Subpanel feeder sizing is determined by the calculated load the subpanel will serve and by the overcurrent device protecting the feeder. It is not the sum of the branch breaker ratings in the subpanel, which would be a meaningless number since those breakers never all carry their full rating at once.
Why the breaker sum is the wrong number
A subpanel with ten twenty-amp breakers does not need a two-hundred-amp feeder. Branch breakers protect their conductors against faults and overload; they do not describe demand. The circuits behind them might collectively draw a small fraction of that on a busy evening. Feeder sizing follows from a load calculation for what the subpanel actually serves, using the same methods and demand factors that govern service calculations. The result is then protected by an overcurrent device suited to the conductor, and the panel itself must have a rating at least equal to that.
Distance and voltage drop
The other consideration on a feeder is voltage drop, which matters because feeders are often long. The code addresses voltage drop as a recommendation rather than a hard requirement in most contexts, but a feeder to a detached garage a hundred feet away can lose enough voltage to affect motor starting and equipment performance. Upsizing conductors for voltage drop is a routine engineering decision, and it costs far less at installation than after the trench is closed. If a feeder is long, run the calculation rather than assuming the ampacity-derived size is sufficient.
Four wires, always
A feeder to a subpanel carries the ungrounded conductors, the grounded conductor and a separate equipment grounding conductor. The equipment grounding conductor is sized by its own rule, based on the rating of the overcurrent device protecting the feeder, not by matching the ungrounded conductors. At the subpanel, grounded conductors and equipment grounding conductors are landed on separate bars, the bonding screw or strap is removed, and the neutral bar is isolated from the enclosure. That separation is what keeps normal neutral current off the grounding system and off metal enclosures throughout the building.
When the subpanel is in a separate structure
A feeder to a detached building adds requirements. That structure needs a disconnecting means at the building and its own grounding electrode system, bonded to the equipment grounding at that panel. Neutrals remain isolated there just as in any subpanel. Older installations sometimes used three-wire feeders with the neutral bonded at the remote building, and that arrangement is no longer permitted for new work. If you are extending or replacing such a feeder, plan for four conductors and an electrode system rather than reproducing what is there.
Panel rating and future capacity
The subpanel enclosure has a bus rating that must equal or exceed the feeder overcurrent device rating, and it has a maximum number of overcurrent devices set by its listing. Beyond the code minimums, the practical advice is to be generous. Feeders are the expensive, disruptive part of any later upgrade, particularly buried ones or those in finished walls, while a larger panel is a modest cost difference at purchase. Consider what the space will be used for in ten years, not what it needs today, and leave spare positions.
What to check before you act
- Calculate the load the subpanel will serve rather than summing breaker ratings.
- Size the equipment grounding conductor by the feeder overcurrent device rating.
- Isolate the neutral bar and remove the bonding screw at every subpanel.
- Run a voltage drop calculation on any long feeder before choosing conductors.
Common questions
Can a subpanel breaker rating exceed the feeder size?
No. The overcurrent device protecting the feeder must suit the feeder conductors, and the panel bus rating must be at least that device rating.
Does a subpanel in the same building need a grounding electrode?
No, it is bonded through the equipment grounding conductor to the service. Separate structures are the case that requires an electrode system.
The practical takeaway
Feeder size follows calculated load, conductor ampacity and voltage drop, never a tally of branch breakers. Run four conductors, isolate the neutral at the subpanel, and size generously because replacing a feeder later is the expensive part.
References and further reading
- NFPA: National Electrical Code overview covering feeders and panelboards
- OSHA: wiring design and protection including feeder conductors and grounding
- Department of Energy: estimating loads to plan subpanel and feeder capacity
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
