Electrical Panel 6

[Reesh14]

I received an RFI from an electrical contractor regarding problems with an electrical panel. They are having problems terminating the feeder cables for a panel (120/208v,3phase,4wire, with a 225 Main Circuit Breaker). The cables are too large and the panel or the main circuit breaker cannot be modified to accomodate them. The panel is being fed through a 112.5kva transformer from another panel. The feeder is 4-600kcmil AWG and 1-#4/0 AWG Gnd. in 3.5" EMT conduit. I believe the sizing is correct.

I was told by a senior electrical engineer that he has never heard of any such problems like this during construction. Any advice/ideas as to why the feeder will not fit into the main breaker would be helpful.Is it simply that the panel that is being used is not capable of such a large feeder size? Thank you!

 

[waross]

The Ampacity of 400 MCM aluminum in raceway at 60C is 225 amps.
I would not expect a 225 amp breaker to accomodate larger wire.
I would normally expect copper cable rated at either 75C or 90C. for 225 amps that would be #4/0.
In your case I suggest exploring methods of installing 4/0 pigtails or pigtails that will fit in the breaker, on the end of the 600 MCM conductors.
reference CEC.
respectfully

 

[davidbeach]

600kcmil????? Why would you have 420A conductors going to a 225A circuit breaker and expect that to work? A 75kVA transformer would probably have been a better size, or a 400A panel, but you just have things too mis-matched.

 

[jraef]

<Playing Devil's Advocte here>

Voltage drop or high ambient perhaps? He made no mention of the distance, that is often a challenge. Also we don't kow the ambient derating factor (if any) that may have required him to derate. He also didn't specify if the cable was 60, 75 or 90 degree rated, or the conductor material. For all we know he has Aluminum conductors, type THW 75deg insulation in a 55deg. C environment.

Termination companies make crimp-on "pins" just for that purpose by the way.

Ilsco link



Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

Breaker lugs are designed to accomodate an expected range of wire sizes. Your conductor size is outside of that range.

Sometimes this happens when conductors are increased in size to minimize voltage drops in long runs or to allow for future capacity expansion. There are a lot of legitimate reasons.

I would contact the breaker and/or panelboard manufacturer and see if they have any options to offer. If not, you will need to use an approved fitting to create a pigtail that is small enough to get into the lug kits that you have - if you have enough room.

 

[mpparent]

I'm not sure....but maybe he can use a compression "finger" reducing fitting. I believe T&B makes them, but I'm not sure about the sizes.

Mike

 

[RalphChristie]

Or add an extra termination box next to the panel. End the big cable in the termination box and take smaller wires from there to the panel. That is if you have to use the big cables.

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[Reesh14]

Thank you everyone!

 

[jraef]

Reesh14,
Don't leave us hanging, why DID you oversize the cable like that? Curious minds want to know.

 

[mpparent]

How about this from Burndy?

https://portal.fciconnect.com/porta...RB&productID=YRB36U31&P=136521,127440&FAM=YRB

Mike

 

[rbulsara]

Or the panel is undersized!! Change the panel and MCB to 400A.

 

[Reesh14]

I am not exactly sure why the cables were oversized or the panel was undersized. This was designed by a former colleague about a year ago. I think he calculated the total connected amps for the panel to be about 197 amps and chose the 225 amp panel. The new panel is a double panel. The left side is designated for kitchen equipment and the right side is for HVAC equipment. He applied a 50% D.F. for non continous kitchen equipment on the panel. The total connected kva was 71 which is probably why the 112.5 kva transformer was selected in the first place. I guess he didn't pay attention to any cable sizing issues in terms of feeding the panel. Distance of the run might have been a factor because the original location for the new panel was three floors below the panel feeding it, but I believe it has since been moved closer.

Thanks for your advice everyone! I really appreciate it.

 

[jraef]

Some people just think "bigger is better" without thinking through the consequences.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[rbulsara]

Well, the design should be consistent.

225A panel and 75 kVA transformer or 400A panel and 112.5kVA tranformer but not the mix-match. The designer should make up his mind (correctly).

225A panel is only good for 180A which is 65 kVA. If this is adequate then pick a 75 kVA transformer and a 225A panel.

If he/she feels 75kVA is not good and picked 112.5 kVA. It makes sense to pick a panel to permit full use of the 112.5kVA (or 311A) and a 400A panel makes most sense.

The design as stated in the original post is inconsistent or indecisive.