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?
The Golden Rule - "The man with the gold makes the rule"
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 !
#29443 - 09/18/0301:42 PMRe: Power Company has there own code
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.
#29444 - 09/18/0302:28 PMRe: Power Company has there own code
"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.