Powering a Trailer Park

[xformer]

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"

IMHO... with the example used... the fused disconnect protects the 500 mcm conductor. Using the split bolt connectors and smaller conductors feeding other discos makes the smaller conductors a tap conductor... I don't have my code book in front of me at the moment for a code reference... but the section I would site states that in general "Conductors shall be protected from over current at the point the conductor receives its supply... something to that effect. Coming off the fused disconnect to the 500 mcm just makes that a feeder. IMHO

 

[GoldDigger]

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 smaller feeders are allowed to connect to the transformer secondary, but not to a single large wire, however short, connected to that secondary. At least that is how I read it.

Tapatalk!

 

[Smart $]

[original post highlighting modified by me]

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, ....

...

You can connect as many sets of conductors as you want to a transformer secondary [read: transformer secondary terminal]...


...but not a transformer secondary CONDUCTOR.

 

[xformer]

IMHO... with the example used... the fused disconnect protects the 500 mcm conductor. Using the split bolt connectors and smaller conductors feeding other discos makes the smaller conductors a tap conductor... I don't have my code book in front of me at the moment for a code reference... but the section I would site states that in general "Conductors shall be protected from over current at the point the conductor receives its supply... something to that effect. Coming off the fused disconnect to the 500 mcm just makes that a feeder. IMHO

The Code section is 240.21

 

[fmtjfw]

Summary

Summary

In the beginning Charlie B. asks what a system of distribution transformers owned by a non-utility entity would look like to serve 2 dozen classroom trailers. The discussion quickly descends into the level of "how many angels can dance on the head of a pin" and he quietly steps away.

The crux of the discussion is "where does the secondary of a transformer physically end and, perhaps, should a secondary be treated as a tap.

here is a summary of the back and forth:
Use Polaris
NEC prohibits Taps
Any wire connected to 2ndary is tap?
Yes 240.21(C) "single load"
No, 2ndary not tap, but often treated as such
No jumper wire -- not permitted indoor xfmr
Use TEE, Polaris busbar
Piece of wire and TEES -- tap[p]ing a tap?
nothing prohibits stacking
2ndary must have OCP before you can tap them
NO read rest of sentence.
OCP required
smaller conductors connected to 2ndardy but not via a single wire.
You can connect as many sets of wires to 2ndary, but not to secondary conductor.

The sections of code that appear to be relevant to me are: 240.21, (B), (B)(5), (C), (C)(4).

Here is what I understand from the Code:

Conductors connected to the secondary lugs of a transformer are feeder wires under (B). They clearly are not branch circuit wires.

Multiple taps to the feeder wires are allowed by (B) and (C).

The taps can be of any length since they are outside according to (B)(5).

The taps are legal under (C)(4).

ARTICLE 240 -- Overcurrent Protection
Part II. Location
240.21 Location in Circuit. Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H). Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.

240.21(B) Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of
240.4(B) shall not be permitted for tap conductors.

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

240.21(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.

240.21(C)(4) Outside Secondary Conductors. Where the conductors are located outdoors of a building or structure, except at the point of load termination, and comply with all of the following conditions:
(1) The conductors are protected from physical damage in an approved manner.
(2) The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the 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 conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.
(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

 

[fmtjfw]

Definitions, etc.

Definitions, etc.

Since conductor and secondary not defined in the NEC one turns to a standard dictionary:

Webster's Collegiate Dictionary
Conductor n. d(1): a material or object that permits an electric current to flow easily ? compare insulator, semiconductor

Usage of conductor in the NEC leads me to believe that it applies to anything designed or used to pass current.

Secondary n. 3: the coil through which the secondary current passes in an induction coil or transformer ? called also secondary coil

This definition leads me to believe that once the conductor(s) in the secondary cease being coiled, we are no longer in the secondary, which means that the conductors between the coil and bushing on the tank are not the secondary of the transformer. Does anyone believe this is a useful interpretation?

If so consider:

240.21(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.

Now this section clearly allows us to connect multiple conductors to a single secondary termination. Must we invade the transformer tank to do so? Or can we treat the exterior bushing termination as the secondary for purposes of 240.21(C)?


