What Size Wire for a 100 Amp Service 200 ft Away? (and at 100 or 300 ft)

For 100 A at 240 V, 200 ft one way, with voltage drop held to 3%: 1 AWG copper or 2/0 AWG aluminum. At that distance voltage drop sets the size. Ampacity alone would allow 3 AWG copper.

The wire is the easy half. The half people get wrong is the ground, and that depends on whether this run is a service or a feeder, which comes down to where the main disconnect sits.

The assumptions

• 120/240 V single-phase, 100 A, distance measured one way.
• 90 °C insulation (THHN/THWN-2, XHHW-2, or USE-2/XHHW-2 underground) landing on terminals marked 75 °C. 110.14(C)(1)(a) holds circuits of 100 A or less to the 60 °C column unless the equipment is marked for 75 °C, so read the label on the lugs (many are marked 60/75 °C).
• 30 °C ambient, no more than three current-carrying conductors together, non-continuous load.
• Voltage drop by the approximate-K method, held to 3%. The informational notes to 210.19(A) and 215.2(A) suggest 3% for a feeder and 5% for feeder and branch circuit together. They’re advice, not requirements, unless your spec or jurisdiction adopts them.

Size it in three steps

Ampacity. In the 75 °C column of Table 310.16, 100 A takes 3 AWG copper (good for 100 A) or 1 AWG aluminum. Distance doesn’t change this.

Voltage drop. On 3 AWG copper, 100 A over 200 ft drops 9.81 V, or 4.09% of 240 V. Go up a size at a time until it’s under 3%: the first copper size that gets there is 1 AWG, at 2.57%.

Take the larger. Ampacity says 3 AWG, voltage drop says 1 AWG: 1 AWG copper.

100 A at 100 to 300 ft

* Voltage drop sets the size; without a star, ampacity does.

At 100 ft, 3 AWG copper does it (ampacity governs). By 300 ft it’s 2/0 AWG copper or 4/0 AWG aluminum. On runs this long, price aluminum before you order: the size goes up, but it is often the cheaper pull.

Allow the full 5% on this one run and 200 ft needs only 3 AWG copper (ampacity governs), but then every branch circuit in the building starts with its drop budget already spent.

Service or feeder? It decides the ground

Service conductors run from the utility’s point of connection to the service disconnect. If the meter is on a pole 200 ft out and the main breaker is in the house, those 200 ft are service conductors: two hots and a neutral, no equipment grounding conductor. Table 250.122 doesn’t apply. At the house you size the grounding electrode conductor from the service conductors with Table 250.66: 6 AWG copper for 1 AWG copper service conductors, 6 AWG copper for 2/0 AWG aluminum. A GEC that only goes to ground rods never has to be larger than 6 AWG copper (250.66(A)).

Feeders start at the load side of the service disconnect. If the service disconnect is at the meter (a meter-main), the run to the house is a feeder: two hots, a neutral and an EGC, with neutral and ground kept apart in the house panel (250.24(A)(5)) and a grounding electrode at the building (250.32). A subpanel is always this case.

Read the label on the outdoor device before you decide. 230.85 requires an outdoor emergency disconnect on one- and two-family dwellings, but it can be installed as the service disconnect or as a device marked “NOT SERVICE EQUIPMENT”. With the second kind, the conductors to the house are still service conductors.

Upsizing the feeder EGC: 250.122(B)

A 100 A feeder normally needs an EGC of 8 AWG copper (Table 250.122). Upsize the hots for voltage drop and 250.122(B) makes the wire-type EGC grow by the same circular-mil ratio. 3 AWG to 1 AWG copper is ×1.59, so the EGC becomes 4 AWG copper. On the aluminum version (1 AWG → 2/0 AWG) the aluminum EGC becomes 4 AWG. The Grounding calculator works out any combination.

Leaving the EGC at the table size after upsizing the hots is the mistake on this kind of job. It’s also one an inspector can check from the sizes printed on the jacket.

The dwelling exception: 310.12

For a 120/240 V service, or a feeder, that carries the entire load of a dwelling unit, 310.12 lets the conductors be sized at 83% of the rating. For 100 A that’s 83 A: 4 AWG copper or 2 AWG aluminum by ampacity. A feeder to a garage, shop or barn doesn’t qualify, because it doesn’t carry the whole dwelling.

It covers ampacity only. On a long run it rarely matters, because voltage drop already sets the size at 200 ft. It does change the 250.122(B) arithmetic: 250.122(B) measures the increase from the minimum size with enough ampacity, and under 310.12 that minimum is 4 AWG. Read that way, going 4 AWG → 1 AWG copper is ×2.01 instead of ×1.59. Here the EGC lands on 4 AWG either way; on other runs the bigger ratio can push it up a size. The Wire Size calculator has a 310.12 option: turn it on and it sizes at 83% of the rating and still checks the drop at the full 100 A, which at 200 ft gives 1 AWG copper.

Run your own numbers

The Wire Size calculator starts at 100 A, 240 V, 200 ft. Change the distance, the material or the drop limit.

To check the drop on wire you already have on the truck, the Voltage Drop calculator is preset to 3 AWG copper at 200 ft:

Enter the one-way length. The formula already doubles it, so a round-trip figure counts the run twice and oversizes the wire. Once the conductors are picked, how many fit in the pipe is the next check.

Every number in this guide is computed by the same engine as the Voltkit app. It cites NEC 2023 sections but doesn't reproduce NFPA text or tables; check the wording in your copy of the code and with your authority having jurisdiction.

Run your own numbers: Wire Size calculator →

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