EGC Sizing Simplified Help!!

[Cletis]

Can someone help with this. I keep getting cross referenced and and conflicting code articles no jiving.

RTU with full load at 75 amps 3 phase 208V

Running 1.25 conduit (flex to seal tight) with 3-#3 Cu and 1 - ???? EGC (25 ft run tops)

I'm seeing run same size #3 Cu (seems like overkill) and I'm seeing #6 0r #8 from 250.95

Can someone straighten me out of this once and for all simplified? I'd rather not run 4-#3's if not needed if needed I'll run it

250.122 doesn't have any column for 75 amps, I'm putting on a 3 pole 90 amp breaker and I'm upsizing conductors so RTU has less resistance also

 

[augie47]

In it's simplest form, the EGC would be sized from Table 250.122 based on your overcurrent device rating.
If that unit had a 100 amp fuse or breaker protecting the circuit, you would use a #8 EGC from Table 250.122

It gets a bit more complicated since you "upsized" the conductor. 250.122 (B) would require an upszie in the EGC proportional to the upsize in phase conductors. You can do the math and determine what size you need but in a case like this where the increase is minimal you could "rule of thumb" to the next larger EGC ( #6)