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NEC Tables Reference

The lookup tables you actually reach for, on one page and free of pop-ups. Ampacity, derating factors, standard breaker ratings, conduit and box fill, and the two grounding tables — every one read from the same source our calculators use, so what you see here is what they compute.

310.16 Ampacity 310.12 Dwelling Services Derating Factors 240.6(A) OCPD Ch. 9 Conduit Fill 314.16 Box Fill 250.66 / 250.122

Table 310.16

Conductor Ampacity

Allowable ampacities of insulated conductors rated up to 2,000 V, not more than three current-carrying conductors in a raceway, cable, or earth, at an ambient of 30 °C (86 °F). Under 110.14(C) the circuit is limited by the lowest-rated termination — usually 75 °C — so the 90 °C column is a derating starting point rather than a usable answer.

Size	Copper			Aluminum			Circular mils
Size	60 °C	75 °C	90 °C	60 °C	75 °C	90 °C	Circular mils
14 AWG	15	20	25	—	—	—	4,110
12 AWG	20	25	30	15	20	25	6,530
10 AWG	30	35	40	25	30	35	10,380
8 AWG	40	50	55	35	40	45	16,510
6 AWG	55	65	75	40	50	55	26,240
4 AWG	70	85	95	55	65	75	41,740
3 AWG	85	100	115	65	75	85	52,620
2 AWG	95	115	130	75	90	100	66,360
1 AWG	110	130	145	85	100	115	83,690
1/0 AWG	125	150	170	100	120	135	105,600
2/0 AWG	145	175	195	115	135	150	133,100
3/0 AWG	165	200	225	130	155	175	167,800
4/0 AWG	195	230	260	150	180	205	211,600
250 kcmil	215	255	290	170	205	230	250,000
300 kcmil	240	285	320	195	230	260	300,000
350 kcmil	260	310	350	210	250	280	350,000
400 kcmil	280	335	380	225	270	305	400,000
500 kcmil	320	380	430	260	310	350	500,000
600 kcmil	350	420	475	285	340	385	600,000

240.4(D) caps small conductors below these figures: 15 A for 14 AWG copper, 20 A for 12 AWG, 30 A for 10 AWG — and 15 A for 12 AWG aluminum, 25 A for 10 AWG. That is why 12 AWG copper reads 25 A here and still sits on a 20-amp breaker.

Wire Size Calculator → Ampacity & Derating Calculator → How to read this table →

Table 310.12

Dwelling Services & Feeders

For a dwelling service, or a feeder carrying the entire load of a one-family dwelling, conductors may be sized at 83% of the rating. This does not reach an ordinary subpanel feeder — that sizes from Table 310.16 above.

Service or feeder rating	Copper	Aluminum / copper-clad	83% of rating
100 A	4 AWG	2 AWG	83.0 A
110 A	3 AWG	1 AWG	91.3 A
125 A	2 AWG	1/0 AWG	103.8 A
150 A	1 AWG	2/0 AWG	124.5 A
175 A	1/0 AWG	3/0 AWG	145.3 A
200 A	2/0 AWG	4/0 AWG	166.0 A
225 A	3/0 AWG	250 kcmil	186.8 A
250 A	4/0 AWG	300 kcmil	207.5 A
300 A	250 kcmil	350 kcmil	249.0 A
350 A	350 kcmil	500 kcmil	290.5 A
400 A	400 kcmil	600 kcmil	332.0 A

Service & Feeder Wire Size → The 83% rule explained →

310.15(B)(1) & 310.15(C)(1)

Derating Factors

Both factors are applied to the ampacity from the conductor's insulation temperature column — normally 90 °C — and the result is then checked against the 75 °C termination limit. Starting from the 90 °C column is often what saves a conductor size.

