How does a subpanel work for a detached ADU?

A detached ADU subpanel receives power from the main house panel through a single feeder circuit and then distributes that power to all of the ADU's individual branch circuits. It is the most common — and usually the most cost-effective — way to electrify a detached ADU when the existing service has spare capacity.

How the feed is built

1. A new two-pole breaker, sized for the ADU's subpanel amperage (for example a 125A breaker for a 125A subpanel), is installed in the main house panel.
2. A feeder — typically four conductors in underground conduit — runs from the main panel out to the ADU.
3. The feeder terminates at the ADU subpanel, mounted inside the unit or on its exterior wall.
4. The subpanel then feeds each ADU branch circuit: lighting, receptacles, HVAC, range, water heater, EV charger, and so on.

The four-wire rule that trips up everyone

This is the single most important — and most commonly botched — detail in detached ADU wiring. A subpanel must use a four-wire feeder: two hots, one neutral, and one separate equipment ground. Inside the subpanel:

• The neutral bus must be isolated from the enclosure (floating).
• The ground bus must be bonded to the enclosure.
• The neutral and ground must not be bonded together.

Neutral-ground bonding belongs only at the service/main panel. Repeating that bond in a downstream subpanel causes neutral current to flow on the grounding system and metal enclosures — a genuine shock and fire hazard, and an automatic inspection failure.

For a detached structure, this also means the ADU typically needs its own grounding electrode system (ground rods, or a Ufer/concrete-encased electrode) bonded to the subpanel's ground bus, in addition to the equipment ground carried in the feeder.

Voltage drop and feeder routing

Long runs matter. As feeder distance grows, voltage drop increases, so conductors usually have to be upsized to keep drop within recommended limits (generally around 3% for feeders). A 125A subpanel 100+ feet from the main panel may require larger copper or aluminum conductors than the breaker size alone would suggest. Our electrical plans show feeder conductor size, conduit size, grounding details, and routing so the inspector sees a complete, code-compliant picture.

Requirements vary by jurisdiction and utility — confirm with your local building department and serving utility. If the run is very long or the main service is maxed out, a separate utility service may be the better path.