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Building Code Reference

Got a CEC 225 correction?

Feeder detached structure

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CEC Article 225 governs feeders and branch circuits that run outside to detached structures — a second building on the same property such as a garage, ADU, workshop, or barn. When you add a sub-panel to a detached structure, Article 225 requires that the feeder be sized for the calculated load of that structure, that conductor size, conduit type, overcurrent protection, and grounding electrode system all be shown on the electrical plans. Most plan-check corrections on this section come down to one issue: the plans don't show the feeder details or the load calculation, so the plan checker can't verify the sub-panel is adequately protected.

How to fix it

  1. Run a load calculation for the detached structure

    Calculate the total connected load for every circuit in the detached structure. Use the standard load calculation method: general lighting at 3 VA per square foot, small-appliance circuits at 1,500 VA each, and nameplate ratings for any fixed appliances (air compressor, welder, EV charger). Apply the demand factors from CEC Article 220 where allowed. The result is the minimum feeder ampacity you must design to. Show the calculation on a dedicated electrical plan sheet — a hand-written note saying 'feeder per load' will not pass.

  2. Size the feeder conductors to the calculated load

    Select conductor gauge from CEC Table 310.16 based on the calculated ampacity, derated for conduit fill and ambient temperature if needed. For a typical detached garage sub-panel in Southern California climate, #6 AWG copper in 1-inch conduit handles up to 65A continuous, #4 AWG handles up to 85A, and #2 AWG handles up to 115A. If the run is long (over 100 feet), check voltage drop — keep it under 3% for the feeder alone. Note the conductor material (copper vs. aluminum), gauge, and conduit type on the electrical plan and panel schedule.

  3. Specify overcurrent protection at the source panel

    The OCPD at the main panel must match or be smaller than the feeder conductor's ampacity rating. A 60A breaker protecting #6 AWG copper is correct. A 100A breaker on #6 AWG is a CEC 240.4 violation. Show the breaker size, pole count (2-pole for 240V feeders), and manufacturer slot in the panel schedule on your plans. If the main panel doesn't have capacity for a new 2-pole breaker, note that a panel upgrade or load management is required before the feeder can be added.

  4. Design a disconnect at the detached structure

    CEC 225.31 requires a disconnect means at each building or structure. CEC 225.39 sets the minimum disconnect rating: 15A for a single branch circuit, 30A for two 2-wire branch circuits, 100A 3-wire for a one-family dwelling, and 60A for all others. The disconnect can be a main breaker in the sub-panel or a separate disconnect switch. Show the disconnect location and rating on the site plan and electrical plan. For ADUs, the disconnect is typically the main breaker in the sub-panel mounted on an exterior wall near the entry.

  5. Install and document the grounding electrode system

    A detached structure served by a feeder from another building needs its own grounding electrode system at the structure — it cannot share the main building's grounding electrodes. Per CEC 250.32(A), install two ground rods at least six feet apart (or one rod with verified 25-ohm or less resistance), connect them with a grounding electrode conductor sized per CEC Table 250.66, and bond it to the grounded neutral bus only if no equipment grounding conductor is run in the feeder. Show the ground rod locations, conductor size, and bonding connections on the electrical plan. This is one of the most commonly missed items on detached-structure corrections.

  6. Specify conduit type and burial depth for underground runs

    If the feeder runs underground between buildings, the plan must call out the conduit type and burial depth. PVC Schedule 40 or 80 requires 18-inch minimum burial depth under non-vehicle areas and under one- and two-family dwelling driveways, and 24 inches under streets and commercial driveways. Rigid metal conduit can be buried at 6 inches. Direct-bury cable (UF-B) requires 24 inches minimum under general areas and 18 inches under a 4-inch exterior concrete slab, and must be in a raceway under a building. Show a conduit schedule and a detail or note on the site plan indicating depth, conduit material, and any changes in depth at slab penetrations.

  7. Update the panel schedule and one-line diagram

    Many plan-check rejections on CEC 225 are purely a documentation problem. The plans may describe the right equipment but put the feeder details in the wrong place. The plan checker needs to see: (1) a one-line diagram showing the feeder from main panel to sub-panel, including conductor size, conduit, OCPD ratings, and sub-panel ampacity; (2) a panel schedule for the sub-panel listing each branch circuit, its load, and the breaker size; and (3) a notation referencing the load calculation. Put all of this on one sheet if possible so the reviewer doesn't have to hunt across multiple pages.

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Why you got flagged

Plan checkers flag CEC 225 most often when the electrical plans show a new sub-panel in a detached garage or ADU but don't include a feeder schedule. The reviewer needs to confirm the wire gauge, conduit type, and OCPD amperage match the calculated load — if that table isn't on the plans, the job gets a correction on the first pass.

