NEC 310.12 Table: Wire Sizes For 100-400 Amp Services (83% Rule)

310.12 NEC table discusses copper and aluminum wire sizes for 100 amp to 400 amp services and feeders. This relates to most households (National Electrical Code refers to homes using these terms: one-family dwellings, 2-family dwellings, and multifamily dwellings). Namely, the NEC section 310.12 includes these 5 distinct parts:

1. Tabel 310.12 Single-Phase Dwelling Services and Feeders. This table tells you what gauge wire (AWG or kcmil) you need to use for services and feeders ranging from 100 amps to 400 amps current. It gives us the AWG or kcmil number for copper and aluminum wires (same as copper-clad aluminum wires). We will look at the complete table further on.
2. 310.12 (A) Services part.  This is the NEC 83% rule for wire ampacity that states “… shall be permitted to have an ampacity not less than 83 percent of the service rating.” This simply means that we can use a smaller wire if it can carry at least 83% of the amp load. Example: 3 AWG copper wire has an ampacity of 100 amps and 4 AWG copper wire has an ampacity of 85 amps. Which wire to choose for 100 amp service? According to the 83% NEC rule, a wire for 100 amp service has to handle at least 83 amps (have at least an 83A ampacity). That means that we don’t have to use the 4 AWG wire; we can use the 3 AWG wire because it has 85A ampacity which is higher than the 83A required ampacity.
3. 310.12 (B) Feeders part. Same as the (A) section but for feeders. In the 100 amp to 400 amp range, we can use thinner wires for feeders that have at least 83% capacity of the feeder specified size.
4. 310.12 (C) Feeder Ampacities part. Specifies that “In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).” Pretty self-efficient; the 83% rule can be observed.
5. 310.12 (D) Grounded Conductors part. This part states that “Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors, if the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.” In short, if we have a grounded wire, we can use a smaller wire size than for ungrounded wires.

Obviously, the most useful part of the NEC 310.12 section is the conductor or wire size for 100A – 400A services or feeders. Here is the whole table:

310.12 Table For Wire Sizes (100-400 Amp Service, Feeders)

In this table, we can see the NEC 83% rule in action:

Let’s take 200 amp service as an example. You will notice that we don’t have to use wires that have a 200A ampacity or higher. In fact, when applying the 83% rule, we see that 200 amp service wire can have as little as 200A × 0.83 = 166A ampacity.

Example: Here are two screenshots of wire ampacity charts for copper wires and aluminum wires in the vicinity of 200A ampacity (we are looking at the median ampacity at 75°C (167°F)):

We hope that the 310.12 table and this example illustrate well how the 83% NEC rule works (for 100 amp to 400 amp services and feeders). If you have any questions relating to this topic, you can use the comment section below, give us some numbers/notes, and we will help you out.