Powering a Trailer Park

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charlie b

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Staff member
Location
Lockport, IL
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Retired Electrical Engineer
I have to provide power to a set of 23 trailers that will serve as temporary classrooms while a school building is being renovated. The school district owns the existing overhead medium voltage distribution system, so the NEC applies. The plan is to run new overhead lines through the ?trailer park,? with step-down transformers every other pole or so. The transformers will be single phase 12.47KV primary, single phase 120/240V secondary. I want each transformer to serve 3 to 5 trailers, each of which will have its own MCB panel. My reading of the NEC tells me that I need to provide a 2-pole fused cutout on the primary of each transformer, and that the total ratings of the MCBs on the trailers? panels cannot exceed 125% of the transformer?s secondary current rating. But the transformer vendor rep is telling me they don?t provide accommodations for multiple connections on their secondary terminals. Are there approved connection kits or make-shift-split-bolt thingies that will allow me to do this? Can I simply put a note on the drawing to instruct the contractor to provide the necessary number of taps, and let the contractor figure out which pieces and parts to use? Like most engineers who have never done hands-on installation work, I cannot quite envision how this will look when it is built.
 

iwire

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Staff member
Location
Massachusetts
This seems odd to me.

Transformers generally provide us with a mounting pad for the secondaries and it is up to us to find suitable hardware and terminations for as many sets as we need.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
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Sorta retired........
This seems odd to me.

Transformers generally provide us with a mounting pad for the secondaries and it is up to us to find suitable hardware and terminations for as many sets as we need.

Are you referring to pad mount transformers Bob?
Charlie is talking about pole mounted transformers.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
But the transformer vendor rep is telling me they don?t provide accommodations for multiple connections on their secondary terminals. Are there approved connection kits or make-shift-split-bolt thingies that will allow me to do this? Can I simply put a note on the drawing to instruct the contractor to provide the necessary number of taps, and let the contractor figure out which pieces and parts to use? Like most engineers who have never done hands-on installation work, I cannot quite envision how this will look when it is built.
How about Polaris connectors with the right number of ports?
 

iwire

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Location
Massachusetts
Are you referring to pad mount transformers Bob?
Charlie is talking about pole mounted transformers.

The pole mount transformers I have worked on had simple flat pads full of holes in them to bolt terminations onto.

Single_Phase_Distribution_Transformer_pole_mounted_oil.jpg


But while doing this image search I did notice that many smaller ones only have a built in wire terminal.
 

GoldDigger

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So any wire connected directly to the secondary terminal is a tap conductor regardless of its ampacity compared to primary side protection level?

Tapatalk!
 

iwire

Moderator
Staff member
Location
Massachusetts
So any wire connected directly to the secondary terminal is a tap conductor regardless of its ampacity compared to primary side protection level?

Tapatalk!

That is my understanding of 240.21(C)

(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary, as specified in 240.21(C)(1) through
(C)(6). The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.
 

Smart $

Esteemed Member
Location
Ohio
So any wire connected directly to the secondary terminal is a tap conductor regardless of its ampacity compared to primary side protection level?
Technically no. Some may disagree, but the Code does not actually distinguish Transformer Secondary Conductors as tap conductors. They are, however, treated quite similarly when considered not protected by transformer primary OCP.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
The pole mount transformers I have worked on had simple flat pads full of holes in them to bolt terminations onto.
But while doing this image search I did notice that many smaller ones only have a built in wire terminal.

Ok, I have never really been around either type but I have seen where pole mounts have only the 1 stud.
 

Smart $

Esteemed Member
Location
Ohio
And it looks like we have a justified disagreement among members as to whether a short wire jumper to a fan out device would be permitted or not.
No disagreement here. It is not permitted. ;)

However, there is nothing prohibiting a multi-conductor termination device being directly connected to the transformer terminal.
 

fmtjfw

Senior Member
My reading of the NEC tells me that I need to provide a 2-pole fused cutout on the primary of each transformer, and that the total ratings of the MCBs on the trailers? panels cannot exceed 125% of the transformer?s secondary current rating. Are there approved connection kits or make-shift-split-bolt thingies that will allow me to do this? Can I simply put a note on the drawing to instruct the contractor to provide the necessary number of taps, and let the contractor figure out which pieces and parts to use? Like most engineers who have never done hands-on installation work, I cannot quite envision how this will look when it is built.

