Wire Sizing Guide: From 14 AWG to 200A Service Cable

Getting wire size right is fundamental to every electrical job. Here's your quick reference guide, based on the NEC and real-world experience.

Standard NM-B Ampacity (60°C Column)

Wire Size	Ampacity	Typical Use

14 AWG	15A	General lighting, bedroom receptacles
12 AWG	20A	Kitchen, bathroom, laundry receptacles
10 AWG	30A	Water heaters, A/C condensers
8 AWG	40A	Range circuits, small sub-panels
6 AWG	55A	Larger sub-panels, EV chargers

Service Entrance Sizing

For residential service conductors, NEC 310.12 allows the 83% rule: - 100A service → #4 copper or #2 aluminum - 200A service → 2/0 copper or 4/0 aluminum

The 80% Rule for Continuous Loads

For continuous loads (running 3+ hours), your breaker must be sized at 125% of the load. In practice, this means you can only use 80% of a breaker's rating: - 15A breaker → 12A max continuous - 20A breaker → 16A max continuous - 30A breaker → 24A max continuous

Voltage Drop: The 3% Rule

NEC recommends voltage drop not exceed 3% for branch circuits. Here's when to upsize:

Distance (ft)	15A Circuit	20A Circuit	30A Circuit

0-50	14 AWG	12 AWG	10 AWG
50-100	12 AWG	10 AWG	8 AWG
100-150	10 AWG	8 AWG	6 AWG

Common Inspection Failures

1. Using 14 AWG on a 20A breaker — instant fail. If you see 14 AWG wire in a panel, it MUST be on a 15A breaker. 2. Overfilled conduit — use a conduit fill calculator. Don't eyeball it. 3. NM cable in wet locations — Romex is NOT rated for wet locations. Use UF-B outdoors.

Pro Tip: Always Label Your Wires

Sharpie the breaker number on the inside of every switch and receptacle plate. Your future self (and the next electrician) will thank you.