different ground wire sizes #6 MC and #6 romex

[Stevenfyeager]

Why is romex #6-3 ground wire size 10 and MC cable ground wire size #8 ? for running conductors in conduit, which size should I use ? Thank you

 

[suemarkp]

It depends on what size over current device is protecting it. 30A to 60A, use #10. 70A to 100A, use #8. NM cable is limited to 60C, so 55A ampacity and maybe a 60A breaker at most. MC cable is probably 75/90C, so it would have an ampacity of 65A in #6 cu and could be on a 70A breaker.

 

[Seven-Delta-FortyOne]

Based on size of OCPD. 250.122

 

[LarryFine]

Why is romex #6-3 ground wire size 10 and MC cable ground wire size #8 ?

As they said above, conductors in MC have a higher ampacity than in NM for a given size.

for running conductors in conduit, which size should I use ?

That depends on the load current. MC cable and individual wires have the same ampacity.