 0.0133 | 0.0133 |
| Total, 10 conductors | 0.1330 | |
| Allowed: 40% of 0.533 in² | 0.2132 |
25.0% full, within the 40% limit. Only the six hots count, so the factor is 80% and each #12 is good for 20 A after the termination cap.
A 50 A EV charger circuit
| Conductors | Area each (in²) | Subtotal (in²) |
|---|---|---|
| 2 × #6 THHN (hots) | 0.0507 | 0.1014 |
| 1 × #10 THHN (ground (Table 250.122, 60 A)) | 0.0211 | 0.0211 |
| Total, 3 conductors | 0.1225 | |
| Allowed: 40% of 0.533 in² | 0.2132 |
23.0% full. Two current-carrying conductors, no adjustment. #6 THHN is 55 A even in the 60 °C column, so the 50 A breaker is covered whatever the terminations are rated.
Short nipples
A 3/4" nipple of 24" or less between two enclosures may be filled to 60%, which allows 24 #12 THHN. Derating does not apply to it either. This is the exception, not a reason to treat a long run as a nipple.
Going to 1" or a second pipe
Once a 3/4" home run hits 9 current-carrying #12, more circuits need a second raceway or a bigger wire. A 1" EMT fixes fill, but not derating; the 9-conductor ceiling for #12 on 20 A is the same in any size pipe.
- Two separate 3/4" runs are counted separately, so each can carry its own 9.
- Upsizing to #10 on 20 A breakers lets one raceway carry up to 20 current-carrying conductors, but 3/4" only fits 10 #10.
- A run with lots of bends, or a fill near 16, pulls far easier in 1". Every pipe size still has the same 360° limit between pull points.
Check any mix with the conduit fill calculator. Values follow NEC 2023; verify against the edition your inspector enforces.
Sources: NEC 2023: Chapter 9, Tables 1, 4 and 5 with Note 7 (fill); 310.15(C)(1) (adjustment for more than three current-carrying conductors), 310.15(E) and (F) (neutrals and grounds), Table 310.16, 240.4(D) and 110.14(C). Method.
Questions electricians ask
How many wires in 3/4 EMT?
16 #12 THHN, 22 #14 THHN or 10 #10 THHN, ground included, per NEC Chapter 9 with Note 7 rounding. XHHW-2 and THW hold fewer because the insulation is thicker.
How many #8 THHN fit in 3/4 EMT?
6 #8 THHN. With #6 THHN it is 4. For a 40 A or 50 A circuit that leaves plenty of room for the ground.
How many 12 gauge wires can be in 3/4 conduit?
16 #12 THHN fit in 3/4" EMT. For 20 A breakers, derating allows only 9 current-carrying #12 conductors in the same raceway, so the useful number is lower than the fill number.
How many circuits can I put in 3/4 conduit?
On #12 THHN and 20 A breakers: 4 circuits that each have their own neutral, or 8 when pairs of hots share a 120/240 V neutral. Each needs to stay at or under 9 current-carrying conductors in total.
How many wires can go in 3/4 conduit before derating?
Three current-carrying conductors with no adjustment. From four to six the factor is 80%, from seven to nine 70%, and from ten to twenty 50%. Grounds never count.
Fill and bends for 3/4" in one app
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- Every bend: offsets, saddles, 90s and kicks
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