Wire Size for a 100-Amp Service: 2026 NEC Guide

A 100-amp residential service uses 4 AWG copper or 2 AWG aluminum service-entrance conductors under the NEC dwelling-service rule, 310.12. Those look small next to the full Table 310.16 numbers because a one-family dwelling service is allowed to be sized at 83 percent of its rating. A 100-amp feeder or sub-panel that is not a service is sized differently, and mixing up the two is the mistake I see most often. This guide covers the service conductor size, the grounding-electrode conductor, cable types, how far you can run each size before voltage drop bites, and whether 100 amps is still enough for a modern home.

100-Amp Service Wire Size Quick Answer

For a single-family dwelling service, use 4 AWG copper or 2 AWG aluminum. NEC 310.12 lets the main service conductors be sized at 83 percent of the service rating, so 100 amps only has to cover 83 amps of design load. If instead you are sizing a 100-amp feeder that does not carry the whole dwelling, use the full Table 310.16 value: 3 AWG copper or 1 AWG aluminum. The grounding-electrode conductor for a 100-amp service is 8 AWG copper or 6 AWG aluminum.

100-Amp Service Wire Size Reference Table

Source: NEC 2023 (NFPA 70) Table 310.12, Table 310.16 (75 °C column), Table 250.66, Table 250.122. Verify against your local code edition.

Why 4 AWG Copper: the 310.12 Dwelling Rule

For a one-family dwelling, or an individual unit of a two-family or multifamily dwelling, NEC 310.12 lets the main service conductors be sized at 83 percent of the service rating instead of the full Table 310.16 ampacity. For a 100-amp service, 83 percent is 83 amps. The conductor that covers 83 amps at the 75 degree column is 4 AWG copper, rated 85 amps, or 2 AWG aluminum, rated 90 amps. That is why an electrician will tell you 4 AWG copper or 2 AWG aluminum for a 100-amp house service, even though those same conductors would be undersized for a general 100-amp feeder. The rule exists because a home almost never pulls its full service rating at once, and decades of load data back it up. It applies to the service and to a feeder that carries the entire load of a dwelling, not to a random sub-panel feeder.

Service Versus Feeder: Do Not Mix Them Up

Here is where projects go sideways. If you are sizing the conductors from the meter to the main 100-amp panel of a house, the 310.12 dwelling rule applies, and you use 4 AWG copper or 2 AWG aluminum. If you are sizing a 100-amp feeder to a detached garage, a workshop, or a sub-panel that does not carry the whole dwelling, the 310.12 allowance does not apply. You size from the full Table 310.16 at the 75 degree column, which is 3 AWG copper or 1 AWG aluminum for 100 amps. The two answers differ by a full wire size. Reaching for the smaller service-rated conductor on an ordinary feeder is a common inspection failure, so before you cut anything, ask whether the run is a dwelling service main or a feeder. Same 100-amp number, different wire.

Copper or Aluminum for a 100-Amp Service?

Both are code-compliant and both are common. Copper (4 AWG) is smaller in diameter, easier to terminate, and slightly more forgiving on long runs. Aluminum (2 AWG) is what most factory-made SE and SER service cable uses, and it is meaningfully cheaper, which matters when the run is long. If you go aluminum, use lugs and breakers rated for aluminum (marked AL or CU-AL), brush an antioxidant compound onto the stripped conductor, and torque the terminals to spec. Most of the aluminum horror stories people repeat come from old solid aluminum branch-circuit wire from the 1960s and 1970s, not from modern AA-8000 series aluminum building wire, which has a solid track record on services and feeders. For a 100-amp service I would pick based on price and the lug ratings on your panel, not on fear.

Grounding and Bonding a 100-Amp Service

A 100-amp service needs a grounding-electrode system: usually two ground rods spaced at least six feet apart, plus a bond to the metal water pipe within five feet of its entry if one is present, and a concrete-encased electrode (a Ufer) in new construction. The grounding-electrode conductor for a 100-amp service is 8 AWG copper or 6 AWG aluminum per NEC Table 250.66. There is a useful exception in 250.66(A): the part of that conductor running only to a driven ground rod never has to be larger than 6 AWG copper, so a single 6 AWG copper run to the rods is fine even though the table starts at 8 AWG. The main bonding jumper ties the neutral to the enclosure at the service disconnect, and that is the one and only place neutral and ground are joined in the whole system. Every downstream sub-panel keeps neutral and ground separate. Get the electrode system and that main bonding jumper right, because they are what clears a fault safely.

Cable and Conduit for the Service

Above-ground service-entrance conductors from the meter to the panel are usually run as SE cable (SER for a four-wire feeder to a main panel, or SE-style service cable), or as individual conductors in conduit. For an underground service lateral from a pad-mount transformer or a meter pedestal, use USE-2 or XHHW-2 rated for wet and direct-burial conditions, pulled through PVC conduit. The exact setup is often dictated by your utility, which has its own service rules for the meter base, the conduit size, and the point where their responsibility ends and yours begins. Because the service conductors sit unprotected by any overcurrent device until they reach the main breaker, they have to be installed exactly to code and to utility spec. That is a big part of why service work is not a typical weekend DIY job.