That would be two separate fused disconnects assuming the transformer is fed from two hots. If the transformer is fed from a single hot and neutral, it would be 9600V primary and a single fused disconnect. I'd also hang lightning arrestors.

A piece of wire connected to each secondary connector and crimped with "TEE" connectors, a Polaris connector, or a hunk of bus bar bolted to each secondary connector and drilled for additional crimped lugs for each tap will all work fine.

I would look at:
240.21(B)(5) allows taps of unlimited length outside, it allows the OCPDs to be at the end, not the beginning of the taps.
240.21(C) allows taps: "A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6)."
240.21(C)(4) may also apply

You could fuse the primary of each transformer to protect it from overload (if you could find fuzes that small) although that is not the general practice for pole mounted transformers.

Underground is the usual practice for feeder to trailers. They are not necessary substantial enough for an overhead drop.
 

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xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
A piece of wire connected to each secondary connector and crimped with "TEE" connectors.

Would this not be defined as taping a tap?
 

xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
The pole mount transformers I have worked on had simple flat pads full of holes in them to bolt terminations onto.

Single_Phase_Distribution_Transformer_pole_mounted_oil.jpg


But while doing this image search I did notice that many smaller ones only have a built in wire terminal.

With this transformer I see a possible 4 wires per terminal....:)
 

fmtjfw

Senior Member
A piece of wire connected to each secondary connector and crimped with "TEE" connectors.

Would this not be defined as taping a tap?

1) If I have a 500mcm conductor for each phase in a trough fed from a fused disconnect and I put multiple split bolt connectors on it with smaller conductors feeding feeder fused disconnect am I tapping a tap? News to me. How is it different if I connect the 500mcm to a transformer secondary? Please quote Code.

2) 240.21 Location in Circuit. .... (C) Transformer Secondary Conductors.
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, ....

This would appear to allow me to connect more than one set of conductors to a secondary.
Is the use of TEE crimps forbidden and the use of bus bars or Polaris connectors OK, or what is the deal? Please quote code.

for "mcm" read "kcmil"
 

Smart $

Esteemed Member
Location
Ohio
1) If I have a 500mcm conductor for each phase in a trough fed from a fused disconnect and I put multiple split bolt connectors on it with smaller conductors feeding feeder fused disconnect am I tapping a tap? News to me. How is it different if I connect the 500mcm to a transformer secondary? Please quote Code.

2) 240.21 Location in Circuit. .... (C) Transformer Secondary Conductors.
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, ....

This would appear to allow me to connect more than one set of conductors to a secondary.
Is the use of TEE crimps forbidden and the use of bus bars or Polaris connectors OK, or what is the deal? Please quote code.

for "mcm" read "kcmil"
The transformer secondary conductors must have ocp before you can tap them. Refer to 240.4(F) and 240.21(C)(1). Otherwise, the [main] secondary conductors have no assured ocp. The sum of tap conductor protection could exceed the ampacity permitted for the [main] secondary conductors.

Granted, a requirement could be implemented such that protection was afforded by tap conductor ocp... but that's not the case to date... :D
 

fmtjfw

Senior Member
The transformer secondary conductors must have ocp before you can tap them. Refer to 240.4(F) and 240.21(C)(1). Otherwise, the [main] secondary conductors have no assured ocp. The sum of tap conductor protection could exceed the ampacity permitted for the [main] secondary conductors.

Granted, a requirement could be implemented such that protection was afforded by tap conductor ocp... but that's not the case to date... :D


NO!

Do you guys read this Code one phrase at a time?

240.21 Location in Circuit. .... (C) Transformer Secondary Conductors.
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, ....

It says, in the code you quoted back to me, that an secondary OCP is NOT REQUIRED.

As far as the NEC is concerned, having the sum of the tap OCPDs greater than the capacity of the transformer is no different than having the sum of the 6 handle OCPDs in a normal service greater than the capacity of the service conductors or transformer. It might well be the same sort of error.

Note: we are outdoors, the rules are different, unlimited length for tap and secondary conductors, OCPDs at load end, not supply end....

Also read 240.21(B)(5) Outside Taps of Unlimited Length.
and 240.21(C)(4) Outside Secondary Conductors.
 
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