Service Tap/Switchboard Question

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icefalkon

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I have a 3 Phase 208V/2500A Rated Main Switchboard and need to tap the lines to run a new 200A feeder. This has been approved, the service can handle the additional load without a problem. My question is...what size does the fuse protection have to be in this case? I have been told that it has to be 10% of the rating of the switchboard (which would be 250A) with wires sized accordingly. However the person telling me this has been wrong before.

Do the fuses have to be 250A or can they be 200A, with wires sized accordingly?
 
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augie47

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my question was answered in the post... sorry
 
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augie47

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Tennessee
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State Electrical Inspector (Retired)
If you are "tapping" service conductors to feed another service disconnect, there in no minimum size. Size the conductors and overcurrent for the load.
Just keep in mind that the new 200 amp overcurrent protection has to be treated as a service {proper bonding, neutral sized per 250.24(C), etc.}

If you were tapping a feeder the rule in 240.21 apply.
 
If you are SPLICING the conductors on the line/supply side of the service disconnecting means, then the conductors are not considered tap conductors - see the definition of taps in 240.2.

With that said, there is no minimum size of the conductors you are splicing in relation to the size of the service. The conductors will be sized according to the overcurrent device they will be supplying.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If you are SPLICING the conductors on the line/supply side of the service disconnecting means, then the conductors are not considered tap conductors - see the definition of taps in 240.2.

With that said, there is no minimum size of the conductors you are splicing in relation to the size of the service. The conductors will be sized according to the overcurrent device they will be supplying.
Of course, in that case, those conductors are service conductors, and the disconnect-outside-or-nearest-the-point-of-entry rules apply.
 

icefalkon

Member
Hmmm

Hmmm

I wasn't asking about the size of the wires...The feed I'm running is for a 200A panel and we're tapping the load side coming out from the main disconnect...ie: tapping the busbars (which are rated for 2500A), mounting the lugs, and running the wire to a fusible disconnect mounted in the same electrical closet, then out to the panel.

What I was told was that the fusible disconnect had to be fused at 10% of the rating of the busbars...I found nothing concerning this anywhere though. I was under the impression in this scenario you:

1. have to have a fused disconnecting means
2. fuse it at the size of the load you're serving (200A main lug panel)

Am I wrong here?
 

don_resqcapt19

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retired electrician
There is no minimum size of the overcurrent protection on the load end of the tap conductors that I know of. There are minimum sizes for the tap conductors.
 
There is no minimum size of the overcurrent protection on the load end of the tap conductors that I know of. There are minimum sizes for the tap conductors.


Don has pointed out a fact, based on the wording of the section,that is often misunderstood.

Since the OP is tapping the load side of the service disconnect, he will be responsible for adhering to 240.21(B).

As an inspector, the first question I would have is:
Are you following the manufacturer's instructions for tapping the bus? Most have no answer for this question. There usually are no instructions for tapping the bus, so I ask them for a letter from the manufacturer that says what they plan on performing is acceptable to the manufacturer. The manufacturer will be looking for certain information to help them decide if your tap is permitted as per their specifications. More than not, the letter comes back not permitting the tap as per the contractor's request.
 

icefalkon

Member
OK

OK

Thanks for that Pierre, I informed the contractor I'm working for about this and he got on the horn with the switchboard manufacturer today. We told them what we wanted to do, gave them the information they requested (dimensions of the buses, the model and dimensions of the switchgear) and they told us they would call us later.

Three hours later we get a phone call from one of their engineers telling us he's emailing us the part numbers for the lugs/hardware they approve and a letter stating that they have been notified of the changes being done and that we (the contractor) are compliant with their requirements. I couldn't believe how fast they replied to us on this. I figured a few days before someone would get to us. However the engineer on the phone said that we are one of the few contractors that have actually called them on an item like this...considering its done all the time.

Go figure...

I still can't find anything mentioning 10% of the buses capacity in the NEC for this issue though so I'm calling it a busted myth.

Thank you all for your input once again!
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I think you will find that the "10%" mentioned comes from the fact that the least the conductor ampacity can be is 10% of the overcurrent ahead of the tap. That comes from 240.21(A)(1). Longer taps have greater ampacity requirements. So the minimum tap conductor on your 2500 amp switchboard will have to have 250 amp ampacity, so the 10% of buss rule has some merit in this situation.
 

icefalkon

Member
HMMM ok Augie, but that refers to the conductor, not the fuse size...

That has to be where the engineer got mixed up and sent me on this wild goose chase.

That being said, we can run 4 - 250MCM THWN (rated 255A@75C), but still only fuse the disconnect at 200A right?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Three hours later we get a phone call from one of their engineers telling us he's emailing us the part numbers for the lugs/hardware they approve and a letter stating that they have been notified of the changes being done and that we (the contractor) are compliant with their requirements.
That was a golden nugget. :) Send them a holiday card for sure.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
HMMM ok Augie, but that refers to the conductor, not the fuse size...

That has to be where the engineer got mixed up and sent me on this wild goose chase.

That being said, we can run 4 - 250MCM THWN (rated 255A@75C), but still only fuse the disconnect at 200A right?

no problem.......
 
Thanks for that Pierre, I informed the contractor I'm working for about this and he got on the horn with the switchboard manufacturer today. We told them what we wanted to do, gave them the information they requested (dimensions of the buses, the model and dimensions of the switchgear) and they told us they would call us later.

Three hours later we get a phone call from one of their engineers telling us he's emailing us the part numbers for the lugs/hardware they approve and a letter stating that they have been notified of the changes being done and that we (the contractor) are compliant with their requirements. I couldn't believe how fast they replied to us on this. I figured a few days before someone would get to us. However the engineer on the phone said that we are one of the few contractors that have actually called them on an item like this...considering its done all the time.

Go figure...

I still can't find anything mentioning 10% of the buses capacity in the NEC for this issue though so I'm calling it a busted myth.

Thank you all for your input once again!

LOL that's what I told the secretary to do yesterday! Send those boys a Holiday card! If I could, I'd buy them all a cup of coffee too!

You are welcome, and I am glad they sent a positive responses as well. It is nice to hear from a manufacturer that you are one of the few...who actually care and are trying to do it correctly.
Most who do not contact the manufacturer,do not because they are afraid of a negative response.
 
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