25' Tap Rule and ampacity of down stream

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123rashab

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project manager
hi all,

I am creating a preliminary design, I'm not a PE yet, here is the thought process:

tapping into a 2400A 480V gear for a 300kVA stepdown Delta-Y transformer(located outdoor ~ 150' from the gear), the load on the transformer is about 234kVA.
with the 25' tap rule i need my tap conductors t o be 1/3 of the feeder conductors which is 800A, I'm looking to install a disconnect within 25' and then run my wires to the transfomer.

Transformer Primary is 361A or 451A continuous; since my tap conductors have to be 800A rated or more can my disconnect still be 400A or do I now have to get an 800A disconnect because of the tap conductors? how about the wires from the disconnect to the transformer, can I reduce the wire size to an appropriate size for the 451A continous primary transformer? I understand being able to go up to 250% on the primary but I dont wish to do that given my load.

thank you,
 
It sounds like you would be using the outside tap rule, which basically allows the conductors to be sized to the single overcurrent device required to be on the load end.
 
123rashab said:
hi all,

I am creating a preliminary design, I'm not a PE yet, here is the thought process:

tapping into a 2400A 480V gear for a 300kVA stepdown Delta-Y transformer(located outdoor ~ 150' from the gear), the load on the transformer is about 234kVA.
with the 25' tap rule i need my tap conductors t o be 1/3 of the feeder conductors which is 800A, I'm looking to install a disconnect within 25' and then run my wires to the transfomer.

Transformer Primary is 361A or 451A continuous; since my tap conductors have to be 800A rated or more can my disconnect still be 400A or do I now have to get an 800A disconnect because of the tap conductors? how about the wires from the disconnect to the transformer, can I reduce the wire size to an appropriate size for the 451A continous primary transformer? I understand being able to go up to 250% on the primary but I dont wish to do that given my load.

thank you,
Click to expand...
The disconnect only has to be sized large enough for the load. Since this will require a 600 amp disconnect, you only need to make sure that the line side terminations are suitable for 800 amps of conductor. That may require optional line side lugs be specified.

If the conductors are all outside the rule in 240.21(B)(5) would apply as mentioned in post #3. In that case, if you use 450 amp fuses, you can use parallel 4/0s or any other conductor or combination of conductors with an ampacity of at least 450 amps.

On the load side of the overcurrent protective device, the conductors need to be sized to the OCPC. With that size transformer, I would install 450 amp fuses and parallel 4/0s to the transformer. You may be able to go smaller with the OCPD, but you have to be careful of the transformer inrush blowing the fuses.
 
Thanks Don for working thru that so well. I dont have the energy to follow along in the book I get the idea from that and while I dont work on that scale can "scale" it down, even apply to using feed thru lugs on simple 200A single ph.,
 
don_resqcapt19 said:
If the conductors are all outside the rule in 240.21(B)(5) would apply as mentioned in post #3. In that case, if you use 450 amp fuses, you can use parallel 4/0s or any other conductor or combination of conductors with an ampacity of at least 450 amps.
Click to expand...
Keep in mind, your EGC need to be 350KCMIL but not required to be larger than your phase conductors (ungrounded conductors) 250.122 (G). I.E if you use 4/0s then your EGC should be also 4/0.
 
tom baker said:
Please clarify where your tap conductors start and end
Click to expand...
my tap starts in the gear inside the electric room, and the disconnect will be inside to comply with 25' rule. and then my transformr is located outside the building
 
Strathead said:
It sounds like you would be using the outside tap rule, which basically allows the conductors to be sized to the single overcurrent device required to be on the load end.
Click to expand...
I thought for the outside rule the gear to be tapped and the transformer has to be outside, am i incorrect in thinking that since my gear is inside the building's electric room and the stepdown transformer is outside the building the outside tap rule wont work?
 
don_resqcapt19 said:
The disconnect only has to be sized large enough for the load. Since this will require a 600 amp disconnect, you only need to make sure that the line side terminations are suitable for 800 amps of conductor. That may require optional line side lugs be specified.

If the conductors are all outside the rule in 240.21(B)(5) would apply as mentioned in post #3. In that case, if you use 450 amp fuses, you can use parallel 4/0s or any other conductor or combination of conductors with an ampacity of at least 450 amps.

