Subpanel Sizing Guide: Feeder Size, Spaces & Load Calc
Reading time: ~10 min read
📋 Key Takeaways
- Three questions size every subpanel: the load calc (feeder amps), the spaces (circuit count now + 25% spare), and the route (conduit vs SER, indoor vs outdoor). Amps come from NEC 220, not from the biggest panel on the shelf.
- The three workhorse residential builds: 60A for garages (6 AWG Cu / 4 AWG Al), 100A for workshops and heavy garages (3 AWG Cu / 1 AWG Al), 125A for ADUs (1 AWG Cu / 2/0 AWG Al).
- Feeders to detached structures are NEC 225 territory: 4-wire feed (hot-hot-neutral-EGC), isolated neutral at the subpanel, and a grounding electrode system at the structure per 250.32.
- If the feeder carries the entire load of a dwelling unit (a true ADU feed), 2023 NEC 310.12(B) lets you size at 83% — a 100A ADU feeder needs only 83A-rated wire: 4 AWG Cu / 2 AWG Al.
- Buy spaces once: a 100A panel with 24 spaces costs barely more than a 12-space, and the second visit for a full panel costs a labor day.
"What size subpanel do I need?" splits into two numbers that have nothing to do with each other: amperage (from the load calculation) and spaces (from the circuit count). Contractors who skip the calc end up either feeding a 60A garage panel on 6 AWG because "that's what we had" or hanging a 125A panel on a barn with two receptacles. The calculator below runs a simplified NEC 220-style estimate — check off the loads, and it returns the demand amps, the standard feeder/breaker size, and the copper or aluminum feeder conductors per Table 310.16 at 75°C.
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Subpanel Feeder Size Calculator
General lighting/receptacle load at 3 VA/sq ft.
1,500 VA each per NEC 220.52.
Calculated demand: —
Feeder breaker & panel: —
Feeder conductors (75°C): —
Simplified NEC 220 method: general load = 3 VA/sq ft + 1,500 VA per small-appliance circuit, with the first 3,000 VA at 100% and the remainder at 35% (Table 220.42); appliances and motors at 100%; EV charging at 125% per 625.41. Feeder conductors from Table 310.16 at 75°C per NEC 215. If the feeder carries the entire load of a dwelling (ADU), NEC 310.12(B) permits 83% sizing — shown in the note when applicable. Informational; the AHJ governs.
The Three Common Builds: 60A, 100A, 125A
Ninety percent of residential subpanel jobs land on one of three feeder sizes. This table is the quoting shorthand — wire sizes per Table 310.16 at 75°C (4-wire feed everywhere: two hots, neutral, EGC):
| Feeder | Cu (THHN/XHHW-2) | Al (XHHW-2/SER) | Panel to buy | Typical scenario |
|---|---|---|---|---|
| 60 A | 6 AWG | 4 AWG | 100A/12–20 space main-lug or MLO | Garage: lights, receptacles, opener, compressor |
| 100 A | 3 AWG | 1 AWG | 100A/20–24 space | Workshop with 240V tools, big garage, pool equipment |
| 125 A | 1 AWG | 2/0 AWG | 125A/24–30 space | ADU / in-law suite, garage + EV charger |
Two practical notes. First, buy the panel with more spaces than the feeder amps suggest — a 60A feed on a 24-space 100A panel is normal and smart; spaces run out long before ampacity does. Second, aluminum feeder is the default economic answer at 100A and up: 1 AWG Al XHHW-2 in conduit or 1-1-1-3 SER beats copper on material cost by a wide margin, and the two-size penalty is already baked into the table.
NEC 225: The Detached-Structure Rules That Fail Inspections
A feeder leaving the house for a garage, shop, or ADU crosses from Article 210/215 branch-circuit thinking into Article 225 (outside branch circuits and feeders). The inspection-critical points:
- Four-wire feed, always. Two hots, an insulated neutral, and a separate equipment grounding conductor. The days of three-wire feeds with a bonded neutral are grandfathered-only under narrow 250.32(B) conditions — new work is four-wire.
- Neutral isolation at the subpanel. The neutral bar floats (no bonding screw or strap), and the EGC lands on the ground bar bonded to the enclosure. This is the single most-failed item on subpanel roughs.