Ambient temperature correction

310.15(B)(1) — base ambient 30 °C

Ambient	60 °C	75 °C	90 °C
≤ 10 °C (50 °F)	1.29	1.20	1.15
11–15 °C (52–59 °F)	1.22	1.15	1.12
16–20 °C (61–68 °F)	1.15	1.11	1.08
21–25 °C (70–77 °F)	1.08	1.05	1.04
26–30 °C (79–86 °F)	1.00	1.00	1.00
31–35 °C (88–95 °F)	0.91	0.94	0.96
36–40 °C (97–104 °F)	0.82	0.88	0.91
41–45 °C (106–113 °F)	0.71	0.82	0.87
46–50 °C (115–122 °F)	0.58	0.75	0.82
51–55 °C (124–131 °F)	0.41	0.67	0.76
56–60 °C (133–140 °F)	—	0.58	0.71
61–70 °C (142–158 °F)	—	0.33	0.58
71–80 °C (160–176 °F)	—	—	0.41

Conductor bundling adjustment

310.15(C)(1) — current-carrying conductors only

Conductors in the raceway	Factor
1–3 conductors	1.00
4–6 conductors	0.80
7–9 conductors	0.70
10–20 conductors	0.50
21–30 conductors	0.45
31–40 conductors	0.40
41 or more conductors	0.35

Equipment grounding conductors never count. A neutral counts where it carries current. A nipple no longer than 24 in is exempt under Chapter 9, Table 1, Note 4.

Ampacity & Derating Calculator → Why derating starts at 90 °C →

240.6(A)

Standard OCPD Ratings

The only ratings fuses and inverse-time breakers come in. A calculation landing between two of them rounds up — there is no 33-amp breaker.

15 A 20 A 25 A 30 A 35 A 40 A 45 A 50 A 60 A 70 A 80 A 90 A 100 A 110 A 125 A 150 A 175 A 200 A 225 A 250 A 300 A 350 A 400 A 450 A 500 A 600 A

240.6(A) also lists additional standard fuse ratings of 1, 3, 6, 10 and 601 A and above, rarely seen on branch circuits.

Breaker Size Calculator → How to size an OCPD →

Chapter 9, Tables 1, 4 & 5

Conduit Fill

Three tables working together: Table 1 gives the allowable percentage by conductor count, Table 4 the raceway's internal area, and Table 5 each conductor's area including insulation.

Table 1 — maximum percent fill

1 conductor

53%

2 conductors

31%

3 or more

40%

Two conductors is stricter than three — that is the code, not a typo, and it means the maximum count cannot be found by a single division. A nipple of 24 in or less goes to 60% under Note 4, and the 310.15(C)(1) adjustment factors do not apply to it.

Table 4 — raceway internal area, in²

Total (100%) area by type and trade size

Trade size	EMT	PVC Sch 40	PVC Sch 80	RMC	IMC
1/2"	0.304	0.285	0.217	0.314	0.342
3/4"	0.533	0.508	0.409	0.549	0.586
1"	0.864	0.832	0.688	0.887	0.959
1-1/4"	1.496	1.453	1.237	1.526	1.647
1-1/2"	2.036	1.986	1.711	2.071	2.225
2"	3.356	3.291	2.874	3.408	3.630
2-1/2"	5.858	4.695	4.119	4.866	5.135
3"	8.846	7.268	6.442	7.499	7.922
4"	14.753	12.554	11.258	12.882	13.457

Table 5 — conductor area, in²

Including insulation

Size	THHN / THWN-2	XHHW-2
1 AWG	0.1562	0.1534
2 AWG	0.1158	0.1146
3 AWG	0.0973	0.0962
4 AWG	0.0824	0.0814
6 AWG	0.0507	0.0590
8 AWG	0.0366	0.0437
10 AWG	0.0211	0.0243
12 AWG	0.0133	0.0181
14 AWG	0.0097	0.0139
1/0 AWG	0.1855	0.1825
2/0 AWG	0.2223	0.2190
3/0 AWG	0.2679	0.2642
4/0 AWG	0.3237	0.3197
250 kcmil	0.3970	0.3904
350 kcmil	0.5242	0.5153
500 kcmil	0.7073	0.6984

Note 7 permits rounding up where a calculation for conductors all of the same size produces a decimal of 0.8 or larger. Annex C bakes that in, which is why a pre-computed Annex C count can read one higher than a strict area calculation. Our conduit fill calculator computes strictly, so it is conservative by one at those boundaries.