I used to have a 2400Vx120/240V 1kVA cast iron tank distribution transformer that was used as the station transformer at a small substation in the dim dark past when rats wore clothes. All 6 connections to the transformers primary and secondary windings were insulated or covered wires passing thought bushings and appearing as wires on the outside of the tank. Now where did the secondary end and the conductor begin? The secondary coil was rectangular cross section wire with varnish as insulation. The secondary "leads" were rubber and fabric insulated stranded, tinned wires. What if I spliced two wires to one of these secondary wires, would it be OK if I used one connector and forbidden if I used two?

 

[Smart $]

I'll make it quick and easy reading...

A transformer secondary ends at the first disconnecting means, OCPD, or premises wiring, whichever comes first. If premises wiring comes first, those wires are transformer secondary conductors and subject to the requirements of 240.21(C).

 

[fmtjfw]

I'll make it quick and easy reading...

A transformer secondary ends at the first disconnecting means, OCPD, or premises wiring, whichever comes first. If premises wiring comes first, those wires are transformer secondary conductors and subject to the requirements of 240.21(C).

Sounds good to me. Thanks.

 

[charlie b]

In the beginning Charlie B. asks what a system of distribution transformers owned by a non-utility entity would look like to serve 2 dozen classroom trailers. The discussion quickly descends into the level of "how many angels can dance on the head of a pin" and he quietly steps away.

And now I will quietly step back in. We have issued the design "for permit," and it is in the hands of the AHJ. I went with the interpretation that 240.21(C) (i.e., the leading paragraph) allows me to feed more than one trailer from the same transformer, and that 240.21(C)(4) allows me to use the main breaker on the panel inside each trailer to protect the feeder that serves that panel. I made sure that the total load on all of the trailers connected to any single transformer was less than 125% of the full current rating of the transformer. I also am taking the stance that suspending the incoming feeders from the transformer to the weatherhead is sufficient to satisfy the requirement to protect the feeder in an approved manner.

Thanks to everyone for the information and support.

 

[USMC1302]

Charlie:
Just a question, since we have had many portable classrooms(not as many in one place, thankfully). Do you have to have a disconnect for each trailer, even though you have an MCB panel? We did something on a smaller scale last year and inspector required disconnect within 30', which I believe is similar to mobile home?

 

[fmtjfw]

Charlie:
Just a question, since we have had many portable classrooms(not as many in one place, thankfully). Do you have to have a disconnect for each trailer, even though you have an MCB panel? We did something on a smaller scale last year and inspector required disconnect within 30', which I believe is similar to mobile home?

We usually used combo meter sockets on poles to feed classroom trailers. We did that because we never set them up on campuses where we owned the primary service and had to put a meter socket in for each service (feeding one or more trailers).

Sounds like he is using the Main breaker on the panel inside the trailer as the disconnect.

550.11 Disconnecting Means and Branch-Circuit Protective Equipment. The branch-circuit equipment shall be permitted to be combined with the disconnecting means as a single assembly. Such a combination shall be permitted to be designated as a panelboard. ....

 

[fmtjfw]

Questions about your classroom trailers.

Questions about your classroom trailers.

Are you getting fully tricked out trailers?

We always got the trailers without a lot of the things required for a classroom building. We would need to add:

Fire alarm pullstations at each door and horn/strobes connected to the central fire alarm system of the nearest permanent building.

Telecom (telephones or intercom circuits)

Emergency lighting inside, and outside at each door; lighted exit signs at each door; and repair or replace broken normal "porch lights".

Internet cabling.

We were always in a race with the carpenters who were installing the ADA compliant walkways and ramps to all the doors. We didn't want to be the ones that held up the project.

Once they decked a 48 inch wide walkway and then added railing that impinged on the 48 inches. We told they that the width between the railing must be 48 inches -- but hey, what do electricians know? The Fire Marshal had them rebuild it before the occupancy permit was issued. They started listening after that.

 

[USMC1302]

I thought the fact that it was a trailer was the reason given for the outside disconnect. Our portable classrooms were lined up in a row of 6. UG feed from main building to a covered wood structure in the middle, with disconnects there. Inspector said the disconnect must be within 30' of the trailer, so had to add an additional disconnect as the two outboard trailers were over that distance. Trailers each had an MCB panel inside.

