Equipment Grounding Conductor
- Thread starter danilo
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petersonra
Senior Member
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- Northern illinois
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- engineer
Re: Equipment Grounding Conductor
The available short circuit current does not affect the ground wire size.
8 mm^2 metric size wire is about #8.
Table 250.122 says that #8 can be used on a circuit with an OCPD of not more than 100A.
However, I am not sure if 8mm^2 wire is just a hair smaller than #8 or just a hair larger. If it is larger than #8, you would be OK (assuming you have not increased the size of your power conductors). If in fact 8mm^2 wire is just a hair smaller than #8, than you would have to use the chart selection for the next size down (#10) which is only good for a 60A OCPD.
<added> I did a little more searching. It appears 8mm^2 wire is slightly smaller than #8, so I would conclude it is too small to use as the egc on a circuit supplied by a 70A OCPD.
[ August 01, 2005, 08:17 AM: Message edited by: petersonra ]
The available short circuit current does not affect the ground wire size.
8 mm^2 metric size wire is about #8.
Table 250.122 says that #8 can be used on a circuit with an OCPD of not more than 100A.
However, I am not sure if 8mm^2 wire is just a hair smaller than #8 or just a hair larger. If it is larger than #8, you would be OK (assuming you have not increased the size of your power conductors). If in fact 8mm^2 wire is just a hair smaller than #8, than you would have to use the chart selection for the next size down (#10) which is only good for a 60A OCPD.
<added> I did a little more searching. It appears 8mm^2 wire is slightly smaller than #8, so I would conclude it is too small to use as the egc on a circuit supplied by a 70A OCPD.
[ August 01, 2005, 08:17 AM: Message edited by: petersonra ]
- Location
- Illinois
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- retired electrician
Re: Equipment Grounding Conductor
Bob,
Don
Bob,
Actually it does. Look at 250.4(A)(5) and the note at the bottom of Table 250.122.
Don
rcwilson
Senior Member
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Re: Equipment Grounding Conductor
Per the IEEE Buff Book, Fig 150, (1986 edition) #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.
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.
A smaller wire may not fuse and melt at these ampacities, but it could get hot enough to do damage to the power wiring in the same raceway.
Note that it only takes 20 feet of #8 wire to reduce the maximum fault current below the wires' 9000 A short circuit rating, assuming the same MCB clearing time.
Per the IEEE Buff Book, Fig 150, (1986 edition) #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.
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.
A smaller wire may not fuse and melt at these ampacities, but it could get hot enough to do damage to the power wiring in the same raceway.
Note that it only takes 20 feet of #8 wire to reduce the maximum fault current below the wires' 9000 A short circuit rating, assuming the same MCB clearing time.
Re: Equipment Grounding Conductor
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?
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
Re: Equipment Grounding Conductor
Wow! And I thought my Weller soldering gun got hot quickly!
petersonra
Senior Member
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- Northern illinois
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- engineer
Re: Equipment Grounding Conductor
I suppose it must be time to buy a new handbook. My 1990 copy does not even have a paragraph 250.4.
Why don't they write these things in an understandable way?
I suppose it must be time to buy a new handbook. My 1990 copy does not even have a paragraph 250.4.
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