Voltage Drop: How Far Can You Run It?

Ampacity gets you the minimum legal size. On a long run, voltage drop is what actually sets the wire. The single-phase drop is:

Worked example: a 100-amp service feeding a realistic 80-amp working load, run 90 feet in 4 AWG copper (41,740 circular mils). Vdrop = 2 × 12.9 × 80 × 90 / 41,740 = 4.45 volts, which is 1.9 percent of 240 volts. That is under the 3 percent that most designers target, so 4 AWG copper is fine to about 90 to 100 feet at that load. Push the same load to 150 feet and the drop climbs past 3 percent, so you step up to 3 AWG or 2 AWG copper. This is exactly why two houses with identical 100-amp panels can call for different wire: the one with the meter 140 feet from the panel needs a fatter conductor than the one with a 20-foot run. Our voltage drop calculator does this math for your exact distance.

Is 100 Amps Still Enough in 2026?

A 100-amp service supplies roughly 24,000 watts at 240 volts. For a small home, an apartment, or a house that heats and cooks with gas, that is plenty. The pressure comes from electrification. Add a heat pump, a Level 2 EV charger, an induction range, and a heat-pump water heater, and a naive load count can blow past 100 amps. The honest way to decide is an NEC Article 220 load calculation, which adds the general lighting and receptacle load, the fixed appliances, the larger of heating or cooling, and any EV or special equipment, then applies the standard demand factors. Plenty of existing homes run fine on 100 amps because the loads never coincide, and load-management devices (an EV charger that backs off when the dryer runs, for example) can keep a 100-amp service viable even with an EV. If the calc lands close to the limit, that is usually the moment people jump to a 200-amp service. Run the numbers with our electrical load calculator before you spend on an upgrade you may not need.

What a 100-Amp Service Upgrade Costs

If you are going from a 60-amp or an old fused service up to 100 amps, expect roughly 1,300 to 3,000 dollars in most of the country, depending on whether the meter location moves, whether the service is overhead or underground, and local labor rates. That covers a new meter base and panel, the correct service conductors, the grounding-electrode system, permits, and both a utility and an AHJ inspection. Many homeowners weighing 100 versus 200 amps find the labor is similar either way, so the upgrade to 200 amps often costs only a few hundred dollars more in materials while roughly doubling capacity. If you expect an EV, a heat pump, or a shop within a few years, price both. Our panel upgrade cost calculator and electrician rate calculator can help you sanity-check a quote.

Temperature Ratings: Why the 75-Degree Column

Every wire size in this guide comes from the 75 degree Celsius column of Table 310.16, and that is not an accident. Modern building wire like THHN or the conductors inside SER cable is rated for 90 degrees, so the insulation itself could carry more current. The limit is the terminals. Almost all residential breakers, lugs, and panels are listed for 75 degree terminations (some small breakers only 60 degrees), and NEC 110.14(C) says you size to the lowest-rated part of the circuit. So even though 4 AWG copper THHN shows a 95-amp value in the 90 degree column, you use its 85-amp value in the 75 degree column because that is what your lugs are rated for. You can still use the 90 degree column as the starting point for derating math when you have several current-carrying conductors bundled together or high ambient heat, but the final answer can never exceed the 75 degree terminal rating. For a straightforward 100-amp service in normal conditions, this is why 4 AWG copper and 2 AWG aluminum are the numbers that matter.

Common 100-Amp Service Mistakes

The first mistake, already covered, is applying the 4 AWG service size to an ordinary feeder that should be 3 AWG copper. The second is ignoring voltage drop on a long run and wondering why lights dim when the AC kicks on. The third is a floating or undersized neutral: a dwelling service neutral can often be reduced under 220.61 because it only carries the unbalanced load, but reducing it too far, or forgetting it entirely, causes real problems. The fourth is a grounding shortcut, either a single ground rod where two are required, or no bond to the water pipe and Ufer, or the neutral-ground bond installed in a sub-panel instead of only at the service. The fifth is aluminum terminated dry, with no antioxidant and lugs that are not AL-rated, which is how you get the loose, oxidized connections that gave old aluminum its bad name. None of these are exotic. They are the exact items an inspector checks, so building to them the first time saves a failed inspection and a second trip.

Electrical work carries safety and legal risk. Verify any calculation with a licensed electrician familiar with your local code amendments before performing work. This article references NEC 2023; many jurisdictions still operate under NEC 2017 or 2020 or have local amendments, so always confirm with your local AHJ (Authority Having Jurisdiction). Service work in particular usually requires a permit and a licensed electrician.

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Data sources: NEC 2023 (NFPA 70) Table 310.12, Table 310.16 (75 °C column), 240.4(D), Table 250.66, Table 250.122, and Article 220; NFPA 70 Handbook 2023; manufacturer termination ratings; r/electricians field reports. Written by Munir Afridi, VoltFlow editorial team. Reviewed against NEC 2023 Article 310 and Article 250.