 

[charlie b]

We are adding the fire alarm pull stations and horn/strobes. Any power and data outlets are being supplied by the trailer manufacturer. I can't answer for the normal or egress lighting, nor for exit signs, as they are being handled by a different group than mine. I don't think 550 applies at all. It defines a mobile home by saying it was designed to be a dwelling. Even though 550.4(A) discusses the possibility of it not being actually used as a dwelling, it starts by saying that it is a mobile home that is not being used as a dwelling. But my facilities were never designed to be a dwelling. They were designed from the start to be classrooms. So to my way of thinking, it is and never has been a mobile home, and therefore 550 is not a player.

 

[USMC1302]

Understood, ours were built as classrooms as well. As usual with summer school projects, it was a rush to get them set and ready. The EC questioned the inspector, but we needed the C/O yesterday, so complied. The newer versions we have are actually fairly nice inside.

 

[fmtjfw]

550

550

I think 550 can easily be applied to classroom trailers, without much stretching.
But other provisions can allow a rational system without extra disconnects.

550.4 General Requirements.
[1](A) Mobile Home Not Intended as a Dwelling Unit. A mobile home not intended as a dwelling unit ? for example, those equipped for sleeping purposes only, contractor?s on-site offices, construction job dormitories, mobile studio dressing rooms, banks, clinics, mobile stores, or intended for the display or demonstration of merchandise or machinery ? shall not be required to meet the provisions of this article pertaining to the number or capacity of circuits required. It shall, however, meet all other applicable requirements of this article if provided with an electrical installation intended to be energized from a 120-volt or 120/240-volt ac power supply system.

[2](B) In Other Than Mobile Home Parks. Mobile homes installed in other than mobile home parks shall comply with the provisions of this article.

[3](C) Connection to Wiring System. The provisions of this article shall apply to mobile homes intended for connection to a wiring system rated 120/240 volts, nominal, 3-wire ac, with a grounded neutral conductor.

550.10 Power Supply.

[4](I) Mast Weatherhead or Raceway. Where the calculated load exceeds 50 amperes or where a permanent feeder is used, the supply shall be by means of either of the following:

[5](1) One mast weatherhead installation, installed in accordance with Article 230, containing four continuous, insulated, color-coded feeder conductors, one of which shall be an equipment grounding conductor

[6](2) A metal raceway or rigid nonmetallic conduit from the disconnecting means in the mobile home to the underside of the mobile home, with provisions for the attachment to a suitable junction box or fitting to the raceway on the underside of the mobile home [with or without conductors as in 550.10(I)(1)]. The manufacturer shall provide written installation instructions stating the proper feeder conductor sizes for the raceway and the size of the junction box to be used.

[7]550.11 Disconnecting Means and Branch-Circuit Protective Equipment. The branch-circuit equipment shall be permitted to be combined with the disconnecting means as a single assembly.
....
[8]The panelboard shall be located in an accessible location but shall not be located in a bathroom or a clothes closet. A clear working space at least 750 mm (30 in.) wide and 750 mm (30 in.) in front of the panelboard shall be provided. This space shall extend from the floor to the top of the panelboard.

[1] I believe includes classrooms. None of the other uses listed typically start out as a mobile home, rather an empty shell that is outfitted as needed.

[2] No exemption from requirements from not being in a mobile home park.

[3] No exemption due to odd power requirements.

[4] This requirement applies as calculated load is above 50A (HVAC + continuous lighting)

[5] But this one is odd, assuming a disconnect is between the mast head and the separately derived supply.

[6] But this one comes to the rescue dumping the previous 4 wire requirement.

[7] This says the disconnect can be the MCB in the panelboard.

[8] And this implies that the MCB can be inside the trailer.

 

[JoeTroubleShoot]

If someone didn't mention these already(sorry I didn't read every word of the thread), then these would do the trick I would imagine.

http://www.hubbellpowersystems.com/connectors/dist/bolted/transformer-poletop/bushing.asp

Also, you can usually buy new/different spades for a transformer or order custom from manufacturer. Most single phase transformers of the size you need do not have spade bushings, like the transformer pictured earlier in the thread. Spade bushings usually only come on higher KVA and three phase.

I believe they make transformer bushing flags that have connectors on them as well so you would not need to press a spade on the end of the wire.

 

[mgookin]

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.

Did you get this part solved Charlie?
You're a good guy on here and you help a lot of people.
If it's not answered yet have you considered a different transformer company?