Does this chart refer to phase-phase voltages or phase-ground? I am trying to size the wire coming from a high voltage line to the primary side of a pole mounted transformer. Everyone I've talked to has told me to use a #6 solid copper with 5kV insulated sheathing. The voltage is 12470v/7200Y. To me it would seem you need a #2 minimum with a 15kV sheathing. What's the correct wire?
NEC 310.5
- Thread starter lizzie14
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Re: NEC 310.5
A conductor's insulation system is rated for the highest voltage at which it can be proven that no significant leakage current to ground will take place.
Phase to ground.
They are saying to use the conductor that is based on a rating up to 8000 volts, because the system operates at 7200 volts (not at 12,470). They are interpreting the table to mean phase to ground voltage. And they are right about that part. So a #6 is big enough, but the insulation system of the selected conductor must be good for 7200 volts or higher.
A conductor's insulation system is rated for the highest voltage at which it can be proven that no significant leakage current to ground will take place.
Re: NEC 310.5
I don't know why they told me 5kV, I assumed you would need 15kV (isn't that the next standard size?). I understand about the chart now, phase to ground so we're only 7200V. I'm having a hard time finding cables that are 15kV rated that come in anything smaller than a #2. Does someone know where I can find this? Is a #8, 15kV, XLP 100% insulation the correct choice?
I don't know why they told me 5kV, I assumed you would need 15kV (isn't that the next standard size?). I understand about the chart now, phase to ground so we're only 7200V. I'm having a hard time finding cables that are 15kV rated that come in anything smaller than a #2. Does someone know where I can find this? Is a #8, 15kV, XLP 100% insulation the correct choice?
physis
Senior Member
Re: NEC 310.5
I see the table values and the 2001 to 8000 volt range. I understand that but still don't see that as explaining 5000 volt insulation for 7200 volts to ground.
I don't work with these voltage ranges so I'm not familiar with common practices. And I also couldn't find whether to use voltage to ground or voltage between phases. But why would you not be concerned with the potential that exists between adjacent conductors. Are the laws of physics different between phases and grounds?
Edit: I'm sure there's a good reason for this Charlie, I just don't what it is.
[ August 09, 2005, 02:44 PM: Message edited by: physis ]
I see the table values and the 2001 to 8000 volt range. I understand that but still don't see that as explaining 5000 volt insulation for 7200 volts to ground.
I don't work with these voltage ranges so I'm not familiar with common practices. And I also couldn't find whether to use voltage to ground or voltage between phases. But why would you not be concerned with the potential that exists between adjacent conductors. Are the laws of physics different between phases and grounds?
Edit: I'm sure there's a good reason for this Charlie, I just don't what it is.
[ August 09, 2005, 02:44 PM: Message edited by: physis ]
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Re: NEC 310.5
The table is intended to mean phase to ground. Think of it this way: If an electron had it in mind to try to escape and see the world
, it would have to get past the insulation systems of two separate conductors. That would be harder, and we don't have to plan for that.
As to the 5kV recommendation: I'm sure they were "mis-quoted" (that's a kinder and gentler way of saying they were wrong).
Lizzie: I think it's time to call a few vendors. But I think you have the right type of cable.
The table is intended to mean phase to ground. Think of it this way: If an electron had it in mind to try to escape and see the world
, it would have to get past the insulation systems of two separate conductors. That would be harder, and we don't have to plan for that. As to the 5kV recommendation: I'm sure they were "mis-quoted" (that's a kinder and gentler way of saying they were wrong).
Lizzie: I think it's time to call a few vendors. But I think you have the right type of cable.
Re: NEC 310.5
Thanks for all of the vendors, I did call a few vendors and it looks like I'm on the right track. Only problem, if you need the 15kV insulation, the smallest they make is a #2. So...we're back up that size. Not a problem though, short length. As for the 'misquoted', yes that is much nicer. That must have been the case.
Thanks for all of the vendors, I did call a few vendors and it looks like I'm on the right track. Only problem, if you need the 15kV insulation, the smallest they make is a #2. So...we're back up that size. Not a problem though, short length. As for the 'misquoted', yes that is much nicer. That must have been the case.
physis
Senior Member
Re: NEC 310.5
But then the adventurous electron would have to overcome both insulations to go visiting in the other conductor.
Hmm. I'll have to give that some thought. My initial reaction is that it wouldn't matter because the same potential exists despite any insulations.But if it had in mind to pay a visit to a good looking electron sitting inside another conductor
But then the adventurous electron would have to overcome both insulations to go visiting in the other conductor.
curt swartz
Electrical Contractor - San Jose, CA
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Re: NEC 310.5
Many 5KV 133% cables are rated for 8KV at 100%
Many 5KV 133% cables are rated for 8KV at 100%
charlie
Senior Member
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- Indianapolis
Re: NEC 310.5
I don't work in our standards or purchasing areas so take my answer with a grain of salt.
The MV cable I have seen is rated for a 15 kV grounded wye system. That is the same as 8.66 kV class cable and it will be a minimum of #2 shielded cable. It is normal to build everything with elbow terminators into the transformers for pad mounter equipment. For pole mounted equipment, you will use cable terminators that have skirts on them to increase the creep distance. Elbows and terminators come with specific instructions that must be followed to the letter if you don't want the terminations to fail. if you are terminating cable in a dry location that is live front, you must use stress cones to terminate the cable with and then take the bare conductor to the terminal.
I don't work in our standards or purchasing areas so take my answer with a grain of salt.
The MV cable I have seen is rated for a 15 kV grounded wye system. That is the same as 8.66 kV class cable and it will be a minimum of #2 shielded cable. It is normal to build everything with elbow terminators into the transformers for pad mounter equipment. For pole mounted equipment, you will use cable terminators that have skirts on them to increase the creep distance. Elbows and terminators come with specific instructions that must be followed to the letter if you don't want the terminations to fail. if you are terminating cable in a dry location that is live front, you must use stress cones to terminate the cable with and then take the bare conductor to the terminal.
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