Conduit Fill Calculator → The method, step by step → Pre-computed fill charts →

314.16

Box Fill

Each conductor entering and terminating counts once. All internal cable clamps together count as one, at the largest conductor present. Each device yoke counts as two. All equipment grounding conductors together count as one. A conductor that both begins and ends inside the box — a pigtail — counts zero.

314.16(B) — volume per conductor

Conductor size	Free space required
6 AWG	5.00 in³
8 AWG	3.00 in³
10 AWG	2.50 in³
12 AWG	2.25 in³
14 AWG	2.00 in³
16 AWG	1.75 in³
18 AWG	1.50 in³

Common box volumes

Table 314.16(A) plus trade sizes

Box	Volume
3" × 2" × 2" device box	10.0 in³
3" × 2" × 2-1/4" device box	10.5 in³
3" × 2" × 2-1/2" device box	12.5 in³
3" × 2" × 2-3/4" device box	14.0 in³
3" × 2" × 3-1/2" device box	18.0 in³
Single-gang plastic box, 20.3 in³	20.3 in³
Single-gang plastic box, 22.5 in³	22.5 in³
4" square × 1-1/2" box	21.0 in³
4" square × 2-1/8" box	30.3 in³
4-11/16" square × 1-1/2" box	29.5 in³
4-11/16" square × 2-1/8" box	42.0 in³

Box Fill Calculator → What counts and what doesn't →

250.66 & 250.122

Grounding Conductors

Two different conductors from two different tables. The equipment grounding conductor is keyed to the overcurrent device; the grounding electrode conductor is keyed to the largest ungrounded service-entrance conductor, then capped by which electrode it lands on.

Table 250.122 — equipment grounding conductor

By OCPD rating. Note there is no 30 A row.

OCPD up to	Copper	Aluminum
15 A	14 AWG	12 AWG
20 A	12 AWG	10 AWG
60 A	10 AWG	8 AWG
100 A	8 AWG	6 AWG
200 A	6 AWG	4 AWG
300 A	4 AWG	2 AWG
400 A	3 AWG	1 AWG
500 A	2 AWG	1/0 AWG
600 A	1 AWG	2/0 AWG
800 A	1/0 AWG	3/0 AWG
1000 A	2/0 AWG	4/0 AWG
1200 A	3/0 AWG	250 kcmil

Table 250.66 — grounding electrode conductor

By largest service-entrance conductor

Copper service	Aluminum service	Cu GEC	Al GEC
2 AWG or smaller	1/0 AWG or smaller	8 AWG	6 AWG
1 or 1/0 AWG	2/0 or 3/0 AWG	6 AWG	4 AWG
2/0 or 3/0 AWG	4/0 AWG or 250 kcmil	4 AWG	2 AWG
over 3/0 through 350 kcmil	over 250 through 500 kcmil	2 AWG	1/0 AWG
over 350 through 600 kcmil	over 500 through 900 kcmil	1/0 AWG	3/0 AWG
over 600 through 1100 kcmil	over 900 through 1750 kcmil	2/0 AWG	4/0 AWG
over 1100 kcmil	over 1750 kcmil	3/0 AWG	250 kcmil

Then the caps: to a rod, pipe or plate the GEC need not exceed 6 AWG copper (250.66(A)); to a concrete-encased electrode, 4 AWG (250.66(B)); to a ground ring, no larger than the ring itself (250.66(C)). A metal water pipe has no cap.

Ground Wire Size Calculator → EGC vs GEC explained →

Sources & standards: all tables are reproduced from NEC (NFPA 70) 2023 — Table 310.16, Table 310.12, 310.15(B)(1), 310.15(C)(1), 240.4(D), 240.6(A), Chapter 9 Tables 1, 4, 5 and 8 with Notes 4 and 7, Annex C, 314.16 and Table 314.16(A), Table 250.66 with 250.66(A)–(C), and Table 250.122. Baseline is NEC 2023; most states are on the 2020 or 2023 cycle, and NEC 2026 was published in 2025. These are planning aids — local amendments override the model code, and a licensed electrician plus the AHJ have final say on anything installed.

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