A second common trigger is an undersized feeder. Contractors sometimes spec the feeder based on the panel amperage alone rather than running an actual load calculation. CEC 225.39 sets minimum disconnect sizes for detached structures (100A for a building with six or more branch circuits), and those figures are a floor, not a design target. If your 100A sub-panel feeds a workshop with a 3-HP air compressor, welder, and lighting circuits, a load calc may show you actually need a larger feeder.

Grounding and bonding is a third source of corrections. Detached structures require their own grounding electrode system — typically two ground rods spaced at least six feet apart — and a separate equipment grounding conductor in the feeder. Plans that show the sub-panel but omit the grounding detail will get flagged under CEC 225.30 and 250.32.

A fourth scenario applies specifically to ADUs: when a detached ADU is served from the main house panel, the feeder often crosses a property line or runs underground. The plans need to call out burial depth (24 inches for direct-bury cable, 6 inches for PVC conduit under a concrete slab per CEC 300.5), conduit material, and any required GFCI protection for the branch circuits inside the ADU. If any of those details are missing or inconsistent with the calculated load, the plan checker will send it back.

The official code

California Electrical Code (Title 24, Part 3) — Article 225: Outside Branch Circuits and Feeders

Read the source →

Common questions

Do I need a separate grounding electrode at my detached garage?
Yes. Per CEC 250.32, a detached structure served by a feeder from another building needs its own grounding electrode system. The most common approach is two 8-foot ground rods driven at least 6 feet apart, connected to the sub-panel's grounding electrode conductor. If you only have one rod, you need to test it — if resistance is 25 ohms or less, one rod is allowed. If over 25 ohms, you must add a second rod. Don't confuse the grounding electrode conductor with the equipment grounding conductor that runs in the feeder conduit — those are two different conductors serving different purposes.
What size feeder do I need for a detached ADU?
It depends on the load calculation for that ADU, not on an arbitrary panel size. A studio ADU with standard lighting, kitchen appliances, and a mini-split HVAC might need only a 60A feeder. A two-bedroom ADU with an EV charger, electric range, and electric water heater could need 100–150A. Run the load calculation per CEC Article 220. For ADUs intended to be all-electric, plan for at least 100A to accommodate future EV charging and appliance upgrades without a second corrections cycle.
Can I use aluminum wire for a feeder to a detached garage?
Yes, aluminum is allowed for feeder conductors (not branch circuits) and is commonly used on longer runs because it's less expensive than copper. Size up per CEC Table 310.16 — aluminum conductors have lower ampacity per gauge than copper. For a 60A feeder, use #4 AWG aluminum vs. #6 AWG copper. Always use aluminum-rated terminals at both ends (the main panel breaker and the sub-panel main lug), and apply anti-oxidant compound at each connection. Note the conductor material explicitly on the electrical plan — the plan checker will want to verify the terminal ratings match.
Does a detached workshop need a 100A sub-panel or can I run a smaller one?
CEC 225.39 sets minimum disconnect ratings based on how many branch circuits the structure has. If you have five or fewer branch circuits, a 60A or even 30A disconnect may be code-minimum. Six or more circuits requires at least 100A. But code-minimum is not always the right answer for a workshop. If you plan to run a table saw, air compressor, welder, or dust collector, your load calculation may require 100A or more regardless. Size for your actual load, not the minimum code threshold — undersizing now means a corrections cycle or a panel upgrade later.
What burial depth is required for the conduit between my house and detached garage?
Burial depth depends on conduit type. PVC conduit under general landscaping areas requires 18 inches. Under driveways or areas subject to vehicle traffic, go to 24 inches. Rigid metal conduit can be buried at 6 inches. If you're using direct-bury (UF-B) cable without conduit, the minimum is 24 inches and direct-bury cable is not allowed under concrete slabs or driveways — use conduit there. Show the conduit path, depth, and any transition points on the site plan. Missing burial depth callouts are a common plan-check flag on CEC 225 corrections.
My garage already has power. Does Article 225 apply if I'm just adding circuits?
If you're adding circuits inside an existing sub-panel without touching the feeder, Article 225 typically doesn't generate a correction — the feeder work was already permitted. But if the existing feeder is too small for the new load, the plan checker may flag a CEC 225 issue during the permit review. Adding an EV charger, for instance, can push a marginally-sized feeder over its rated ampacity. Rerun the load calc with the new circuits included. If the feeder is undersized, upgrading it is required before the permit can be approved.
Does a feeder from the house to a detached ADU count as a service entrance?
No. A feeder from the main service panel in the house to a sub-panel in the detached ADU is a feeder, not a second service entrance. This matters because Article 225 (outside feeders) applies, not Article 230 (services). The ADU sub-panel must have a main disconnect rated for the calculated load (100A minimum if there are six or more branch circuits), and it needs its own grounding electrode system. You don't need a separate utility meter for the ADU unless the city requires it for rental units — check your jurisdiction's ADU ordinance.

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