Feed Thru Sub Panel

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luke13340

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Wisconsin
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Electrician
Question about a possible setup here
Customer has a 200 amp feed thru panel which is currently full except for the feed thru lugs. They want to add a 100 amp sub panel in the garage of the building. They want to avoid using tandem breakers to free up space for a 2-pole 100 amp breaker so here is my theoretical solution.

I want to come out of the feed thru lugs of the 200 amp panel with #2 aluminum conductors, and hit a 100 amp breaker in the sub panel to feed that panel. Is there anything preventing this? In my mind the conductors would be protected by the 100 amp breaker in the sub panel.

Any suggestions or answers are appreciated.
Thank you in advance
 
Welcome to the forum.

There are tap rules that allow such arrangements, but there's more to it than it physically fitting.

What is the present calculated load demand, and what is the new load?
 
LarryFine said:
Welcome to the forum.

There are tap rules that allow such arrangements, but there's more to it than it physically fitting.

What is the present calculated load demand, and what is the new load?
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I can take the time to do a load calculation, but there is very little load. the 200 amp service is on a garage that is 20x40 with a small kitchen included in there. Current load is very minimal, it was calculated at 6.5 kw
the new load would be for a bunk house that would be detached from the garage. As I run the calculation the total load on that end would be 4.7 kw
 
Another option would be to move one 2p or two 1p circuits to the new panel to make room for a 100a.

Then you could use a main-lug sub-panel and not be limited to feeder length and other limitations.
 
LarryFine said:
Another option would be to move one 2p or two 1p circuits to the new panel to make room for a 100a.

Then you could use a main-lug sub-panel and not be limited to feeder length and other limitations.
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I couldnt move any circuits to the new sup panel as it's going to be in a detached building.
If I end up having to use tandems then it's no biggie I was just looking to save some cost.

Is there a cost article for what I'm trying to accomplish? If so I'd love to study it myself
 
luke13340 said:
I want to come out of the feed thru lugs of the 200 amp panel with #2 aluminum conductors, and hit a 100 amp breaker in the sub panel to feed that panel. Is there anything preventing this? In my mind the conductors would be protected by the 100 amp breaker in the sub panel.
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There is a few issues with this scenario. As Larry stated you would need to comply with the 25' tap rule so that limits the length of the tap condcutors unless they are outside of the structure. Secondly #2 Al is only rated for 90 amps so you could not use a 100 amp OCPD at the end of the tap.
 
As @LarryFine mentions, there are 'tap rules' which might permit a version of what you describe.

The issue is that there are many details you need to get correct to do this.

Read section 240.21 in your code book, and see if one of the rules there fits your situation.

Jon
 
Outside feeder taps can be unlimited length, and depending on more details could be a way to do what you are proposing, otherwise you are going to be limited to 25 feet tap length. You would need at least 1 AWG aluminum for the tap conductors either way though if the breaker is 100 amps.
 
kwired said:
Outside feeder taps can be unlimited length, and depending on more details could be a way to do what you are proposing, otherwise you are going to be limited to 25 feet tap length. You would need at least 1 AWG aluminum for the tap conductors either way though if the breaker is 100 amps.
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Outside feeder taps require that the tap originate outside, and that does not sound like the case we have here.

Could come off the feed through lugs with 200 amp wire, to a splice box external to the building and then run 100 amp wire to the new panel with a 100 amp main in the second building. The 100 amp main would have to be outside, or inside nearest the point of entry of the conductors at the second building.
 
Did you look into what type of load is currently on the branch circuits that are in the feed through panel?

Many times it’s possible to combine a couple or even a few circuits to free up space for your added 2 pole breaker.

Just a thought
 
don_resqcapt19 said:
Outside feeder taps require that the tap originate outside, and that does not sound like the case we have here.

Could come off the feed through lugs with 200 amp wire, to a splice box external to the building and then run 100 amp wire to the new panel with a 100 amp main in the second building. The 100 amp main would have to be outside, or inside nearest the point of entry of the conductors at the second building.
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yes.

NEC may not directly say it for this instance but many AHJ's will let you consider the conductors to be outside if they are in raceway/cable that is encased in at least 2" of concrete as well. But that origin of the tap thing still is not "outside" is still a problem here.

Just some ideas for OP to think about since we don't have all the details.
 
kwired said:
NEC may not directly say it for this instance but many AHJ's will let you consider the conductors to be outside if they are in raceway/cable that is encased in at least 2" of concrete as well. But that origin of the tap thing still is not "outside" is still a problem here.
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Interesting point. IMO the definition of Conductors Considered Outside the Building should be in Article 100 so it applies universally to all code articles. 240.21(B)(5) does references 230.6.


240.21(B)(5) Outside Taps of Unlimited Length.
Where the conductors are located outside of a building or structure, except at the point of load termination, and comply with all of the following conditions:
(1) The tap conductors are protected from physical damage in an approved manner.
(2) The tap conductors terminate at a single circuit breaker or a single set of fuses that limits the load to the ampacity of the tap conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.
(3) The overcurrent device for the tap conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.
(4)The disconnecting means for the tap conductors is instal⁠led at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the tap conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the tap conductors


230.6 Conductors Considered Outside the Building.
Conductors shall be considered outside of a building or other structure under any of the following conditions:
(1) Where installed under not less than 50 mm (2 in.) of concrete beneath a building or other structure
(2) Where installed within a building or other structure in a raceway that is encased in concrete or brick not less than 50 mm (2 in.) thick
(3) Where installed in any vault that meets the construction requirements of Article 450, Part III
(4)Where installed in conduit and under not less than 450 mm (18 in.) of earth beneath a building or other structure
(5)Where installed within rigid metal conduit (Type RMC) or intermediate metal conduit (Type IMC) used to accommodate the clearance requirements in 230.24 and routed directly through an eave but not a wall
of a building
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infinity said:
Interesting point. IMO the definition of Conductors Considered Outside the Building should be in Article 100 so it applies universally to all code articles. 240.21(B)(5) does references 230.6.
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But only for the load end of the tap conductors, not the supply end.
 
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