Cable of medium voltage loop

[m sleem]

We have 13.8KV loop feeding no of transformers, the first transformer is connected to the MV switchgear through 600MCM, from transformer to another the wire size is 350MCM.
The loop is existing and anstalled before 20 years without any problem.

Does NEC permit different wire sizes at the same loop?

 

[weressl]

We have 13.8KV loop feeding no of transformers, the first transformer is connected to the MV switchgear through 600MCM, from transformer to another the wire size is 350MCM.
The loop is existing and anstalled before 20 years without any problem.

Does NEC permit different wire sizes at the same loop?

The feeder protection should be sized to the 350kCMil.

 

[hurk27]

I would think so as long as the OCPD was sized for the smallest wire in the loop? never seen that large of wire on a MV primary before:?

 

[masterinbama]

Are your loadbreak elbows fused?

 

[m sleem]

I would think so as long as the OCPD was sized for the smallest wire in the loop? never seen that large of wire on a MV primary before:?

Shall the transformer's primary conductor match the loop conductor?

The 300mm2(600mcm) is very common for such application, i don't know reason exactly, but i heared many from different sources that the medium voltage cable size match the short circuit level (based on IEC) requirements.

 

[m sleem]

Are your loadbreak elbows fused?

Yes, for the 350mcm cable.

No, for the 600mcm cable where it is protected by ACB in the MV switchgear.

 

[Smart $]

Shall the transformer's primary conductor match the loop conductor?

Is it a feeder loop with taps as primary conductors, or is it a feed-through loop (i.e. can be switched in and out of loop with transformer primary switches)? Your comment makes it sound like the former of the two. If so, under NEC it does not have to match, but the taps if smaller in size must be afforded protection by the loop OCPD.

The 300mm2(600mcm) is very common for such application, i don't know reason exactly, but i heared many from different sources that the medium voltage cable size match the short circuit level (based on IEC) requirements.

Have no idea how IEC requirements compare to NEC requirements... which are a short read, about a half page of text at the end of Article 240.

 

[weressl]

I would think so as long as the OCPD was sized for the smallest wire in the loop? never seen that large of wire on a MV primary before:?

500kCMil was what we were using for loop feed @ 13.8kV. There were a number of large transformers with primary selective disconnect. The loop WAS a true loop - having feeders at both ends - and the selective - one switch from either direction - disconnect would 'decide' which end would feed the transformer. All switches would be closed, except one, unless one or more of the transformers needed to be taken out of service or one of the segments of cable needed service. We no longer use loop though.....

 

[mayanees]

Look at NEC 240.100 & 101.
As long at the ocpd protects the lowest ampacity conductor, as laszlo and hurk state, it's okay.
But the level of protection isn't at the ampacity of the conductor, and can be as high as 3X for a fuse, or 6X for an electronic device.
240.100(C) provides a qualifier that provides assurance that the conductor won't be damaged during a fault.
John M

 

[m sleem]

Look at NEC 240.100 & 101.
As long at the ocpd protects the lowest ampacity conductor, as laszlo and hurk state, it's okay.
But the level of protection isn't at the ampacity of the conductor, and can be as high as 3X for a fuse, or 6X for an electronic device.
240.100(C) provides a qualifier that provides assurance that the conductor won't be damaged during a fault.
John M

From your opinion, either use of fuse or breaker doesn't change the conductor size.

And the tr's primary conductor shall be sized as the FLA as 240.4 & table 310.15 where the 240.4(b) could be applied.

So, from 240.100(c) & the above sections the OCPD on primary protects the conductor against short circuit and the conductor size has nothing to do with short circuit level.

 

[mayanees]

?

?

I'm not sure I understand your statement, but the sections you're referencing apply to conductors under 600 Volts, and your application is at 13.8 kV.
At that voltage, your limit is 300% of the ampacity using a fuse.
240.100(C) keeps it logical by forcing you to go to a Time Current Characteristic curve to ensure that the fuse opens before the thermal damage curve (line) for the cable is exceeded.
John M

 

[m sleem]

I'm not sure I understand your statement, but the sections you're referencing apply to conductors under 600 Volts, and your application is at 13.8 kV.
At that voltage, your limit is 300% of the ampacity using a fuse.
240.100(C) keeps it logical by forcing you to go to a Time Current Characteristic curve to ensure that the fuse opens before the thermal damage curve (line) for the cable is exceeded.
John M

You are right,

Anyway it seems to me there is no much information regarding this issue.

 

[mayanees]

m sleem,
I know at first glance it appears that feeders over 600 Volts are susceptible to damage from overload since you can have a protective device set as high as 600%. But it's there for a reason... protective device coordination.
The MV cables are sized per NEC 230, such that the amount of connected load drives the size of the feeders. Everything can come on at full load, and the feeder must be sized to accommodate that load.
The protective devices are allowed to be so large relative to feeder capacity per 240.100 because they are there to protect against short circuits, and by allowing them to be 3-6X, it gives room for the downstream devices to operate first. Note that 240.100(C) forces you to make sure the conductor can't be damaged during a short-circuit condition.
If you sum up all of the transformers or loads that are connected to your 13.8 kV 350 kCM cable, you should find that the full-load amperage is within the capacity of the cable.
I understand how it appears flawed, but it does make sense when you look at the big picture.
John M