100-Amp Subpanel for a Garage, Shop or ADU: Wire Size, Grounding and Cost

What size wire for a 100 amp subpanel

Feeder conductors are sized from NEC Table 310.16. For residential work the 75°C column is the one that matters, because breaker and panel terminals are rated 75°C. Here are the values for the sizes that come up with 100 amp subpanels.

So for a true 100 amp subpanel feeder: #3 copper or #1 aluminum for the hot conductors, the neutral usually the same size, and an equipment grounding conductor sized from Table 250.122 (#8 copper or #6 aluminum for a 100 amp breaker). Long runs can need a bigger size to keep voltage drop reasonable, which we cover below.

Can you use 2-2-2-4 aluminum for a 100 amp subpanel?

Not on a 100 amp breaker. 2-2-2-4 aluminum (sold as SER, MHF or URD) has #2 aluminum hots, and #2 aluminum is rated 90 amps at 75°C. The "next size up" rule in 240.4(B) lets you round up to the next standard breaker only when the ampacity does not match a standard breaker size. 90 amps is itself a standard size, so there is nothing to round up to. The feeder gets a 90 amp breaker.

The confusion comes from NEC 310.12, the dwelling service rule. It lets conductors that carry the entire load of a single-family home be sized at 83% of the service rating, and its table shows #2 aluminum for a 100 amp service. That rule applies to the service or the main feeder of the whole dwelling. It does not apply to a subpanel that serves only part of the house, a garage or a shop. A feeder that carries the entire load of a separate dwelling unit, such as a whole ADU, can arguably use it; that one is the inspector's call.

In practice a 90 amp subpanel feeder is plenty for most garages and shops. If you need the full 100 amps, step up to #1 aluminum or #3 copper.

What size conduit for a 100 amp feeder

Conduit size comes from NEC Chapter 9: with more than two conductors, the wires may fill at most 40% of the conduit's inside area. Here is how the common 100 amp combinations work out in EMT and Schedule 40 PVC.

Schedule 80 PVC has a smaller inside diameter, so a combination that just fits 1 inch Schedule 40 can fail in 1 inch Schedule 80. Most of our underground feeders to detached buildings go in 1-1/4 inch or larger PVC anyway: it pulls far more easily and leaves room to upsize later.

For buried runs, NEC Table 300.5 sets the minimum cover: generally 24 inches for direct-burial cable, 18 inches for PVC conduit, and 6 inches for rigid metal conduit, with different rules under driveways and for some residential circuits.

Grounding: four wires, a separate neutral, and no bond at the subpanel

The most common subpanel defect we find is a neutral bonded to ground in the subpanel. In a subpanel, the neutral bar and the ground bar must be separate, the bonding screw or strap that comes with the panel is left out, and the feeder has four conductors: two hots, an insulated neutral and an equipment ground. The neutral and ground are bonded only at the service. Bonding them again downstream puts normal neutral current on ground wires and metal parts.

A detached garage, shop or ADU also needs its own grounding electrode system at that building (NEC 250.32), connected to the equipment ground of the feeder, still with no neutral-to-ground bond. And it needs a disconnecting means at the building, which is often the subpanel's own main breaker.

• Attached garage or basement subpanel: four-wire feeder, neutral isolated, no ground rods needed at the subpanel.
• Detached building: four-wire feeder, neutral isolated, ground rods or other electrodes at the building, and a disconnect at the building.
• A single circuit to a detached shed (including one multiwire circuit): the building can skip its own ground rods; two or more circuits need them. Ask before you trench.

Do you need two ground rods?

Usually, yes. A single ground rod has to be supplemented by a second electrode unless it is tested at 25 ohms or less to earth (NEC 250.53(A)(2)). Testing takes a meter most homeowners do not have, so the normal practice is simply to drive two rods.

The rods have to be at least 6 feet apart and have at least 8 feet in contact with soil. The grounding electrode conductor to rods does not need to be larger than #6 copper, whatever the size of the service. The same rules apply to a 100 amp service, a 200 amp service, and a detached building's subpanel.

Oregon's amendments to the 2026 code, as published in draft, add a narrow exception for single-phase temporary services of 200 amps or less, which can use a single rod. It does not change the rule for permanent services or detached buildings.

Feeding subpanels from 100 and 200 amp mains

A 100 amp subpanel off a 200 amp main is routine. Two 100 amp subpanels off a 200 amp main is also allowed. There is no rule that feeder breakers have to add up to less than the service rating. Each breaker protects its own conductors, the main still protects the service, and the load calculation for the whole house, including everything the subpanels feed, has to fit 200 amps.

A 100 amp subpanel off a 100 amp main is allowed too. The feeder breaker can equal the main breaker; it protects the feeder conductors and the subpanel, and the main protects the service. It is common when the main panel is full and the subpanel exists to add spaces rather than capacity. It does not add any capacity: the house is still limited to 100 amps.

That is the key point about subpanels. A subpanel solves a space problem or a distance problem. It does not solve a capacity problem. If the load calculation does not fit the service, the answer is load management or a service upgrade, which we cover in is 100 amp service enough.

Long runs: voltage drop

Detached shops and ADUs are often 75 to 200 feet from the main panel. Over that distance, wire that is legal for the ampacity can still lose enough voltage to make motors and heat pumps work harder. The NEC recommends keeping feeder and branch circuit drop to about 5% combined, in an informational note rather than a requirement.

As an example, #1 aluminum carrying 80 amps over a 100 foot run loses roughly 4 volts, under 2% at 240 volts. At 200 feet it is about double that, and stepping up to 1/0 or 2/0 aluminum is often the cheaper fix than copper. We size long feeders for the actual load and distance, not just the breaker.

What a subpanel costs in Portland and Vancouver

Our published planning range for a subpanel for a garage, shop or ADU is $800 to $1,800 and up. The main things that move it are feeder length, whether the feeder has to be trenched or run through finished walls, the subpanel's size, and whether the building needs its own ground rods and disconnect.

Trenching a long run and any service upgrade needed to make room for the new load are priced separately. For comparison, a 100 to 200 amp service upgrade typically ran $2,500 to $3,500 on our invoices from August 2024 to August 2026. The permit covers the feeder and the new panel; in Portland a feeder of 200 amps or less falls under the same $212 service-and-feeder fee plus the 12% state surcharge. See our permit guide.

The subpanel mistakes inspectors catch most

1. Neutral and ground bonded in the subpanel, or a three-wire feeder with no separate ground.
2. 2-2-2-4 aluminum on a 100 amp breaker for a subpanel.
3. No ground rods or no disconnect at a detached building.
4. Aluminum terminations without the right torque, or on lugs not rated for aluminum.
5. Feeder cable run where it is not allowed, such as SE cable buried without conduit.
6. Panel mounted without working clearance: 30 inches wide, 36 inches deep in front, 6.5 feet of headroom.
7. A subpanel added to fix a capacity problem that is really a service problem.

Sources

• NEC Table 310.16 ampacities, reprinted by LAPP Tannehill
• IAEI: 2023 NEC 310.12 and subpanels
• EC&M: 310.12 dwelling service and feeder conductors
• EC&M: next size up, 240.4(B)
• Square D QO2100 submittal sheet, lug range
• Eaton BR2100 product page, lug range
• EE Power: 2023 NEC grounding electrodes (250.52, 250.53)
• Oregon BCD draft 2026 OESC amendments (Table 1-E)