The 125% rule and how it relates to the requirements for branch circuit ratings
Part II of Art. 210 provides the requirements for branch circuit ratings. The first requirement is that branch circuit conductors must have an ampacity at least as large as the load being served [291.1(A)(1)]. However, there's a twist on the ampacity requirements. It's called the 125% rule, and it applies to continuous loads. You have acontinuous load when "the maximum current is expected to continue for three hours or more” [Art. 100].
If a circuit has only noncontinuous loads, just add up the loads to get your total load (for ampacity). Before you add any continuous loads, you must multiply them by 125%. This rule applies to branch circuit overcurrent protection [210.20] as well.
The ampacity tables address only the minimum ampacities required for safeguarding personnel and property [90.1]. For operational efficiency, you also need to address voltage drop. You may address it in one of two ways:
- Increasing the conductor size. Exceeding the ampacity table sizing by one or two sizes is usually practical. However, costs and connector dimensions put real limits on how far you can go.
- Decreasing the conductor length. A best practice for 120V power distribution in a plant is to place the step-down transformer as close to the point of use as is practical, so your long runs are at 480V rather than 120V.