wireway ampacity question

[310 BLAZE IT]

I have a confusing service entrance scenario that I really need a second opinion on...

we have an existing 1600A service entrance panel, a ~10' wireway, and a 1600A MDP.
The 10' wireway has what looks like three triplexed sets of 500kcmil conductors, 4 parallel 500kcmil non-triplexed neutral conductors, and what looks like a 500kcmil ground if the wireway is not rated for or used as one.
Temperature derating does not apply, number of conductor derating does not apply (under 30, wireway). I am using the table for this 'raceway' as wireway is defined as, 310.15(B)(16).

I am assuming this is 75C, rather than 90C. It seems like these conductors are undersized. Does anyone know of any way that the original engineer could have installed like this? Even if I miscounted 4 sets would not be enough. It can't be a delta because the GFCI at the service wouldn't work. 240.4(B) can't apply, over 800A and also a tap. I'm stumped. Only thing I can think of is that it was designed as a cable tray and installed as wireway.

any help is appreciated.

 

[augie47]

Does your MDP have a Main or is it MLO ?

 

[oldsparky52]

Was it installed before the 800-amp rule came into the code?

 

[310 BLAZE IT]

MLO <6 discs.

What is that? And when was that? Most likely the case. There are other existing condition code violations too

 

[310 BLAZE IT]

It is MLO.

I believe it is before the 800A rule. Can you enlighten me on the NEC's past? I only know 2008 to now. I guess 240.4(D) is a new section? the next breaker size is 1200A so makes sense if this is the case.

 

[don_resqcapt19]

It is MLO.

I believe it is before the 800A rule. Can you enlighten me on the NEC's past? I only know 2008 to now. I guess 240.4(D) is a new section? the next breaker size is 1200A so makes sense if this is the case.

That rule existed as Exception #1 to 240-3 in the 1978 NEC. Not sure how long it has been in the code as the 78 is the oldest code book I have.

 

[310 BLAZE IT]

That's pretty old. I'd say your book is older than this building, maybe there is another 800 amp rule he was referring to.

Any other ideas?

 

[kwired]

Augie may not have asked the question quite correctly - do you have a single 1600 amp main breaker or fuses as the service disconnecting means or do you have multiple service disconnecting means supplied by these conductors?

A single service disconnecting means would need 1600 amp conductors supplying it.

Four 400 amp service disconnecting means only would need supplied with conductors that have an ampacity equal or greater then the calculated load. That would be the common conductors supplying all four - each individual disconnect would still need at least 351 amp conductors or up to 400 if load calculation on that particular disconnect were over 350.

 

[310 BLAZE IT]

There is one service disconnecting means in the service panel. I drew a nice sketch but unfortunately it's over the attachment limit size.

There are two panels tapped to the service panel this bus. a landlord panel, with a 800A main breaker (<<80A load max) and the building MDP. The MDP is 1600A bus with 6 disconnects.

 

[augie47]

With the landlord Main Breaker panel and the 6 breakers in the 1600 MLO panel you are in violation of the 6 disconnect rule but as far as the serviced conductors are concerned they only need to be sized sufficient for the load since they are supplying more than one main.
The original job may have been to Code and a breaker was added.

 

[kwired]

There is one service disconnecting means in the service panel. I drew a nice sketch but unfortunately it's over the attachment limit size.

There are two panels tapped to the service panel this bus. a landlord panel, with a 800A main breaker (<<80A load max) and the building MDP. The MDP is 1600A bus with 6 disconnects.

The 800 amp panel could be supplied with a feeder tap - provided it meets all requirements that apply in 240.21(C)

The MDP needs 1600 amp or higher conductors supplying it if the overcurrent protection is 1600 amps.

The service conductors also need to be 1600 amp or higher if the single overcurrent protection device is 1600 amps.

 

[Carultch]

Was it installed before the 800-amp rule came into the code?

What is the history of the "next size up rule", 240.4(B), even existing in the first place? Never mind the 800A maximum.

It would seem that intuitively by default, you would need at least as much wire as you have overcurrent protection. So at some point, someone went out of their way to put the "next size up rule" in the code. Content that as long as you round up the number to the nearest standard size, it is OK.

It also makes me wonder just how close in ampacity the wire can be after all calculations, to the previous size down. 1 Amp? 1/10 of an Amp? You OK with 250.01 Amps worth of wire being protected by a 300A breaker?

 

[iwire]

It also makes me wonder just how close in ampacity the wire can be after all calculations, to the previous size down. 1 Amp? 1/10 of an Amp? You OK with 250.01 Amps worth of wire being protected by a 300A breaker?

Yes, as long as the calculated load is 250.01 amps or less.

As a side note when dealing with service conductors supplying multiple service disconnects we could have 100 amps of service conductor 'protected by' six 200 amp breakers as long as the calculated load is under 100 amps. See exception 3 of 230.90

As far as the NEC is concerned it is up to the future designers / installers to ensure the calculated load remains under the ampacity of the conductors.