310.4 nec

[contactm]

I installed 7 sets of 300mcm for a 2000 amp ct cabinet which totals 1995 amps, i was told to add 1 set OF 300mcm because i'm short 5 amps:rant:, i couldn't run the new set underground as the old one so i ran it in rigid conduit above ground, the original sets of conduits are about 9 feet long and the new one is about 8 foot long, now i got a violation that it does not conorm with 310.4 that it has to be the same length, i took a reading on the wire and the new wire is with in a 3 amp range of the old, can someone advise how i can convince the inspector that i am conforming with 310.4:angel:

 

[Dennis Alwon]

Was it not possible to leave an extra foot on the last run? I am not sure there is any support for you other than show him the readings. I would think in a short run there could be more problems than a longer run with a difference of a foot.

 

[don_resqcapt19]

Even if the currents are the same, just the fact that one raceway is above ground and the others are underground makes it a code violation.

Are these service conductors and is there more than one service disconnect (overcurrent protective device)?

 

[raider1]

I agree with Don, if one raceway is in RMC and the other sets are underground and most likely PVC that would be a violation of 310.4 (310.10(H) 2011 NEC).

Chris

 

[augie47]

I agree the conduit run is a violation, but just to cover all bases... exactly what are you feeding ? CTs I know, but a panelboard/switchboard with a Main ? How did you determine the 2000 amp "load" ?

 

[iwire]

I agree the conduit run is a violation, but just to cover all bases... exactly what are you feeding ? CTs I know, but a panelboard/switchboard with a Main ? How did you determine the 2000 amp "load" ?

He is over 800 amps so the load is irelvent it will be the overcurrent device that matters, maybe he can reduce the overcurrent device size ....... Or add a second service disconnect.

 

[augie47]

I was not clear from the OP that he was feeding a singular main overcurrent device (I may be overlooking something)...
Thought possibly he was feeding multiple OCP devices.

 

[mayanees]

At this point your best bet may be to dial the LTPU on the breaker down to protect the cable.
John M

 

[iwire]

I was not clear from the OP that he was feeding a singular main overcurrent device (I may be overlooking something)...
Thought possibly he was feeding multiple OCP devices.

Ahh, got ya.

I had assumed one device due to the inspectors request.

 

[dicklaxt]

Is .25% current loss /set really a problem? Can you use 90* wire for your calcs somehow or maybe application of 310.15C will buy you a few amps?That math is beyond me but?????

dick

 

[infinity]

Since the 7 sets are underground can you use the ambient temperature correction factor of 1.05 for less than 78??

285 * 7 *1.05 = 2095 amps.

As an inspector I would accept this.

 

[augie47]

Since the 7 sets are underground can you use the ambient temperature correction factor of 1.05 for less than 78??

285 * 7 *1.05 = 2095 amps.

As an inspector I would accept this.

I was leaning that route also as I would want to accept the install...trying to find a reason to do so...
Thought about 310.15(A)(2) Exception, but the runs are so short, 10% of the length is nil.

 

[Ponchik]

Is this an engineered job or you did the design and build? How come it was not caught during plan review or maybe during plan check it was noted somewhere that 7 sets of 300MCM are ok???

 

[Dennis Alwon]

I understand the code issue but 5 amps... perhaps the inspector can give a variance which would be a better situation then the fix that may get done. Certainly the load is no where near the capacity anyway.

 

[jumper]

Even if the currents are the same, just the fact that one raceway is above ground and the others are underground makes it a code violation.

Are these service conductors and is there more than one service disconnect (overcurrent protective device)?

I agree with Don, if one raceway is in RMC and the other sets are underground and most likely PVC that would be a violation of 310.4 (310.10(H) 2011 NEC).

Chris

I must be dense this morning, not unusual, but where does it say that each raceway or conduit must be the same materiel or be ran in the same fashion? Conductor characteristics must be the same, but raceway.....:?

 

[iwire]

Here


C) Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.

 

[jumper]

Here


C) Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.

Thanks, I missed that little part.:slaphead:

I saw this:

the cables or [...........] conductors shall have the same number of conductors and shall have the same electrical characteristics.

:ashamed1:

 

[George Stolz]

Another option is likely 90 degree terminations, which would put you way over the top.

 

[kwired]

I installed 7 sets of 300mcm for a 2000 amp ct cabinet which totals 1995 amps, i was told to add 1 set OF 300mcm because i'm short 5 amps:rant:, i couldn't run the new set underground as the old one so i ran it in rigid conduit above ground, the original sets of conduits are about 9 feet long and the new one is about 8 foot long, now i got a violation that it does not conorm with 310.4 that it has to be the same length, i took a reading on the wire and the new wire is with in a 3 amp range of the old, can someone advise how i can convince the inspector that i am conforming with 310.4:angel:

I am basing this response on 2008 NEC because 2011 does not have a section 310.4.

I did not check your conductor ampacities, but assuming you do have an ampacity of 1995, that is a violation of 240.4(C). If the overcurrent device is over 800 amps you can not round up to next higher overcurrent device - you must have conductor that meets or exceeds the overcurrent device in ampacity.

Next part, 310.4(C) says:"Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics".

You said you ran RMC above ground for the additional set of conductors you installed. Not only do the conductors need to be same size, type, length, they need to be in same type of raceway to achieve same electrical characteristics. Should your underground raceways be PVC and your above ground raceway be RMC - you will have a definite difference in impedance in conductors in one of the underground raceways than you have in the above ground RMC - especially during high current instances like a short circuit or ground fault.


When you compared amps on the additional conductors to that of the original conductors, what kind of load was on the service? If it was lightly loaded your reading of only 3 amps difference may have very little value compared to if you were running near full capacity.

Other possible directions to go with this?

Do you have a single 2000 amp overcurrent device that this is protecting this?

It would be possible to have your original 1995 amps worth of conductors supplying multiple service disconnecting means as long as the calculated load was 1995 or less amps, and you could even have more than 2000 amps total service disconnecting means and still be ok with those same conductors.

I know, all that over 5 amps of capacity, but that is what is written in the book.

 

[bullheimer]

i dont think you have any other choice than to run with the 1.05 correction factor and removing the run you added. best of luck. ps. i thought that idea was a brilliant solution! cudos! infinity.