What size wire for every amp rating, 15 to 4000 amps

Wire size chart by amps, 15 to 400 A

Each cell is the smallest conductor in NEC Table 310.16 whose ampacity at that temperature column is at least the circuit rating, with the small conductor caps of NEC 240.4(D) applied, so 12 AWG copper stops at 20 A and 10 AWG copper at 30 A. Up to 100 A use the 60°C columns unless every terminal on the circuit is marked 75°C. Over 100 A the 75°C columns apply, which is why the 60°C cells end there.

The chart assumes no more than three current carrying conductors in a raceway or cable at a 30°C (86°F) ambient. Hotter spaces, rooftops and bundles of more conductors need the correction and adjustment factors on the wire ampacity chart, and the result can be a size larger.

Computed from NEC Table 310.16 values and the 240.4(D) limits listed on our wire ampacity chart. Aluminum is not used at 14 AWG. Type NM cable is held to the 60°C column whatever the breaker size.

Dwelling services and feeders, the 83 percent rule in NEC 310.12

For a single family dwelling, NEC 310.12 lets the service conductors, or a feeder that carries the whole load of the dwelling unit, be sized at 83 percent of the service rating. The reason is load diversity, every load in a house is never on at once. A 200 A service therefore needs 166 A of conductor, which 2/0 copper at 175 A or 4/0 aluminum at 180 A covers. That is why both answers appear on every 200 amp wire size page.

The rule applies from 100 to 400 A at 120/240 V single phase, and only when the conductors carry the entire load of the dwelling. A feeder to a garage or basement subpanel that carries part of the load is sized at 100 percent from the first table. The 2026 NEC adds that the 83 percent sizing may only be used with conductors rated 75°C or more, which in practice rules out Type NM cable for it. Correction and adjustment factors still apply.

Computed as 83 percent of the rating, then the smallest 75°C conductor in Table 310.16 at or above it. This reproduces the published 200 A answer of 2/0 copper or 4/0 aluminum.

Commercial services from 400 to 4000 A in parallel sets

Large services and the secondaries of pad mounted transformers are run in parallel sets, several identical conductors per phase, each set in its own raceway. The NEC paralleling rule (310.4 in the 2005 code, renumbered in later editions) allows copper or aluminum in size 1/0 and larger to be paralleled, and every conductor in a phase must match in material, length, insulation and size. Running each set in its own conduit keeps it at three current carrying conductors, so no bundling derating applies.

The table gives the fewest sets that reach the rating at the 75°C column, with each conductor capped at 600 kcmil copper or 750 kcmil aluminum. Other combinations work too, for example 2 sets of 250 kcmil aluminum at 205 A each is a common way to build a 400 A service.

Computed from Table 310.16 at 75°C, 30°C ambient, three current carrying conductors per raceway. No next size up rounding is used, the total always meets or exceeds the rating.

Ground wire to pair with the circuit

Two different ground conductors are sized two different ways. The equipment grounding conductor (EGC), the wire run with the circuit, comes from NEC Table 250.122 and is keyed to the rating of the breaker or fuse. The grounding electrode conductor (GEC), from the service to the ground rods or water pipe, comes from NEC Table 250.66 and is keyed to the size of the service conductors.

With parallel sets in separate raceways, NEC 250.122(F) requires a full size EGC in every raceway, sized on the breaker that protects the whole set. The 1/0 minimum for paralleled conductors does not apply to the EGC.

Rows quoted from the sources listed below. For a 200 A house service with 2/0 copper, the GEC is commonly 4 AWG copper, and with 4/0 aluminum 2 AWG copper or 1/0 aluminum. Check both tables in the edition your inspector enforces.

Subpanels and common circuits

A 60 A subpanel feeder needs 4 AWG copper on 60°C terminals or 6 AWG copper where the breaker and panel lugs are both marked 75°C. A 100 A subpanel needs 1 AWG copper at 60°C or 3 AWG copper at 75°C, and 1/0 or 1 AWG aluminum. A 125 A feeder to a subpanel that does not carry the whole house load takes 1 AWG copper or 2/0 aluminum at 75°C, not the 83 percent sizes.

A 50 A range or welder circuit on 60°C terminals needs 6 AWG copper, and a 30 A dryer or water heater circuit 10 AWG copper.

Long runs and voltage drop

Ampacity sets the minimum, voltage drop often sets the real size. The NEC recommends, in informational notes rather than rules, keeping feeder drop to 3 percent and the total of feeder and branch circuit to 5 percent. On a long service lateral or a feeder to a detached building, run the drop calculation and go up a size or two if it is over. One common example is upsizing a long 200 A service from 2/0 to 3/0 copper, or from 4/0 to 250 kcmil aluminum.

Which transformer serves each service size

A three phase service draws its current from a pad mounted transformer, and the transformer kVA must at least match the service. kVA equals amps x volts x 1.732 divided by 1,000. The table gives the kVA each service represents at full rating and the smallest standard pad mount rating that covers it. Real services are sized on the calculated load, so the transformer is often smaller than the full service rating, and the utility or engineer makes that call.

Computed. Standard three phase pad mounted ratings run 45 to 2,500 kVA with 3,000 to 5,000 kVA units built as large pad mounts or unit substations.

What size wire for 200 amp service

For a single family house, 2/0 AWG copper or 4/0 AWG aluminum, because NEC 310.12 lets dwelling service conductors be sized at 83 percent of the rating, 166 A. For a 200 A commercial service or a subpanel feeder that does not carry the whole house load, the 100 percent rule applies and you need 3/0 copper or 250 kcmil aluminum at 75°C.

What size wire for 100 amps

At 100 percent, 1 AWG copper on 60°C terminals or 3 AWG copper where both ends are marked 75°C, and 1/0 or 1 AWG aluminum. As the main service to a house under the 83 percent rule, 4 AWG copper or 2 AWG aluminum.

What size wire for 50 amps

6 AWG copper. It is rated 55 A at 60°C and 65 A at 75°C, so it covers 50 A on either kind of terminal. In aluminum, 4 AWG at 60°C or 6 AWG at 75°C.

What size wire for 60 amps

4 AWG copper on 60°C terminals or 6 AWG copper on 75°C terminals. In aluminum, 3 AWG at 60°C or 4 AWG at 75°C.

What size wire for 30 amps

10 AWG copper. 10 AWG is capped at 30 A by NEC 240.4(D) even though its table ampacity is higher. In aluminum, 8 AWG.

What size wire for a 400 amp service

Usually two parallel sets. For a house under the 83 percent rule you need 332 A, which 2 sets of 250 kcmil aluminum or 2 sets of 3/0 copper cover. At 100 percent for a commercial service, a single 600 kcmil copper at 420 A, 2 sets of 4/0 copper at 460 A, or 2 sets of 250 kcmil aluminum at 410 A.

Can I use the 83 percent rule for a subpanel

Only if the subpanel feeder carries the entire load of the dwelling unit. A feeder to a garage or basement subpanel that serves part of the house is sized at 100 percent.