Question about Table 250.122.

reyamkram

Senior Member
Location
Hanover park, il
What size the conductors feeding the 75 amp breaker? You need 100 amp or larger conductors on the load side of the bus duct disconnect and an EGC selected from T250.122 as Dennis stated.
There are 3-#6 THHN and one # 10 THHN, EGC feeding from the 3 phase 208 disconnect with 3-100 amp fuses.
I believe, there should be #3-THHN and one # 8 EGC , Because, 75 amps X 1.25 = 93.75 amps and the next higher OCPD is 100 amps.
the 75 amp disconnect (non- fused) is for the CNC machine from the manufacturer.
PLEASE, Correct me if I am wrong
thank you. for the information.
 

kwired

Electron manager
Location
NE Nebraska
There are 3-#6 THHN and one # 10 THHN, EGC feeding from the 3 phase 208 disconnect with 3-100 amp fuses.
I believe, there should be #3-THHN and one # 8 EGC , Because, 75 amps X 1.25 = 93.75 amps and the next higher OCPD is 100 amps.
the 75 amp disconnect (non- fused) is for the CNC machine from the manufacturer.
PLEASE, Correct me if I am wrong
thank you. for the information.
The #6 conductors might be fine, particularly if there are motor loads, which in a CNC there probably is.

But if 100 amp OCPD is used the EGC needs to be #8 even if ungrounded are not full (by rule/exceptions) 100 amp conductors.

The EGC never has to be larger than the ungrounded conductors it is associate with though, but you don't run into that too often other than maybe with a feeder tap situation.
 
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