Reference · Electrical · NEC 310.16 · NEC 240.6 · NEC 314.16 · ASTM B258

Size — Wire, Breaker, Conduit & Box Sizing Reference

A printable size chart for the four sizing decisions every electrical job depends on: the AWG size chart (4/0 down to 40 AWG with ampacity at 60/75/90 °C insulation), standard breaker size values per NEC 240.6, conduit size per NEC Chapter 9 Tables 4 and 5, and box size by NEC 314.16 fill. The embedded wire-size calculator picks an AWG and matching breaker for any load, and the worked examples show how the four sizing decisions chain together for a real branch circuit. Reviewed by a licensed PE.

Figure 1 — Fourteen AWG sizes drawn at true relative diameter — from 4/0 (11.7 mm) to 24 AWG (0.51 mm). ASTM B258.

Wire size + breaker size calculator

The breaker size calculator below combines NEC 310.16 ampacity, the standard breaker and fuse list from NEC 240.6, and conductor temperature derating. Enter your load and run length, and the calculator returns the AWG, the standard breaker rating, and the voltage drop. It works for single-phase or three-phase circuits with copper or aluminium conductors, so the same tool covers determining breaker size, calculating fuse size, and 3-phase breaker sizing. For HVAC equipment use the dedicated MCA / MOP calculator instead.

CALC.003 Wire Size · NEC 310.16 · 6 presets · ampacity + VD

Application[App]

NEC 210.19(A) recommends ≤3% VD on branch, ≤5% combined feeder + branch.

Material[Mat]

Size standard[Std]

Insulation rating[°C]

Ambient temp[Ta]

°C

Load current[I]

or power[P]

System voltage[V]

Power factor[cos φ]

cos φ

Length one-way[L]

VD limit[%]

Conductors in raceway[N]

Parallel sets[Par]

Continuous load (3+ hrs at full current → ×1.25 NEC 210.19)

Recommended size

12 AWG

Both ampacity and voltage drop pass with margin.

0%3%6%

Voltage drop: — V (—%)
Ampacity (derated): —
Required ampacity: —
Recommended OCPD: —
Min EGC (NEC 250.122): —
Power loss in run: — W
V at load: — V

PASS · NEC 310.16 + 210.19(A)

A_min = max( A_vd , A_ampacity ) · NEC 310.16 NEC 240.4(D) · 250.122 · 310.15(B)

Sizing formulas

Eq. 01 — AWG diameter (Brown & Sharpe) SI

d_n = 0.005 \cdot 92^{(36 - n)/39} \,\, \text{inches}

·: n = AWG number (smaller → larger wire). Range −3 (= 0000 = 4/0) to 40.
·: d₀ ≡ 0.460" ; d₃₆ ≡ 0.005". The 92^(1/39) ratio fixes 39 size steps between them.
·: Each 6 AWG step roughly halves the diameter; each 3 AWG step roughly halves the area.

Eq. 02 — Cross-sectional area (kcmil) SI

A_{kcmil} = 1{,}000 \times d^2 \,\, \text{(d in inches)}; \quad A_{mm^2} = A_{kcmil} \times 0.5067

·: Circular mil = area of a circle 1 mil (0.001") in diameter; defined to make round-wire algebra integer-friendly.
·: 1 kcmil = 1 000 cmil = 0.5067 mm² exactly.
·: NEC ampacity tables are indexed by kcmil for sizes ≥ 250 kcmil.

Eq. 03 — Continuous-load sizing (NEC 210.19, 215.2) SI

I_{wire} \geq 1.25 \cdot I_{cont} + I_{noncont}; \quad I_{breaker} \geq 1.25 \cdot I_{cont} + I_{noncont}

·: Continuous load = ≥ 3 hours at maximum current (lighting, EV charging, electric heat).
·: Wire sized to the 75 °C column unless terminations are 60 °C-rated.
·: Round up to next standard NEC 240.6 breaker / fuse rating.

