Detached Garage Subpanel Calculator
This free garage subpanel calculator sizes the feeder the way inspections are actually passed. The wire itself is the easy part — 6 AWG copper or 4 aluminum for a 60 A panel, 3 copper or 1 aluminum for 100 A — but detached buildings carry a chain of rules that fail more inspections than any conductor choice: the feeder must be 4-wire with the neutral isolated at the subpanel (the 3-wire shortcut left the code in 2008), the building needs its own grounding electrode, and a disconnect must sit at or near where the feeder enters.
Distance is the quiet variable. Detached-building runs are long, and voltage drop — not ampacity — usually decides the conductor: 60 A over 150 feet pushes past the 3% recommendation on 6 AWG copper and steps you to 4. Enter your trench length and the calculator checks it against the same K-factor math as the voltage-drop tool. It also flags the misapplied "83% rule": Table 310.12 covers dwelling service conductors, not this sub-feeder.
The trench itself is honest DIY territory — dig it, lay the conduit, call for the open-trench inspection — while the feeder breaker in the main panel is electrician work nearly everywhere. Burial depths per Table 300.5 (PVC 18″, rigid metal 6″, direct-burial UF 24″) are in the results. Reference only; verify with a licensed electrician and your local AHJ. Free, no signup.
Garage Subpanel Calculator
What wire to a detached garage or shed? Pick the subpanel size and trench length for the 4-wire feeder, ground rods, disconnect, burial depth, and the voltage-drop check that usually governs. Free, no signup.
The feeder
Enter the trench length — on detached-building feeders, voltage drop usually governs the wire size before ampacity does.
How deep the trench has to be
The wiring method sets the digging: direct-burial UF cable needs a full two feet of cover, PVC conduit gets by with eighteen inches, and rigid metal conduit only six. Most detached-garage feeders go in PVC — the 18″ trench is the sweet spot between digging effort and conduit cost.
Calculation Formulas
Sized like any feeder; the 310.12 dwelling 83% allowance does NOT apply to a detached-building sub-feeder.
Example:
60 A: 6 AWG Cu or 4 AWG Al · 100 A: 3 AWG Cu or 1 AWG Al.
The 3-wire allowance was removed in the 2008 NEC; existing compliant installs are grandfathered. Separation backwards = the #1 inspection failure.
Example:
Pull the bonding screw in the subpanel; bond the ground bar to the can.
One rod suffices only at ≤25 Ω (250.53(A)(2)) — in practice, drive two rods ≥6 ft apart.
Example:
Two 8-ft rods, 6+ ft apart, bonded to the ground bar.
On detached-building runs, drop usually governs before ampacity.
Example:
60 A over 150 ft on 6 AWG Cu ≈ 3.7% → step to 4 AWG.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| 60 A feeder | 6 AWG Cu / 4 AWG Al | The typical garage-shop subpanel (75°C). |
| 100 A feeder | 3 AWG Cu / 1 AWG Al | Heavy shop / detached EV charging. |
| Burial | UF 24″ · PVC 18″ · RMC 6″ | Table 300.5 minimum cover; trench inspected open. |
| Disconnect | 225.31/.32 | At/near feeder entry; snap switch OK for a single-circuit outbuilding (225.36 Exc.). |
| One-circuit option | 225.30 | A single branch circuit (or MWBC) to a shed needs no subpanel at all. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
NEC 250.32 — Buildings supplied by a feeder(NFPA 70 (2023) 250.32(A)/(B))
View Standard4-wire feeder with EGC, neutral isolated at the sub, grounding electrode system at the building. 3-wire removed 2008.
Key Requirements:
- •4-wire
- •Separation at the sub
- •Electrode at the building
NEC 225.30–225.36 — Outside feeders(NFPA 70 (2023) Art. 225 Part II)
View StandardOne feeder per building (with the 2020+ common-supply exception), disconnect at a readily accessible location near entry, snap-switch exception for outbuildings.
Key Requirements:
- •Disconnect at the building
- •≤6 devices
NEC Table 300.5 — Burial(NFPA 70 (2023) Table 300.5)
View StandardMinimum cover by wiring method for the trench.
Key Requirements:
- •24/18/6 inch by method
NEC 250.53(A)(2) — Rod electrodes(NFPA 70 (2023) 250.53(A)(2))
View StandardSupplement a single rod unless it tests ≤25 Ω — the origin of the two-rod field practice.
