What size aluminum wire do I need for a 200 amp residential service?

4/0 AWG aluminum. NEC 310.12 allows dwelling service conductors at 83 percent of the rating, which is 166 amps, and 4/0 aluminum is rated 180 amps at 75 degrees in Table 310.16.

When does a 200 amp service require 250 kcmil aluminum?

When the 83 percent dwelling rule does not apply, such as commercial services or feeders that do not carry the entire dwelling load. The conductor must then carry the full 200 amps, which requires 250 kcmil aluminum at 75 degrees.

What Size Aluminum Wire for a 200 Amp Service at 75 Degrees?

A user asked the app for the wire size on a 200 amp service using aluminum conductors with 75 degree terminations. For a single family dwelling service, the answer is 4/0 AWG aluminum. For anything that does not qualify for the dwelling allowance, the answer grows to 250 kcmil. Knowing which rule you are under is the whole game.

The 83 percent dwelling rule

NEC 2023 310.12(A) lets the service conductors for a one family dwelling, or the individual dwelling services of a two family or multifamily building, be sized at 83 percent of the service rating. For 200 amps that is 166 amps. Table 310.16 in the 75 degree column gives 4/0 aluminum an ampacity of 180 amps, which comfortably covers 166. That is why 4/0 SER or 4/0 XHHW aluminum is the standard 200 amp residential service and feeder conductor across the country. The same 83 percent allowance applies to the feeder between the meter main and an interior panel when it carries the entire dwelling load, per 310.12(B).

When 4/0 is not enough

Strip away the dwelling allowance and the conductors must carry the full 200 amps on their own. In the 75 degree column, 4/0 aluminum at 180 amps falls short, and the next standard size that works is 250 kcmil aluminum at 205 amps. That applies to commercial services, feeders to detached shops and barns sized at 200 amps, and any installation where the conductor does not supply the entire load of a dwelling. Voltage drop on long runs can push the size up again; past roughly 150 feet it is worth running the numbers before ordering wire.

Grounding and neutral sizes that ride along

With 4/0 aluminum service conductors, Table 250.66 calls for a 2 AWG aluminum or 4 AWG copper grounding electrode conductor. The neutral can be reduced below the ungrounded conductors only as far as the calculated unbalanced load from 220.61 allows, and never smaller than the grounding electrode conductor size from 250.24(C)(1). Aluminum terminations get antioxidant paste where the manufacturer requires it, and every lug must be torqued to spec, which 110.14(D) now makes an enforceable requirement, not a suggestion.

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