Standards governing sizing

Sizing in North America is governed by the NFPA 70 / National Electrical Code (NEC); the metric world (most of Europe, ANZ, Asia) follows IEC and ISO. The four sizing decisions reference different code articles:

Standard	What it covers	Region
NFPA 70 / NEC 310.16	Wire ampacity (60 / 75 / 90 °C) for 0–2 000 V conductors in raceway / direct buried	North America
NEC 240.6(A)	Standard fuse and inverse-time breaker sizes — the only ratings you may install	North America
NEC Chapter 9, Tables 4 & 5	Conduit fill: cross-sectional area of every conduit and conductor type	North America
NEC 314.16	Box-fill calculation in cubic inches	North America
ASTM B258	Standard nominal diameters of solid round copper wire (AWG sizes)	USA
IEC 60228	Conductor cross-sectional areas (mm²) — metric replacement for AWG	Worldwide (non-NEC)
IEC 61386	Conduit systems for cable management — metric trade sizes 16–63 mm	EU / IEC countries
BS 7671 (UK)	UK wiring regulations — uses IEC 60228 mm² and BS EN 50525 cable types	United Kingdom

AWG size chart — full table

The pillar reference: every AWG size from 4/0 (largest practical) to 40 (hair-thin), with diameter, area, and ampacity for the most common copper insulations at 75 °C (THW, THWN, THHN dry). For aluminium ampacity, multiply column "75 °C Cu" by ≈ 0.78. For 90 °C (THHN dry, XHHW-2) ampacity use the "90 °C" column. 4mm size ≈ 11 AWG; size 6 mm / size of 6mm ≈ 9 AWG.

AWG	Ø (in)	Ø (mm)	Area (kcmil)	Area (mm²)	60 °C Cu (A)	75 °C Cu (A)	90 °C Cu (A)
4/0	0.4600	11.68	211.6	107.2	195	230	260
3/0	0.4096	10.40	167.8	85.0	165	200	225
2/0	0.3648	9.27	133.1	67.4	145	175	195
1/0	0.3249	8.25	105.6	53.5	125	150	170
1	0.2893	7.35	83.69	42.4	110	130	145
2	0.2576	6.54	66.36	33.6	95	115	130
3	0.2294	5.83	52.62	26.7	85	100	115
4	0.2043	5.19	41.74	21.2	70	85	95
6	0.1620	4.11	26.24	13.3	55	65	75
8	0.1285	3.26	16.51	8.37	40	50	55
10	0.1019	2.59	10.38	5.26	30	35	40
12	0.0808	2.05	6.530	3.31	20	25	30
14	0.0641	1.63	4.107	2.08	15	20	25
16	0.0508	1.29	2.583	1.31	—	—	18*
18	0.0403	1.02	1.624	0.823	—	—	14*
20	0.0320	0.812	1.022	0.518	—	—	11*
22	0.0253	0.644	0.6424	0.326	—	—	7*
24	0.0201	0.511	0.4040	0.205	—	—	3.5*
26	0.0159	0.405	0.2540	0.129	—	—	2.2*
28	0.0126	0.321	0.1597	0.0810	—	—	1.4*
30	0.0100	0.255	0.1003	0.0509	—	—	0.86*
40	0.00314	0.0799	0.00989	0.00501	—	—	0.09*

* Sizes 16 AWG and smaller are not in NEC 310.16 — values shown are typical fixture-wire / chassis-wire ampacities (NEC 402.5 / chassis-wiring practice). Power circuits in NEC scope start at #14 AWG (15 A) for general use, #12 AWG (20 A) for required minimums on most receptacle circuits.

Standard breaker / fuse sizes — NEC 240.6(A)

The NEC restricts protective devices to a fixed list. Determining breaker size is therefore a "round up to the next allowed value" operation, never an arbitrary number. Use this table together with the AWG chart above.

Bracket	Standard ratings (A)
Branch (residential)	15, 20, 25, 30, 35, 40, 45, 50, 60
Feeder / sub-panel	70, 80, 90, 100, 110, 125, 150, 175, 200, 225
Service	250, 300, 350, 400, 450, 500, 600, 700, 800
Industrial	1 000, 1 200, 1 600, 2 000, 2 500, 3 000, 4 000, 5 000, 6 000

