An electrical subpanel is a smaller distribution panel fed from your main panel, used to extend circuits to an addition, ADU, garage or workshop. In San Diego homes the most common problems are bonded neutrals and grounds, undersized feeders, and missing permits – all of which a home inspector flags during a visual review.
Why so many San Diego homes have subpanels
San Diego County has a huge stock of homes built in the 1950s through the 1980s, and a wave of recent additions and accessory dwelling units (ADUs) layered on top of them. When a homeowner converts a garage, builds a granny flat in the backyard, or finishes a detached workshop, the existing main panel usually does not sit close enough to feed the new space directly. The practical solution is a subpanel: a feeder runs from the main panel to the new structure, and branch circuits fan out from there.
California’s aggressive ADU laws have only accelerated this. A lot of the subpanels we see in newer ADUs were installed correctly by licensed electricians. A lot of the ones in older garage conversions were not – they were added by a previous owner or a handyman, often without a permit. That distinction matters, because a subpanel that is wired like a main panel creates a genuine shock and fire hazard.
The single most important rule: separate neutrals and grounds
In a main service panel, the neutral (grounded) conductor and the equipment ground are bonded together – tied to the same bus and to the grounding electrode system. In a subpanel, they must be kept completely separate. The neutrals land on an isolated neutral bar, the grounds land on a separate grounding bar bonded to the enclosure, and there is no bonding jumper or bonding screw tying the neutral bar to the cabinet.
This is the error we find more than any other in San Diego subpanels. If neutrals and grounds are bonded at the subpanel, normal return current travels on the ground wires and on metal parts – conduit, panel cans, even gas and water piping. That means a metal box or appliance can become energized, and a ground fault may not clear the way it should. It is invisible from across the room and only becomes obvious when the deadfront cover comes off.
Two related details inspectors look for:
- A four-wire feeder. A modern subpanel feed needs two hots, a neutral, and a separate equipment ground – four conductors. Older installs sometimes used a three-wire feed with the neutral doing double duty, which is no longer acceptable for a separate structure or a new feeder.
- The bonding screw removed. Many panels ship with a green bonding screw that ties the neutral bar to the can. In a subpanel that screw must be backed out. We routinely find it still driven home.
Feeder sizing and the conductors feeding the panel
The feeder is the cable running from the main panel to the subpanel, and it has to be sized to the breaker protecting it and to the load the new space will carry. A 60-amp subpanel and a 100-amp subpanel use different wire gauges, and using wire that is too small for the breaker is a fire risk because the breaker will not trip before the wire overheats.
Common feeder problems we document on inspections:
- Undersized wire for the breaker. A 100-amp breaker feeding a panel wired with conductors rated for 60 amps is a classic DIY shortcut.
- No main disconnect at a detached structure. A subpanel in a separate building – a detached garage, workshop or ADU – generally needs its own disconnecting means and its own grounding electrode (a ground rod or rods at that structure). Backyard ADUs frequently miss this.
- Aluminum feeders without proper terminations. Aluminum feeder conductors are common and acceptable, but they need the right anti-oxidant treatment and torque. Loose aluminum lugs are a recurring heat-damage source.
- Buried or undersized direct-burial cable to the ADU. Outdoor feeders need correct burial depth, conduit where required, and conductors rated for wet locations.
A general home inspection is visual and non-invasive. An inspector can read the breaker size, identify the visible conductor, look for scorching and corrosion, and flag obvious mismatches. Confirming exact ampacity, load calculations, and code compliance is the job of a licensed electrician or the permitting authority – and that is exactly the referral we make when something looks off.
Common DIY subpanel errors we flag
Beyond bonded neutrals and bad feeders, the recurring offenders in owner-installed subpanels include:
- Double-tapped breakers – two wires under one breaker lug that is not rated for two.
- Missing or wrong-amperage breakers – 14-gauge wire on a 20-amp breaker, or a 15-amp breaker feeding a high-draw circuit.
- Open knockouts and missing covers – holes in the enclosure that let rodents and fingers in, common in garage installs.
- No GFCI or AFCI protection where the room type now requires it – a real issue in finished garages and ADU kitchens and baths.
- Improper neutral and ground sharing on a bar, with multiple grounds jammed under one terminal.
- No labeling – an unlabeled subpanel makes it dangerous to know what you are shutting off.
Many of these overlap with what we see in aging service equipment generally; if you want the bigger picture on older gear, see our guide to electrical panel problems in older San Diego homes.
Permits: the paper trail that protects you
This is the part buyers underestimate. A subpanel feeding an addition or ADU is permitted electrical work, and the permit record tells you whether a licensed contractor and a city or county inspector ever signed off. When we find a subpanel with no corresponding permit, two things follow: the work was never independently inspected, and the addition itself may be unpermitted.
Unpermitted electrical does not just create safety questions – it can complicate your insurance, your appraisal, and your eventual resale. We always recommend buyers verify permit history with the City of San Diego or the County of San Diego (whichever has jurisdiction) for any visible addition or ADU. If the subpanel is unpermitted, budget for an electrician to evaluate it and, where needed, bring it to permit.
Buying a home with an ADU or addition? Inspect it deliberately
If the property you are buying has an ADU, a garage conversion, or a bonus room, the electrical behind it deserves attention rather than a glance. Our ADU and granny flat inspection looks closely at how the secondary space ties back into the home’s electrical, plumbing and structure, and a standard buyer’s inspection documents the main panel and subpanel condition so you can make decisions before closing.
To talk through a specific property anywhere in San Diego County – coastal, inland, or backcountry – reach The Real Estate Inspection Company at (619) 752-4399. Lead inspector Joseph Romeo is an InterNACHI Certified Professional Inspector and holds CSLB General Contractor License #1113143, so subpanels and additions get a genuinely informed look. As always, verify anything safety-related and consult a licensed electrician before you rely on it.
