If you are working to the NEC, table 250.122 gives the required sizes.Originally posted by danilo:
...is it safe to use the minimum 8mm2 TW wire as equipment grounding for 70A,MCCB ...
I've not ever seen where the available fault current effects the size of an equipment grounding conductor.Originally posted by danilo:
...equipment grounding for 70A,MCCB if the bus symmetrical fault current calculation is 18ka?...
molded case circuit breaker (MCCB)Originally posted by coulter:
What's an "MCCB"?
carl
Actually it does. Look at 250.4(A)(5) and the note at the bottom of Table 250.122.The available short circuit current does not affect the ground wire size.
yeah i must agree with this analysis coz in my opinion, the values in Table 250.122 might have been computed differently or a higher rating of short circuit fault might not have considered.,just imagine if a 100A,MCCB will use #8 for its EGC connected to a bus with 22ka..could it be that this #8 is way below the minimum already?Originally posted by rcwilson:
If the fault current is really 18,000 A, a #4 AWG (25 mm) would be needed to keep the temperature below 200C. Assumptions: MCB clearing time of .02 seconds, initial temperature 75C, final temperature 200 C, rubber insulation.
Wow! And I thought my Weller soldering gun got hot quickly!Originally posted by rcwilson:
...#8 awg can handle 18000 A for 0.01 seconds (less than a cycle) before the insulated copper conductor reaches 200C and melts the insulation beyond repair.
Why don't they write these things in an understandable way?Originally posted by don_resqcapt19:
Bob,
Actually it does. Look at 250.4(A)(5) and the note at the bottom of Table 250.122.The available short circuit current does not affect the ground wire size.
Don