Key Requirements:
- •2 rods ≥6 ft apart in practice
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Grandfathered 3-wire feeders
Pre-2008 installs
Existing compliant 3-wire feeders may remain, but ANY new feeder is 4-wire — and extending/modifying usually triggers the current rule.
Regional Examples:
Frost and trench depth
Cold climates
Table 300.5 sets electrical minimums; frost heave and local amendments can push conduit deeper.
Regional Examples:
DIY boundary
Trench vs tie-in
Trenching, conduit, and setting the subpanel enclosure are homeowner-friendly; the feeder breaker in the main panel and final terminations are electrician work in most places.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
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Voltage Drop Calculator
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How to Use This Calculator
- Pick the subpanel size — 60 A covers most garage shops; 100 A if heavy equipment or EV charging lives out there.
- Choose copper or aluminum (aluminum is common and economical for feeders) and enter the one-way trench length.
- Click Calculate for the feeder wire, the voltage-drop verdict, and the inspection checklist: 4-wire, separation, rods, disconnect, burial.
- Have the trench inspected open, before backfill — and leave the main-panel tie-in to the electrician.
The Subpanel Mistake Inspectors Catch Every Week
Inside your main panel, the neutral and ground bars are bonded together — that is correct there, and exactly wrong in a detached subpanel. Since the 2008 NEC removed the 3-wire allowance, every new feeder to a detached building carries four conductors, and at the subpanel the neutral bar must float (bonding screw removed) while the ground bar bonds to the enclosure and to the building's own grounding electrode — two rods, at least six feet apart, in universal practice, because the single-rod option requires a 25-ohm test nobody performs. Get the separation backwards and normal neutral current flows on the grounding conductor and every bonded metal surface in the building; it works fine right up until it doesn't, which is why it is the first thing inspectors check. The disconnect rule (225.31/.32) rounds out the chain: a means to kill everything at the building, at or near where the feeder enters — the subpanel's own main breaker satisfies it, and on a single-circuit shed, even a snap switch does.
Frequently Asked Questions
What size wire for a 60 amp subpanel in a detached garage?
6 AWG copper or 4 AWG aluminum THWN-2 (75°C) — as a FOUR-wire feeder: two hots, a neutral, and an equipment grounding conductor. But check your distance before buying: over 150 feet, 6 AWG copper at full load pushes past the 3% voltage-drop recommendation and 4 AWG is the better pull. For a 100 A subpanel the answers are 3 AWG copper or 1 AWG aluminum. Enter your trench length in the calculator — distance governs these runs more often than ampacity.
Can I run a 3-wire feeder to my garage like the old days?
Not anymore. The 3-wire allowance (re-grounding the neutral at the detached building) was removed in the 2008 NEC; every new feeder must be 4-wire per 250.32(B), with the neutral isolated at the subpanel and a separate equipment grounding conductor in the trench. Existing compliant 3-wire installs are grandfathered, but extending or modifying one typically triggers the current rule. Plan the fourth conductor from the start.
Do I need ground rods at a detached garage?
Yes — every detached structure supplied by a feeder needs its own grounding electrode system (250.32(A)), separate from the rods at the house. One rod suffices only if it measures 25 ohms or less to earth (250.53(A)(2)), and since nobody performs that test, universal practice is two rods driven at least 6 feet apart, bonded to the subpanel's ground bar. The rods do not replace the equipment grounding conductor in the feeder — you need both.
Why do grounds and neutrals separate in a subpanel?
Because bonding them anywhere except the main service creates parallel paths for normal neutral current — through the grounding conductor and every bonded metal surface. In the main panel the bars are bonded (correctly, once); in the subpanel the neutral bar must float (bonding screw removed) with the ground bar bonded to the enclosure. Reversing this is the most common detached-building inspection failure, and it's invisible in operation until a fault finds the wrong path.
How deep do I bury the feeder to my garage?
By wiring method, per NEC Table 300.5: PVC conduit needs 18 inches of cover, rigid or intermediate metal conduit needs just 6, and direct-burial UF cable needs 24. Cover is measured from grade to the TOP of the conduit. Cold-climate note: local amendments sometimes push conduit below the frost line — deeper than the electrical minimum — so check before renting the trencher. And schedule the inspection while the trench is open; backfilling first is an expensive mistake.