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#29441 - 09/18/03 12:42 AM Power Company has there own code  
aldav53  Offline
Member
Joined: Aug 2001
Posts: 545
Chandler, AZ USA
Not sure why the power company has there own rules. They say their 4/0 feeder wire is good for a 400 amp service, but by NEC standards its a lot less, (around 250a depending on the insulation). I realize they do not have to follow the NEC but why such a difference?


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#29442 - 09/18/03 12:44 PM Re: Power Company has there own code  
DougW  Offline
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Joined: Jun 2003
Posts: 1,143
North Chicago, IL
Probably a difference in interpretation of manufacturer's data (familiarity) between NFPA and the PoCo.

The NEC has a much higher safety margin, to compensate for IF (Idiot Factor). Like the difference between ratings on extinguishers for non-FF's and FF's.

Remember the gas company guys who would smoke in the trench with an open line? It was because the atmosphere was above the UEL (too rich to burn)... they knew this, of course, but the first time you saw it you almost soiled yourself [Linked Image] !


#29443 - 09/18/03 01:42 PM Re: Power Company has there own code  
rmiell  Offline
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Joined: Nov 2000
Posts: 242
La Junta, Co. USA
Most utilities use the NESC (Nation Electrical Safety Code), which references the NEC. As for the ampacity of utility conductors, ask yourself, if a conductor will handle a certain ampacity, and you put that conductor in a conduit, can it handle more or less before damage. The answer is less. Now how about going the other way, where the conductor is in the open air. Can it handle more or less. The answer is, of course, more. The NEC also allows for this, in it's ampacity tables.

While it seems wrong that a power company will run #2 aluminimum to a 200amp service, when you are required to install 4/0, running it in free air does make a difference. The utility also does not have to install conducotrs for the worst case, as we do. If it is a 200amp (or 400amp) service, we need to install conductors that can handle this amount. The utility will install conductors that are rated less, due to the fact that it is in free air, and that the actual load will not be that much (cost savings measure). In the event of a fire, the conductor will burn in two, with no other damage except power loss, whereas if our wiring burns, more than the wire will burn, such as the building it is in.

If you look at table 310.17, you can see the differences.

We are not that different, the utilities and us end-users of electricity.

Rick Miell


#29444 - 09/18/03 02:28 PM Re: Power Company has there own code  
iwire  Offline
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Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
On another forum I have asked questions like this of a utility power guy and he had some great info.

As Rick pointed out if you look at NEC 310.17 4/0 copper is 300 to 400 amp rated 4/0 Aluminum is 235 to 315 amp rated.

Not that the POCOs use this table but it gives us an idea what the conductors can carry.

The POCOs know from experience that a service calculated by NEC standards will not draw the amount of load that the service size is.

Ask yourself, have you ever seen the POCOs conductors fail from overload?

The POCOs conductors are outside of the building if for some reason the conductors where to overheat they can not damage anything but POCOs equipment.

I agree its frustrating to pay for and install large conductors only to see them connected to much smaller conductors, but history shows the POCOs and the NESC know what they are doing.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#29445 - 09/18/03 03:13 PM Re: Power Company has there own code  
aldav53  Offline
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Joined: Aug 2001
Posts: 545
Chandler, AZ USA
The PoCo feeders are underground.
Actually 3/0 is good for 215 amps, should be ok for the 200 amp sub-panel.


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#29446 - 09/18/03 03:32 PM Re: Power Company has there own code  
rmiell  Offline
Member
Joined: Nov 2000
Posts: 242
La Junta, Co. USA
"The PoCo feeders are underground.
Actually 3/0 is good for 215 amps, should be ok for the 200 amp sub-panel."

Careful, aldav53, make sure to read the right table. 310.17 is for free air, not conduit, but even in 310.16 3/0 is good for 200amps, so it is ok for this subpanel.

If I read your last post right, the service lateral is underground, probably in PVC, and single phase. right? Then there will be not more than 3 current carring conductors, in a raceway, therefore the table 310.16 still applies. Allowances can be made for temp, but it will be nowhere close to 400 amps for a 4/0 copper wire.

Unfortunaly, utility workers tend to use the same ampacity of overhead wires for underground feeders, but there is a major difference.

Bring it to their attention, in writing. Then, when the feeders fail, you are not liable for any damage. I say when they fail bacause, with the wire rated at only 230 amps (copper) but the service rated 400amp, the continuious load most likely will be greater than 230amps, and failure will occur.

Good luck.



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