The National Electrical Code tables that get looked up by number, reproduced in full with the code sections that modify them. Values follow the 2020 and 2023 NEC.
Allowable ampacity of insulated conductors, copper and aluminum, at 60°C, 75°C and 90°C. The starting point for every branch circuit and feeder.
Ambient temperature correction and the derating factors for more than three current-carrying conductors in a raceway — 80%, 70%, 50% and below.
Sizes the conductor from the service enclosure to the ground rods, water pipe, or Ufer, based on the largest service-entrance conductor.
Sizes the fault-current path from the rating of the overcurrent device — 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG up to 60 A.
Minimum cover for underground wiring: 24 inches direct burial, 18 inches PVC, 6 inches rigid metal, 12 inches for GFCI-protected residential circuits.
| NEC reference | Answers |
|---|---|
| 310.16 | What ampacity does this conductor have? |
| 310.15(B)(1) | It is hotter than 30°C — how much ampacity do I lose? |
| 310.15(C)(1) | More than three current-carrying conductors in the pipe — derate by how much? |
| 240.4(D) | What is the biggest breaker allowed on 14, 12 or 10 AWG? |
| 250.66 | What size wire goes to the ground rods? |
| 250.122 | What size ground wire goes in this circuit? |
| 300.5 | How deep do I have to bury it? |
| Chapter 9, Tables 1, 4, 5 | How many wires fit in this conduit? |
On code editions: the NEC is published every three years, but adoption is a state and local decision and often lags by a full cycle or more. Some jurisdictions also amend specific articles. These pages are a reference, not a substitute for the adopted code book in your area or for a licensed electrician.