If I have 3 - 1/C #4/0 AWG power feeds to the an equipment, how do I determine the ground cable?

My FLA is approximately 150 Amps.

If I have 3 - 1/C #4/0 AWG power feeds to the an equipment, how do I determine the ground cable?
If your feeds to the equipment are each 150 Amps, and you have 3 of them, then you have a total of 450 Amps. However, I am assuming that your load only draws 150 amps, in which case 0 AWG should be OK. I would put a 150A fuse on your lines though just to be safe... by the way, I am getting my info from the powerstream website. I have a BASc in Mechanical Engineering. I am NOT a licensed engineer!!!
Reply:Your conductor rating should be OK for your FLA whether it is copper or aluminum. The ground cable should be colored green. I presume that you are feeding a single phase load with two conductors for the load and one for grounding.
Reply:Look at the National Electrical Code Table 250.122.





Per the NEC equipment ground sizing is determined by the rating or setting of the overcurrent device ahead of the equipment.





You don't exactly say what your feeding or what your overcurrent setting is but lets use the ampacity rating of the 4/0 from the NEC. Table 310.16 using the 75C column (the 90C column can not be used for ampacity because even though the wire is rated 90C, the terminations are only rated 75C. There are no 90C low voltage terminations) the ampacity of 4/0 is 230 amps.





This does not correspond to a standard fuse or breaker size so we are allowed to go up to the next standard size. In this case the next standard size is 250 amps.





Using the table 250.122 to determine grounding conductor size we see that 250 amps is not a standard rating here, so again we must go the next size up, 300 amps.





This give you a minimum grounding conductor size of #4 AWG copper or #2 AWG aluminum.





According to the NEC in no case shall the equipment grounding conductor be required to be larger than the phase conductors.





The reason a smaller grounding conductor can be allowed is due to the duration of the fault.





Conductors are actually rated according to two parameters. Current and time. Most times all anyone ever concerns themselves with is continuous current (time = infinity), this is where the ratings given by cable manufacturers comes from. However conductors do have a withstand rating and can carry extremely large currents but for a short period of time.





The National Electrical Code recognizes this and allows grounding conductors to be sized smaller than the phase conductors as they will carry a very large fault current, but only long enough for the breaker to open or the fuse to blow.





As most power circuit breakers open in 3 cycles or less and current limiting fuses open in 1/4 cycle or less you can see that the current the conductor can carry for this short amount of time can be very very large.





To get a good handle on equipment grounding consult the NEC Article 250. Be aware that this article also covers service grounding (grounding electrode conductors). These are two entirely different types of grounding. Don't confuse them.





Some cable manufacturers will sometimes go a size larger on their ground wire size but never smaller than dictated in Article 250.

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