On the load side of the overcurrent protective device, the conductors need to be sized to the OCPC. With that size transformer, I would install 450 amp fuses and parallel 4/0s to the transformer. You may be able to go smaller with the OCPD, but you have to be careful of the transformer inrush blowing the fuses.
Click to expand...
thank you Don. so I can theoretically tap my , inside electric room gear with parallel 4/0 and since they would terminate outside the building, i can just install my disconnect on the outside near the transformer and continue with my installation without needing conductors of 1/3 the 2400A gear.

with the below code- I think it seems feasible to run it as stipulated above?

*240.21(B)(5)
(4) The disconnecting means for the 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 conductors

c. Where installed in accordance with 230.6, nearest
the point of entrance of the conductors
 
123rashab said:
thank you Don. so I can theoretically tap my , inside electric room gear with parallel 4/0 and since they would terminate outside the building, i can just install my disconnect on the outside near the transformer and continue with my installation without needing conductors of 1/3 the 2400A gear.

with the below code- I think it seems feasible to run it as stipulated above?

*240.21(B)(5)
(4) The disconnecting means for the 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 conductors

c. Where installed in accordance with 230.6, nearest
the point of entrance of the conductors
Click to expand...
no there was a misunerstanding. you rconductors would need to be entirely outside, except for the point of termination. So you are stuck with the 1/3 rule. Can you use the 10' rule to drop the size a bit?
 
Engser18 said:
Keep in mind, your EGC need to be 350KCMIL but not required to be larger than your phase conductors (ungrounded conductors) 250.122 (G). I.E if you use 4/0s then your EGC should be also 4/0.
Click to expand...
If the ungrounded conductors are two parallel 4/0 - the effective conductor size is 423200 circular mils. Still need full sized EGC.
 
123rashab said:
I thought for the outside rule the gear to be tapped and the transformer has to be outside, am i incorrect in thinking that since my gear is inside the building's electric room and the stepdown transformer is outside the building the outside tap rule wont work?
Click to expand...
Yes, but until you clarified, I thought you were outside.
 
123rashab said:
thank you Don. so I can theoretically tap my , inside electric room gear with parallel 4/0 and since they would terminate outside the building, i can just install my disconnect on the outside near the transformer and continue with my installation without needing conductors of 1/3 the 2400A gear.

with the below code- I think it seems feasible to run it as stipulated above?

*240.21(B)(5)
(4) The disconnecting means for the 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 conductors

c. Where installed in accordance with 230.6, nearest
the point of entrance of the conductors
Click to expand...
The tap cannot originate inside for the outside tap rule.
(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: ...
Click to expand...
 
kwired said:
Running full size feeder conductor to the outside then making the tap should be compliant though. Maybe not always practical but should be compliant.
Click to expand...
Yes, that would be compliant, but highly unlikely it would be practical for this installation.
 
electrofelon said:
no there was a misunerstanding. you rconductors would need to be entirely outside, except for the point of termination. So you are stuck with the 1/3 rule. Can you use the 10' rule to drop the size a bit?
Click to expand...
the Electric room is packed with equipment and the only 10' distance will end us up in the stairwell which the fire marshall has disagreed to put any equipment there
 
kwired said:
Running full size feeder conductor to the outside then making the tap should be compliant though. Maybe not always practical but should be compliant.
Click to expand...
can you please elaborate on this.. full size feeder conductor? full size of what exactly, the 1/3 ampacity of the gear size, so 800A feeders tapped from interior gear and ran all the away to the exterior and place a disconnect outside?
 
123rashab said:
can you please elaborate on this.. full size feeder conductor? full size of what exactly, the 1/3 ampacity of the gear size, so 800A feeders tapped from interior gear and ran all the away to the exterior and place a disconnect outside?
Click to expand...
2400 amp of conductor from the tap connection to the outside of the building, and spliced conductors sized for your load and run to a load end OCPD.

800 amps to a disconnect with no more than 25' of wire is the 25' tap rule.

If you can get the disconnect with in 10' wire feet of the connection to the 2400 amp circuit, the wire would only have to be 240 amps to meet the tap rule, but your circuit needs more current than that.
 
don_resqcapt19 said:
2400 amp of conductor from the tap connection to the outside of the building, and spliced conductors sized for your load and run to a load end OCPD.

800 amps to a disconnect with no more than 25' of wire is the 25' tap rule.

If you can get the disconnect with in 10' wire feet of the connection to the 2400 amp circuit, the wire would only have to be 240 amps to meet the tap rule, but your circuit needs more current than that.
Click to expand...
Thank you, Don! you are absolutely correct in this being not so practical of an installation. appreciated!
 
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