For planning a subpanel for an addition or garage, size the subpanel ampacity to meet your expected demand plus about twenty five percent spare capacity.

Start by calculating the expected demand by listing every load you will serve including lighting, receptacles, HVAC equipment, electric vehicle charging if any, and major appliances, then convert those loads into continuous and noncontinuous currents using the nameplate ratings and typical duty cycles. Apply the general thumb rule used in residential construction which is to choose a subpanel ampacity roughly equal to the calculated demand plus twenty five percent to allow for future changes and margin; for many typical garages and small additions that ends up in the range of sixty to one hundred amperes depending on loads. When you size the feeder conductors, pick copper or aluminum gauge consistent with the ampacity and the length; increase conductor size if voltage drop to the farthest load exceeds three percent for sensitive equipment or five percent for total feeder run. Allocate physical breaker spaces so you leave about twenty to thirty percent of the panel bus free; that means if your panel can accept twelve breakers plan to use no more than eight to nine spaces and reserve the remainder for future circuits. Balance the single phase loads across the two legs of the panel by grouping similar loads on opposite poles to reduce neutral current and avoid chronic unbalance which can raise heating and reduce efficiency. Secure correct grounding and bonding by installing the subpanel with isolated neutral bus and a separate grounding bar tied to the main grounding electrode system per code practices, and label neutrals and grounds to avoid accidental bonding.

Common mistakes are undersizing the ampacity because of optimistic assumptions about loads which causes nuisance tripping and heat issues, so calculate conservatively and add the twenty five percent margin and check nameplate currents. People also overcrowd panel spaces which leaves no room for future circuits and forces unsafe workarounds, so choose a panel with room for at least twenty to thirty percent spare breaker slots or plan for tandem breakers designed for the panel. Another frequent error is running undersized conductors or ignoring voltage drop on long runs, which produces poor equipment performance and overheating; solve this by using the correct gauge for the ampacity and length and by upsizing conductors when runs exceed about one hundred feet to keep voltage drop within acceptable limits.

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