Table 250.122 "Automatic Overcurrent Device"

[Stonewall]

When using table 250.122, the overcurrent setting doesn't explicitly state instantenous so does this mean a tirp setting with a long time delay can be used?

For instance, lets say we have 50/51 protection on the circuit with the 51 set at 100A and the 50 set at 1000A. Is the intent of the NEC for us to use 100A as the setting for the automatic overcurrent device or 1000A?

 

[coulter]

I sure hope so. I've never seen a design that bases EGC on the Instantaneous setting.

Consider a coordination scheme where the large upstrean CB's have the instantaneous turned off and are using only an LT and ST. The LT setting is the one used to size the ECG.

Another note. For large feeders, often the coordination study shows the cable damage curves - the Trip curves are generally set under that (but not always). I've never seen an EGC cable damage curve on a coordination study.

I'm curious why you are asking. The question doesn't come up with QOs. The number on the CB handle is the one you use for sizing the EGC. Perhaps there is something context sensitive about your suituation.

carl

 

[Stonewall]

I'm asking because I design large feeders with power circuit breakers. The trip to the breaker comes from 50/51 device. I have always used the 51 setting as the overcurrent setting applied to table250.122, but I'm curious if others feel a need to use the 50 setting. Just FYI, since these are large feeders, I do go a step futher and confirm the EGC can handle the max ground fault current till the relay trips the breaker.

 

[don_resqcapt19]

That is a very good question as you really don't have an overcurrent device...that is a single device that provides, short circuit, ground fault and overload protection. It appears that the code rule was written with a single overcurrent device in mind and would expect that you would size the EGC based on the setting of the overload device.