Compute load current (A) — start with the load, not the wire. Single-phase: I = P / (V × PF). 5 kW resistive heater on 240 V: I = 5 000 / 240 = 20.8 A. Three-phase motor: use full-load amps from NEC 430.250 or motor nameplate. Continuous loads (≥ 3 hours) are sized at 125 % per NEC 210.19(A) — so 20.8 A continuous → design for 26 A.
Pick the wire size from NEC 310.16 (or IEC 60228). Match the design current to the 75 °C column of NEC Table 310.16 for copper THWN/THHN. 26 A → #10 AWG copper (35 A rated). Round up if voltage drop on the run > 3 % — see /wire-size/ for the full sizing engine. Aluminium needs about two AWG sizes larger for the same ampacity.
Pick the breaker / fuse size from NEC 240.6. Standard breaker sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 400, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 A. Pick the smallest standard size at or above the design current that does not exceed the wire ampacity. 26 A design current → 30 A breaker on #10 AWG. Continuous loads: breaker rated 125 % of load.
Pick the conduit size from NEC Chapter 9 Tables 4 and 5. Compute total conductor cross-sectional area (sum of THWN areas in in²), then look up the smallest conduit ≥ that area at 40 % fill (3+ conductors). Three #10 THWN = 3 × 0.0211 = 0.0633 in² → ½" EMT (0.122 in² @ 40 %) is sufficient. Use 1" EMT for three #4 THWN.
Pick the box size from NEC 314.16 (box-fill calculation). Sum allowances: each #14 = 2.0 in³, each #12 = 2.25 in³, each #10 = 2.5 in³, each #8 = 3.0 in³. Add one allowance per device (counted as the largest conductor connected). 4 × #12 wires + 1 receptacle = 5 × 2.25 = 11.25 in³ — needs a 4" square × 1½" deep box (21.0 in³) or 3" × 2" × 2¾" device box (12.5 in³).

Worked example — 60 A oven branch circuit

A built-in electric range is rated 11 kW at 240 V single-phase, served from a residential subpanel 50 ft away. Walk through all four sizing decisions:

Step 1 — load current: I = 11 000 / 240 = 45.8 A. NEC 220.55 demand factors apply for ranges ≥ 8.75 kW; for a single 11 kW range demand = 8 kW per Column A → I_demand = 8 000 / 240 = 33.3 A. Use 33.3 A as the design current (for ranges only — most other appliances use full nameplate).

Step 2 — wire size: from NEC 310.16, 75 °C Cu column: 33.3 A → #8 AWG copper THWN-2 (50 A rated) is sufficient. Voltage drop on 50 ft round-trip at 33 A through #8 (R = 0.778 Ω/kft) = 33 × 2 × 50 × 0.778 / 1000 = 2.6 V (1.1 % — well within 3 %).

Step 3 — breaker size: per NEC 422.10(A), branch ≥ 100 % nameplate (no continuous-load multiplier — ranges are not continuous). 33.3 A demand → next standard NEC 240.6 size = 40 A breaker. The breaker can also be sized up to the wire ampacity (50 A) per NEC 240.4(B)(3). A 40 A or 50 A breaker is code-compliant.

Step 4 — conduit size: 3 × #8 THWN (the two ungrounded + grounded) = 3 × 0.0366 = 0.1098 in². NEC Chapter 9 Table 4 — ½" EMT @ 40 % fill = 0.122 in² (just sufficient); ¾" EMT @ 40 % = 0.213 in² (recommended).

Step 5 — box size: junction box for the range whip with three #8 conductors + grounding: 3 × 3.0 + 1 × 3.0 = 12.0 in³ minimum. A 4" square × 1½" deep box (21.0 in³) is more than enough.

AWG vs metric (mm²) — when to use which

Outside North America, conductor sizes are specified in cross-sectional area (mm²) per IEC 60228. The two systems do not map cleanly — common conversions:

AWG	Area (mm²)	Closest IEC 60228 size	IEC ampacity (≈ 30 °C, 70 °C insul.)
4/0	107.2	120 mm²	~ 240 A
1/0	53.5	50 mm²	~ 145 A
2	33.6	35 mm²	~ 110 A
4	21.2	25 mm²	~ 85 A
6	13.3	16 mm²	~ 65 A
8	8.4	10 mm²	~ 50 A
10	5.3	6 mm²	~ 35 A
12	3.3	4 mm²	~ 25 A
14	2.1	2.5 mm²	~ 18 A
16	1.3	1.5 mm²	~ 13 A

Ampacities differ because the IEC uses a 30 °C ambient and shorter laying-condition tables (BS 7671 Tables 4D1A through 4F3A); NEC 310.16 uses 30 °C ambient but groups by raceway type. Always size with the local code, never via cross-conversion.

Variants and special sizing cases

Breaker size — for AC, oven, heat pump, transformer

Residential air-conditioning condensers run roughly 20 A on #12 AWG for a 2-ton unit, 30 A on #10 for a 3-ton, 40 A on #8 for a 4-ton, and 50 A on #6 for a 5-ton — but always use the equipment MOP from the nameplate, not a generic rule. Electric ovens follow a similar pattern: 30 A on #10 for a 7 kW range, 40 A on #8 for ranges up to 11 k