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NEC Quick-Reference Guide

A working-reference summary of commonly used National Electrical Code tables and rules of thumb. Based on the 2023 NEC. Always verify against the current adopted code cycle and local amendments before applying to a real design.

Conductor Ampacity (Copper, 3 or fewer in raceway)

NEC Table 310.16

AWG / kcmil	60°C (140°F)	75°C (167°F)	90°C (194°F)
14	15 A	20 A	25 A
12	20 A	25 A	30 A
10	30 A	35 A	40 A
8	40 A	50 A	55 A
6	55 A	65 A	75 A
4	70 A	85 A	95 A
2	95 A	115 A	130 A
1/0	125 A	150 A	170 A
2/0	145 A	175 A	195 A
4/0	195 A	230 A	260 A
250 kcmil	215 A	255 A	290 A
500 kcmil	320 A	380 A	430 A

Values assume 30°C ambient, THHN/THWN-2 or equivalent 90°C-rated insulation. Apply ambient-temperature and conductor-count derating (Table 310.15(B) & (C)) for anything outside this base case, and terminate at the temperature rating of the equipment (usually 60°C ≤100 A, 75°C >100 A per 110.14(C)).

Conduit Fill

NEC Chapter 9, Table 1

# of conductors	Max fill
1	53%
2	31%
3 or more	40%

Fill is based on conductor OD (with insulation), not copper area. Use Annex C tables for a direct "conductors per trade size" lookup once conductor type and conduit type are known.

Box Fill

NEC 314.16(B)

Item	Volume allowance
Each conductor	1× conductor size
All grounds (combined)	1× largest ground
Each device (2 max csv)	2× largest conductor
Internal clamp	1× largest conductor

"Volume allowance" is per conductor size, then summed against the box's rated cubic-inch capacity (Table 314.16(A)).

Grounding Electrode & Equipment Grounding Conductors

NEC Table 250.66 (GEC) & Table 250.122 (EGC)

Service / OCPD rating	Copper GEC	Copper EGC
≤100 A	#8 AWG	#8 AWG
≤200 A	#4 AWG	#6 AWG
≤400 A	#2 AWG	#3 AWG
≤600 A	#1/0 AWG	#1 AWG
≤1000 A	#2/0 AWG	#2/0 AWG

GEC sizing is driven by the largest ungrounded service-entrance conductor (250.66). EGC sizing is driven by the rating of the overcurrent device ahead of the equipment (250.122), and is a separate calculation — don't conflate the two.

Voltage Drop

VD = (2 × K × I × D) ÷ CM Single-phase. K ≈ 12.9 (copper) or 21.2 (aluminum) · I = load current (A) · D = one-way distance (ft) · CM = conductor circular-mil area. For three-phase, multiply by 0.866 instead of 2.

NEC doesn't mandate a voltage-drop limit outright, but the informational note under 210.19(A) and 215.2(A) recommends ≤3% on branch circuits or feeders, and ≤5% combined feeder + branch circuit.

Rules of Thumb Worth Remembering

Continuous loads (on 3+ hours): size conductor and OCPD at 125% of continuous load, per 210.19(A)(1) / 215.2(A)(1).
Standard OCPD sizes: 240.6(A) — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300…
Small-conductor rule: 14 AWG copper OCPD max 15A, 12 AWG max 20A, 10 AWG max 30A (240.4(D)).
GFCI required: kitchens, bathrooms, garages, outdoors, unfinished basements, crawl spaces, within 6 ft of a sink (210.8).
AFCI required: most 120V, 15/20A branch circuits supplying dwelling-unit outlets (210.12).
Working clearance: typically 36 in depth (Condition 1) in front of electrical equipment 0–150V to ground (110.26(A)).
Panel dedicated space: 6 ft above the panel, width of the panel, kept clear of foreign systems (110.26(E)).