What Wire Size for a 20A Branch Circuit?
A 20 amp branch circuit โ kitchen small appliances, bathroom outlets, garage tools โ takes 12 AWG copper on a 20A breaker, full stop. Twelve-gauge THHN is rated 25A at 75C, but the breaker caps the circuit at 20A, and continuous loads (3+ hours) are limited to 80%, or 16A. This guide works the sizing, the derating exceptions, and the inspection points.
Step-by-Step Takeoff
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1
List the load: five 1800W general outlets at 120V draw 15A each worst-case, but a 20A circuit serves them together under diversity.
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2
Pick the breaker first: 20A for general receptacles; size the wire to match or exceed it โ 12 AWG copper (25A at 75C).
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3
Apply the 80% rule for continuous loads: a heater or grow light running 3+ hours may not exceed 16A on this circuit.
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4
Check ambient and bundling: above 30C or more than 3 current-carrying conductors in one conduit, derate โ often forcing 10 AWG.
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5
Verify terminal ratings: most receptacles are 60/75C โ size to the lowest-rated termination in the chain.
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6
Add AFCI protection where required (bedrooms, living areas) and GFCI in wet locations.
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7
Price the run (wire per foot plus breaker) in the Electrical Calculator and export the materials list.
Guide FAQ
Can I use 14 AWG on a 20A breaker?
No โ 14 AWG is limited to 15A circuits. The breaker must protect the smallest wire on the circuit.
When do I need 10 AWG instead of 12?
Long runs with voltage drop over 3%, hot attics above 40C, or 4+ conductors bundled in one conduit.
How many outlets on a 20A circuit?
Residential code allows generous counts under diversity, but plan ~8โ10 general receptacles so vacuum cleaners Plus heaters do not trip it.
What does the 80% rule mean exactly?
Continuous loads (3+ hours) may use only 80% of breaker rating: 16A on a 20A circuit, 12A on a 15A.
Aluminum instead of copper?
Possible with CO/ALR devices and anti-oxidant, but upsize one gauge and expect inspector questions.
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How Many Wires Fit in 3/4" PVC Conduit?
The 40% fill rule worked: THHN areas by gauge, derating limits, and conduit upsizing calls.
Is My 200A Panel Overloaded? Do the Math
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