Tap Rule

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sbutler

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Came across something I have never seen before and want some input.

We have a 400A MLO panel directly fed from a DTT. The panel is old and there are no breakers available to purchase to add in the panel. So, the thought is to tap the existing feeder and add a separate, 100A panel. If the tap is more than 10' but less than 25', 240.21(2)(1) says "the ampacity of the tap conductors is not less than 1/3 of the rating of the over-current device protecting the feeder conductors."

In this case, there is no OCPD so how would you determine what size the tap conductor should be?

Thanks for any discussion this may generate.

Steve
 
sbutler said:
Came across something I have never seen before and want some input.

We have a 400A MLO panel directly fed from a DTT. The panel is old and there are no breakers available to purchase to add in the panel. So, the thought is to tap the existing feeder and add a separate, 100A panel. If the tap is more than 10' but less than 25', 240.21(2)(1) says "the ampacity of the tap conductors is not less than 1/3 of the rating of the over-current device protecting the feeder conductors."

In this case, there is no OCPD so how would you determine what size the tap conductor should be?

Thanks for any discussion this may generate.

Steve
Click to expand...

What's a DTT?
 
sbutler said:
Came across something I have never seen before and want some input.

We have a 400A MLO panel directly fed from a DTT. The panel is old and there are no breakers available to purchase to add in the panel. So, the thought is to tap the existing feeder and add a separate, 100A panel. If the tap is more than 10' but less than 25', 240.21(2)(1) says "the ampacity of the tap conductors is not less than 1/3 of the rating of the over-current device protecting the feeder conductors."

In this case, there is no OCPD so how would you determine what size the tap conductor should be?

Thanks for any discussion this may generate.

Steve
Click to expand...

If it is a feeder it has to have OCPD somewhere upstream.

Since it is a MLO panel, code would require some kind of upstream OCPD to protect the panel board bus.

If no OCPD than they would have to be service conductors. But again being a MLO panel, there would have to be upstream OCPD so they aren't service conductors.
 
How many breakers are there in the MLO panel? If it is more than 6, you have an existing code violation that you would need to address with the owner, during the process of designing whatever project you are doing for the owner now.
 
charlie b said:
How many breakers are there in the MLO panel? If it is more than 6, you have an existing code violation that you would need to address with the owner, during the process of designing whatever project you are doing for the owner now.
Click to expand...
Depends on a few conditions. Need input from sbutler.

Obviously, if this is a service transformer the 6 breaker rule would apply. Being a DTT, it is quite likely not a service transformer.

If it is two-wire or delta-delta, the upstream protection may suffice... but otherwise is he definitely looking at a panel overcurrent protection violation [408.3(B)].
 
sbutler said:
Came across something I have never seen before and want some input.

We have a 400A MLO panel directly fed from a DTT. The panel is old and there are no breakers available to purchase to add in the panel. So, the thought is to tap the existing feeder and add a separate, 100A panel. If the tap is more than 10' but less than 25', 240.21(2)(1) says "the ampacity of the tap conductors is not less than 1/3 of the rating of the over-current device protecting the feeder conductors."

In this case, there is no OCPD so how would you determine what size the tap conductor should be?

Thanks for any discussion this may generate.

Steve
Click to expand...

If it were me, I would remove the old existing DP that is fed by the DTT and install a new DP with a MCB so I could install more MWBC's but that's just me JAP>
 
charlie b said:
How many breakers are there in the MLO panel? If it is more than 6, you have an existing code violation ...
Click to expand...

Hold on there.
Decades ago MLO panels were commonly connected to transformers. It has only been relatively recent that the NEC has specific language prohibiting the majority of these installations.
 
jim dungar said:
Hold on there.
Decades ago MLO panels were commonly connected to transformers. It has only been relatively recent that the NEC has specific language prohibiting the majority of these installations.
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Perhaps but I don't think that you would be allowed to hack into it in the way being described even if it might have been legal 20 years ago.

I don't see anything in the code that would allow tap conductors to the secondary conductors to be protected by the primary protection. The code just doesn't allow for it.
 
petersonra said:
...I don't see anything in the code that would allow tap conductors to the secondary conductors to be protected by the primary protection. The code just doesn't allow for it.
Click to expand...

I agree, there is no allowance for tapping a tap.
These are transformer secondary conductors so the OP needs to be looking at 240.21(C), which effectively always requires an OCPD for each set of conductors.
 
sbutler said:
Came across something I have never seen before and want some input.

We have a 400A MLO panel directly fed from a DTT. The panel is old and there are no breakers available to purchase to add in the panel. So, the thought is to tap the existing feeder and add a separate, 100A panel. If the tap is more than 10' but less than 25', 240.21(2)(1) says "the ampacity of the tap conductors is not less than 1/3 of the rating of the over-current device protecting the feeder conductors."

In this case, there is no OCPD so how would you determine what size the tap conductor should be?

Thanks for any discussion this may generate.

Steve
Click to expand...

If you look hard enough I bet you could find some NOS or used/remfg breakers for the panel.
 
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