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.
NEC Table 310.16. It lists allowable ampacity for insulated conductors rated up to 2000 volts with not more than three current-carrying conductors in a raceway or cable, at 30°C ambient, for copper and aluminum at 60°C, 75°C and 90°C.
Two different tables. Table 250.122 sizes the equipment grounding conductor from the overcurrent device rating. Table 250.66 sizes the grounding electrode conductor from the service-entrance conductors. They answer different questions and are frequently confused.
The values shown follow the 2020 and 2023 National Electrical Code, which share the same numbering for these tables. The 2017 edition numbered the derating tables differently — 310.15(B)(2)(a) and 310.15(B)(3)(a) instead of 310.15(B)(1) and 310.15(C)(1) — with the same values. Always confirm which edition your local jurisdiction has adopted.