A branch circuit supplying a fixed storage-type water heater with a capacity up to 120 gallons must have a rating of at least what percentage of the nameplate rating?

For a branch circuit supplying a fixed storage-type water heater with a capacity of up to 120 gallons, the National Electrical Code (NEC) specifies that the branch circuit must be rated at least 125 percent of the nameplate rating. This requirement ensures that the circuit can handle the starting current draw and the continuous operation of the water heater without tripping or causing overheating.

Water heaters often have a high inrush current when they first start, which can be several times higher than their normal operating current. By sizing the branch circuit to 125 percent of the nameplate rating, the installation allows for this initial surge without compromising safety or performance.

In contrast, other percentages, like 100 percent, do not provide the necessary buffer for the inrush current, which could lead to issues in operation, such as nuisance tripping of circuit breakers or extension of heating times. Similarly, 150 and 200 percent ratings would exceed the requirements and potentially lead to unnecessarily oversized wiring, increasing installation costs without providing additional safety or efficiency benefits. Hence, 125 percent is the optimal standard as defined by code.