What is the minimum standard size overcurrent protective device required for a 480-volt, 3-phase feeder with 85 amperes continuous and 50 amperes noncontinuous load?

To determine the minimum standard size overcurrent protective device for a feeder, it’s important to understand the difference between continuous and non-continuous loads, as well as the calculation method for sizing the protective device.

In this scenario, you have a continuous load of 85 amperes and a non-continuous load of 50 amperes. According to the National Electrical Code (NEC), a continuous load is defined as a load where the maximum current is expected to continue for three hours or more. Non-continuous loads are those that do not meet this criterion.

To size the overcurrent protective device, the continuous load must be multiplied by 1.25 to ensure that the device can handle the sustained current without tripping. So, the calculation would be as follows:

1. Calculate the adjusted continuous load:

• 1.25 x 85 A = 106.25 A

2. Add the non-continuous load to the adjusted continuous load:

• 106.25 A + 50 A = 156.25 A

Once the total load has been calculated, the next step is to choose a standard device size that is equal to or exceeds this calculated value. The closest standard size from the options