Introduction
Electrical codes don’t exist to make wire sizing harder — they exist because undersized wire starts fires, and the rules encode a century of lessons about heat, terminations, and human error. The sizing math (ampacity, voltage drop) lives inside a framework of code requirements: which temperature column governs, when derating applies, how continuous loads are treated, and what your local authority having jurisdiction (AHJ) demands on top.
The Wire Size Calculator applies the standard ampacity and voltage-drop calculations. This guide covers the code framework around them — the NEC rules that shape every sizing decision, and why local rules always get the last word. Code citations here are descriptive, not legal advice: confirm everything with your AHJ and a licensed electrician.
The Core Sizing Rules
Table 310.16 and the temperature columns
NEC Table 310.16 lists conductor ampacity at 60°C, 75°C, and 90°C ratings. The governing principle: the lowest temperature rating in the circuit wins — wire, terminations, and equipment must all be rated for the column you use. Residential work typically lands on the 60°C column because NM-B cable and standard devices are 60°C-rated.
Derating: ambient temperature and conduit fill
Two conditions reduce a conductor’s effective ampacity:
- High ambient temperature — attics, rooftops, and hot climates. Correction factors scale the ampacity down as temperature rises.
- Conduit fill — more than three current-carrying conductors in a raceway. Adjustment factors reduce each conductor’s ampacity because bundled wires can’t shed heat.
Derating calculations start from the 90°C column ampacity, apply the correction factors, and the result must not exceed the termination column’s value. Both factors can apply simultaneously — and commonly do in attic conduit runs.
Continuous loads at 125%
Loads operating 3 hours or more (EV chargers, water heaters, fixed lighting, HVAC) are calculated at 125% of nameplate current for both conductor and overcurrent-device sizing. A 40A EV charger is a 50A design load — full stop.
Overcurrent protection limits
Branch-circuit rules cap protection at 15A for 14 AWG, 20A for 12 AWG, 30A for 10 AWG — even where the ampacity table lists higher numbers. The breaker protects the wire; the wire must never be the fuse.
Voltage Drop: Guidance, Not Mandate
The NEC addresses voltage drop in informational notes recommending 3% maximum on branch circuits and 5% total (feeder + branch). Informational notes are explanatory, not enforceable requirements — but treat them as the design target anyway, because:
- Equipment warranties and performance specs often assume reasonable voltage.
- Some local codes and project specifications do make drop limits mandatory.
- Chronic under-voltage shortens motor and appliance life — the physics doesn’t care about the note’s legal status.
The formula guide derives the calculation; the point here is regulatory status: design to 3%/5%, verify against local rules.
Permits, Inspections, and the AHJ
The AHJ has the final word
The NEC is a model code — it becomes law only as adopted (sometimes amended) by your state, county, or city. Your authority having jurisdiction decides which edition applies, what amendments exist, and what homeowner work is allowed. When this guide and your AHJ disagree, the AHJ wins.
The inspection sequence
Where permits are required, expect roughly:
- Permit application — scope of work, sometimes a plan review for larger jobs.
- Rough-in inspection — wiring installed but walls open; the inspector checks gauge, routing, boxes, and grounding.
- Final inspection — devices installed, covers on; everything energized and tested.
Closing walls before rough-in inspection is a classic failure — the inspector can’t verify what they can’t see.
Grounding and bonding
Equipment grounding conductors, sized from the overcurrent device rating, are required on every circuit. Grounding electrode systems and bonding at services and subpanels have their own detailed rules — beyond sizing scope, but non-negotiable at inspection. Flag them for your electrician early.
Worked Example: Sizing to Code on a Continuous Load
48A EV charger (continuous), 240V, 18 m (59 ft) one-way, copper, THWN-2 in conduit, 75°C-rated terminations verified.
Step 1 — Continuous load. 48 × 125% = 60A design current.
Step 2 — Ampacity at 75°C column. 6 AWG = 65A ✓ (60°C column would give only 55A — the verified 75°C terminations matter here). Breaker: 60A.
Step 3 — Voltage drop (6 AWG, 26,240 CM, actual 48A). VD = 2 × 12.9 × 48 × 59 ÷ 26,240 = 73,094 ÷ 26,240 = 2.79V → 2.79 ÷ 240 = 1.16% ✓.
Step 4 — Metric cross-check (6 AWG = 13.3 mm², 18 m). VD = 2 × 0.0215 × 48 × 18 ÷ 13.3 = 37.15 ÷ 13.3 = 2.79V ✓, identical.
Step 5 — Code review. Continuous-load rule applied ✓; termination column verified at 75°C ✓; conduit fill — if this conduit carries more than three current-carrying conductors, derating from the 90°C column (75A for 6 AWG) must still clear 60A ✓ in most cases, but run the actual factors.
Practical takeaway: code-compliant sizing is a checklist, not a single lookup — continuous-load factor, correct column, drop check, derating review. The sizing checklist encodes this sequence.
Common Code-Related Mistakes
- Assuming the latest NEC applies. Your jurisdiction may be an edition or two behind — check the adopted version.
- Skipping derating on “just a few extra wires.” Four current-carrying conductors in a conduit is already derating territory.
- 75°C terminations assumed, never verified. The marking is on the equipment — check it before using the column.
- No permit, no inspection. Unpermitted work fails at sale, claim, or fire-investigation time.
- Closing walls before rough-in. The inspector needs to see the wire.
- Grounding as an afterthought. Grounds are inspected as rigorously as hots.
More in the common mistakes guide; the legal boundaries of DIY are in DIY vs pro.
Related Guides
- Complete Guide — ampacity, voltage drop, and the sizing workflow.
- Sizing Checklist — the code-aware verification sequence.
- DIY vs Pro — permits and legal boundaries.
- Formula Explained — the voltage-drop math.
- Common Mistakes — errors and their fixes.
Frequently Asked Questions
Which NEC table gives wire ampacity?
Table 310.16 (in recent NEC editions) lists ampacity by gauge at 60°C, 75°C, and 90°C ratings, for copper and aluminum separately.
Is the 3% voltage drop limit legally required?
In the NEC it’s an informational recommendation, not an enforceable rule. Local codes or project specs can make it mandatory — check both.
What is an AHJ?
The Authority Having Jurisdiction — the local body (city, county, state) that adopts and enforces the electrical code. Their amendments and interpretations are final.
When does derating apply?
With high ambient temperatures or more than three current-carrying conductors in a raceway. Calculate from the 90°C column, capped by the termination column.
Do I need a permit to replace a breaker panel?
In virtually all jurisdictions, yes — panel work always requires permits, inspections, and usually a licensed electrician plus utility coordination.
Can local code be stricter than the NEC?
Yes — jurisdictions routinely amend the NEC with stricter local requirements. The local amendment always governs.
Get Your Number
Size to the code, not just the math: continuous loads at 125%, the correct temperature column, derating where it applies, drop within guidance, permits pulled, inspections passed. The Wire Size Calculator handles the arithmetic — your AHJ and a licensed electrician handle the authority. Never substitute a web tool for either.
Disclaimer: Calculator results are planning estimates. Code requirements vary by jurisdiction and NEC edition; descriptions here are educational, not legal advice. Electrical work is regulated — consult a licensed electrician and your local authority having jurisdiction (AHJ) for code-compliant design and installation.