- Grounding electrode at the structure. Per 250.32, the detached building gets its own electrode (typically two ground rods 6 ft apart) bonded to the subpanel's grounding bar — the EGC from the house ties in alongside, not instead of.
- Disconnect at the structure. 225.31/225.32: a disconnecting means at the building, either the panel's main breaker or a separate disconnect ahead of a main-lug panel. For a single circuit, the branch breaker can serve; multi-circuit subpanels want the main.
- Burial depth and raceway. Direct-burial USE-2 or URD at 24 in; PVC conduit at 18 in; rigid metal at 6 in (Table 300.5). Warning tape 12 in above is cheap insurance on every trench.
Scenario Math: Garage vs. Workshop vs. ADU
Basic garage (60A): 800 sq ft × 3 VA = 2,400 VA + two receptacle circuits at 3,000 VA = 5,400 VA, demand-factored to ~3,800 VA ≈ 16A. Even with a compressor and an opener, a 60A feeder is generously sized — but 60A costs almost nothing over 30A in wire, so it is the floor for any new garage feed.
Serious workshop (100A): the same general load plus 240V tools at 7,200 VA and a mini-split at 5,000 VA lands near 14,000 VA ≈ 58A calculated. You could legally feed it at 60A — until the customer adds a welder or a second mini-split. Quote 100A: 1 AWG Al and a 24-space panel is the workshop standard.
ADU / in-law suite (125A): full kitchen (range 8,000 VA), laundry (dryer 5,000 VA + washer circuit), water heater 4,500 VA, 2.5-ton HVAC 5,000 VA, plus 900 sq ft of general load — roughly 28,000 VA before demand factors, landing near 19,000 VA ≈ 79A after diversity. A 100A feed covers it, and if the feeder truly carries the entire dwelling load of the ADU, 2023 NEC 310.12(B) lets you run 4 AWG Cu / 2 AWG Al at the 83% allowance. Where an EV charger enters the picture, add 12,000 VA and the honest answer becomes 125A — our EV charger collection covers the 40–48A units that drive that math.
Frequently Asked Questions
What size wire do I need for a 100-amp subpanel?
Per NEC Table 310.16 at 75°C: 3 AWG copper THHN/XHHW-2 or 1 AWG aluminum, run as a 4-wire feed (two hots, insulated neutral, EGC). If the feeder carries the entire load of a dwelling unit such as an ADU, 2023 NEC 310.12(B) allows the 83% allowance — 4 AWG Cu or 2 AWG Al. On runs over about 100 ft, check voltage drop and expect to upsize one step.
What size breaker do I need for a subpanel?
The feeder breaker in the main panel must protect the feeder conductors at their ampacity — a 60A feeder on 6 AWG Cu gets a 60A two-pole breaker; a 100A feeder on 1 AWG Al gets a 100A two-pole. The subpanel itself can be rated higher than the feeder (a 100A panel on a 60A feeder is fine and common); the breaker size follows the wire, not the panel label.
Does a subpanel need a ground rod?
At a detached structure, yes — NEC 250.32 requires a grounding electrode system at the outbuilding (typically two 8-ft rods at least 6 ft apart) bonded to the subpanel ground bar. The EGC in the 4-wire feeder still runs back to the main panel; both connections exist together. A subpanel in the same building needs no new electrode — just the EGC back to the service equipment.
Can a subpanel be rated higher than the feeder breaker?
Yes — the panel rating is a maximum, not a requirement. A 125A or 200A-rated panel fed by a 60A or 100A feeder breaker is perfectly legal and often smart: the ampacity limit lives at the feeder breaker in the main panel, and the larger panel gives you spaces and bus headroom for future circuits. The reverse (panel rated below the feeder) is the violation.
How many circuits can I put in a 100-amp subpanel?
As many as the panel's space count and load calculation allow — there is no breaker-count limit tied to the amp rating. A 100A/24-space panel holds 24 full-size breakers (more with listed tandems where the panel allows CTL twins). The real constraint is the NEC 220 demand calculation on the feeder: the sum of breaker ratings routinely exceeds 100A because loads are not simultaneous.
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