Tapping rules and reducing breaker size in an existing system

[Sidetrackd]

This is my first time posting to this forum, so please forgive me if I am posting a question that has already been answered.

I have been tasked to solve the following problem on one of our projects.

--The customer wants to replace the transformer and the existing main OCPD. The plan is to remove the existing 1200 amp main breaker and replace it with a 600 amp breaker (same frame type).

My problem is that the existing feeders are 3 runs of 500Kcmil (4-wire) THHN and are fused at 800 amps.

I plan to use polaris terminals to "tap" off of the ends of the existing feeders to convert the feeders to the proper configuration for the new breaker.

So 3 x 500s converted to 2 x 350s (breaker has double lugs) (350s less than 10 ft). Is this a legitimate means to adapt to the existing system?

 

[augie47]

Seems legit to me.

 

[iwire]

My first thought is to order the breaker with lugs on it for three 500s

If they can't provide that get lugs for two 500s and just tape off the extra unneeded conductors.

 

[Smart $]

My first thought is to order the breaker with lugs on it for three 500s

If they can't provide that get lugs for two 500s and just tape off the extra unneeded conductors.

The latter, though potentially compliant, may break the tap rules...???

 

[iwire]

I may be mistaken but I don't see the tap.

Two sets of 500s would be fine on a 600 P breaker

 

[Sidetrackd]

I may be mistaken but I don't see the tap.

Two sets of 500s would be fine on a 600 P breaker

I think I forgot to mention that this is a government contract. The civil service employee that engineered the project either over-designed the system intentionally or doesn't know how to calculate feeder sizes in parallel runs and went with the safe bet (re-pulling the original conductors that are rated for the original 1200 amp service).

According to their drawings, we must use all 3 sets of 500s (fused at 800 amps). Since I plan landing 2 conductors, for each phase, thus reducing the over-all ampacity to less than 800 amps, I think it would be considered a tap.

I could be totally wrong though.

I might actually RFI and see if I can leave one set "abandoned" in place and just use two sets.**

**I am also not sure that two 500's would fit under the lugs on the breaker if I ordered it with double lugs, hence the original idea to use 350s for my tap.**

 

[Carultch]

This is my first time posting to this forum, so please forgive me if I am posting a question that has already been answered.

I have been tasked to solve the following problem on one of our projects.

--The customer wants to replace the transformer and the existing main OCPD. The plan is to remove the existing 1200 amp main breaker and replace it with a 600 amp breaker (same frame type).

My problem is that the existing feeders are 3 runs of 500Kcmil (4-wire) THHN and are fused at 800 amps.

I plan to use polaris terminals to "tap" off of the ends of the existing feeders to convert the feeders to the proper configuration for the new breaker.

So 3 x 500s converted to 2 x 350s (breaker has double lugs) (350s less than 10 ft). Is this a legitimate means to adapt to the existing system?

Since you are dealing with parallel sets of conductors, make sure you do not do anything to intentionally break the symmetry of the sets. This means you'd need a 5-terminal tap connector to accomplish your goal. 3 terminals to bring the incoming three sets together, and the remaining terminals for the outgoing 2 sets.

 

[Smart $]

I may be mistaken but I don't see the tap.

Two sets of 500s would be fine on a 600 P breaker

The two 500's landing on the line side of a 600A breaker is fine... it's the same two 500's leaving the load side of 800A fuses... without the third 500 in parallel (isolated at load end). With all three 500's they're a feeder. With just two 500's, they're a feeder tap.

 

[Smart $]

...I might actually RFI and see if I can leave one set "abandoned" in place and just use two sets.**

Two sets of 500's make them a parallel feeder tap from the 800A fuses (parallel 500's ampacity is less than 800A). How long are they? There's some concession for supervised industrial. Does the facility qualify?

**I am also not sure that two 500's would fit under the lugs on the breaker if I ordered it with double lugs, hence the original idea to use 350s for my tap.**

If your 600A is the same frame size as the former 1200A, I do not see why you couldn't get it with tri-lugs for 500's... :happyyes:

 

[jim dungar]

Two sets of 500's make them a parallel feeder tap from the 800A fuses (parallel 500's ampacity is less than 800A).

Paralleled 500's are rated for 760A and so may be protected at 800A per 240.4(B), therefore they are not a tap.
The 600A breaker ensures the load does not exceed the conductor rating.

 

[Smart $]

Paralleled 500's are rated for 760A and so may be protected at 800A per 240.4(B), therefore they are not a tap.
The 600A breaker ensures the load does not exceed the conductor rating.

You are correct.

I sometimes forget 240.4(B) & (C) thresholds are "800A and less" and "over 800A", respectively... a difference of 1A.

The 600A breaker is moot regarding the supply feeder.

 

[kwired]

I am seeing service>>>>>600 amp main breaker (used to be 1200 amp) >>>>>>>800 amp breaker>>>>>>> parallel 500MCM conductors.

The parallel 500's are essentially protected at 600 amps not 800.

 

[Smart $]

I am seeing service>>>>>600 amp main breaker (used to be 1200 amp) >>>>>>>800 amp breaker>>>>>>> parallel 500MCM conductors.

The parallel 500's are essentially protected at 600 amps not 800.

I'm of the impression it's...

service
|||
800A fused disconenct
|||
600A breaker (formerly 1200A)​

 

[iwire]

The two 500's landing on the line side of a 600A breaker is fine... it's the same two 500's leaving the load side of 800A fuses... without the third 500 in parallel (isolated at load end). With all three 500's they're a feeder. With just two 500's, they're a feeder tap.

I agree with you .... if your veiw of the configuration is correct.

To me the OP has not been clear, that was why I put in my disclaimer saying I could be mistaken.

 

[iwire]

**I am also not sure that two 500's would fit under the lugs on the breaker if I ordered it with double lugs, hence the original idea to use 350s for my tap.**

There is no reason that a 600 amp breaker in a 1200 amp frame size could not be ordered with lugs to fit three 500s.

 

[Smart $]

I agree with you .... if your veiw of the configuration is correct.

To me the OP has not been clear, that was why I put in my disclaimer saying I could be mistaken.

Well I got trumped on that anyway. Jim Dungar pointed out 800A supply side OCPD is just under the wire of 240.4(B)... so two 500's would be a feeder and not a feeder tap (assuming any applicable derating keeps the ampacity above 700A).