FEEDER SIZE

[frostback]

ON A 3000A 277/480V SERVICE WE AND THE ENGINEERING COMMUNITY USE 8 SETS OF 4#500 CU WITH
4WIRES PER CONDUIT NOT COUNTING THE NEUTRAL AS A CURRENT CARRYING CONDUCTOR THEREFORE NO DERATING
BASED ON 310.15B4
WE HAVE JUST HAD AN INSPECTOR SAY WE MUST USE 6OO
CU BECAUSE HE IS COUNTING THE NEUTRAL
THE PROJECT IS A TYPICAL MULTIUSE BUILDING WITH
MIXED LTG,MECH LOADS AND XFMRS FOR 120/208VLOADS
IS HE CORRECT

 

[charlie]

Re: FEEDER SIZE

PLEASE TURN OFF YOU CAPS LOCK, it looks like you are shouting at us.

The AHJ is correct but not for the reason stated. I am assuming this is in a duct bank and is installed underground. I am guessing that the conductors will fail if loaded to their 380 amperes capacity since the duct bank will not permit the dissipation of the heat from the center of the configuration. In other words, the insulation will fail and the phases will short out in a catastrophic manner.

 

[marketman]

Re: FEEDER SIZE

Your specification of 8 sets of 500 kcmil CU should be acceptable. If the inspector is taking exception to this by considering the neutral as current carrying I would assume that it is in reference to 310.15(B)(4)(c). I would review the load content of the project to consider how much equipment is capable if injecting harmonic current into the system. Electronic ballasts usually contribute a small amount, switch mode power supplies for PC workstations, variable frequency motor drives all have impact. The inspector will have to clarify what he feels constitutes a "major" portion. Perhaps if you can clarify that the portion of the load that is capable of injecting harmonics is minimal this will appease him.

If the article referenced above is the correct one being applied by the inspector I would point out that it is worded as a "circuit" requirement and not a "feeder" requirement.

Also of note is the fact that 600kcmil at 420 amperes derated 80% (for the 4 to 6 current carrying qualification) is good for 336 amps.
336 x 8 sets = 2,688 ampere feeder capacity.
His suggestion doesn't satisfy the code section he is enforcing. Careful putting that one out there as it could come back to bite ya...

 

[marketman]

Re: FEEDER SIZE

After review of additional material it seems that the NEC task group Ad Hoc Subcomitte on Non Linear Loads evaluated both branch circuits and feeders, therefore the codes literal wording of "circuit" in 310.15(B)(4)(c) could be enforced as "feeder" as well.

Therefore, I would strike the second statement from my previous post.

 

[charlie b]

Re: FEEDER SIZE

My take is that there is not enough information to give you an answer. You did not tell us the calculated load, and you did not describe the type of installation. Is it (as the other Charlie conjectured) underground? If so, what is the configuration and the spacing?

If the results of a calculation (per NEC 220) shows a load of under 3040 amps, and if you have 2 parallel sets of 4 ducts spaced 7.5 inches apart center-to-center, then I would sign off on the use of 500 MCM copper.