Transformer primary and secondary conductors.

[canon_in_a]

Good morning. I am confused about how long transformer primary and secondary feeder conductors are allowed to be, without having an overcurrent protection device (OPD). I have always thought that there is no limit on how long the primary conductors can be, and that the secondary feeder conductors are allowed to be ten feet without an OPD. Am I correct? Thank you all for your help.

 

[david luchini]

You generally wouldn't see transformer primary conductors without an OCPD, unless you we're supplying the transformer from a tap.

When supplying the transformer from a tap, the primary plus secondary length cannot exceed 25ft. See 240.21(B)(3).

For secondary conductor (not from a feeder tapped transformer) the length may be 10ft, or 25ft, (or unlimited for outside secondary conductors) based on 240.21(C).

 

[augie47]

Not disagreeing with David...
Point of clarification: Other than primary tap conductors installed in accordance with 240.21(B)(3), there is no limit on the length of the primary conductors between the OCP device and the transformer.
Secondary conductors should meet 240.21(C).

 

[GoldDigger]

Not disagreeing with David...
Point of clarification: Other than primary tap conductors installed in accordance with 240.21(B)(3), there is no limit on the length of the primary conductors between the OCP device and the transformer.
Secondary conductors should meet 240.21(C).

Yes, it still is not clear whether the OP was asking about how long the conductors could be from an OCPD or how long they could be from somewhere (a tap) without any OCPD on the supply side protecting the primary conductors for their wire size.

 

[IRAQ TIGER]

EGC installed from utililty transformer to first means of OCP.

EGC installed from utililty transformer to first means of OCP.

My question is slightly off topic, but this location seems the best place to pose my question.

If I have a utility transformer with a 120/208V secondary and bond and ground inside. Would my EGC need to be sized to the full capacity of the secondary side of the transformer and not the OCPD at my first enclosure? my thinking is if a fault did occur between these two points and the EGC sized to table 250.122 for example a 400A circuit breaker #3AWG I would assume that it would melt in two before the fuse of the transformer secondary would open?

Thank You

 

[david luchini]

My question is slightly off topic, but this location seems the best place to pose my question.

If I have a utility transformer with a 120/208V secondary and bond and ground inside. Would my EGC need to be sized to the full capacity of the secondary side of the transformer and not the OCPD at my first enclosure? my thinking is if a fault did occur between these two points and the EGC sized to table 250.122 for example a 400A circuit breaker #3AWG I would assume that it would melt in two before the fuse of the transformer secondary would open?

Thank You

From a utility transformer to your service disconnecting means (the OCPD at your first enclosure), you would not need an EGC.

In the service disconnect enclosure, you would need to bond the enclosure and equipment grounding conductors to the grounded conductor.

The connection of the grounding electrode to the grounded service conductor can be made at any accessible point from the load end of the service lateral to the terminal in the disconnect enclosure.

If the transformer is Outdoors, you need a second grounding connection to the grounded conductor, located outside the building.

 

[IRAQ TIGER]

EGC installed from utililty transformer to first means of OCP

EGC installed from utililty transformer to first means of OCP

Sir,

Allow me to give you some additional insight as to what I am dealing with. I am a licensed electrical contractor in the United States working on a military installation in Afghanistan. My job entails taking spot generation loads and connecting them to what we call prime power, this is provided by a large central power plant. The distribution system for prime power is 4160V installed underground throughout the base and stepped down with a typical transformer that you would see in the United States to 120/208 Volt 60HZ. These are the circumstances I am dealing with and are incorrect but the persons above me have the opinion that I am in error.

1 - The persons above me are applying Article 450 to what we would call in the States the utility transformer connection on the secondary side.

2 - The persons above me are are not allowing the use of Article 230.90 (A) Exception No.2 for the sizing of service laterals.

3 - The persons above me are insisting that a EGC be run with the service lateral between the utility transformer and the first OCPD based on Article 250.122.

4 - The persons above me are insisting that in addition to the EGC being ran with the service lateral that a bonding jumper be installed within the enclosure of the OCPD? Creating objectionable current and potential hazard.

5 - Because this is a military installation the only difference with the service equipment is that there is NO metering equipment for each structure.

6 - My belief with the EGC installed with the service lateral is a very poor practice because the only overcurrent protection of the lateral is the secondary fuse of the transformer, which is not designed for this purpose. It is my belief that the installed EGC will fail during a fault allowing the condition to remain.

I do have a very good understanding of the NEC but looking for guidance to deal with this unusual situation.

Thank You

 

[GoldDigger]

Sir,

Allow me to give you some additional insight as to what I am dealing with. I am a licensed electrical contractor in the United States working on a military installation in Afghanistan. My job entails taking spot generation loads and connecting them to what we call prime power, this is provided by a large central power plant. The distribution system for prime power is 4160V installed underground throughout the base and stepped down with a typical transformer that you would see in the United States to 120/208 Volt 60HZ. These are the circumstances I am dealing with and are incorrect but the persons above me have the opinion that I am in error.

1 - The persons above me are applying Article 450 to what we would call in the States the utility transformer connection on the secondary side.

3 - The persons above me are insisting that a EGC be run with the service lateral between the utility transformer and the first OCPD based on Article 250.122.

4 - The persons above me are insisting that in addition to the EGC being ran with the service lateral that a bonding jumper be installed within the enclosure of the OCPD? Creating objectionable current and potential hazard.

6 - My belief with the EGC installed with the service lateral is a very poor practice because the only overcurrent protection of the lateral is the secondary fuse of the transformer, which is not designed for this purpose. It is my belief that the installed EGC will fail during a fault allowing the condition to remain.

I do have a very good understanding of the NEC but looking for guidance to deal with this unusual situation.

Thank You

Sounds like an awkward situation at best.
Under the NEC the transformer would fit in as a POCO service transformer or as a customer operated SDS depending on where it fell on the dividing line between NEC and NESC. In your case I would have to say that which approach you take will have to depend in part on what wiring practices were used for the primary feed to the transformer.

You have not stated whether the 4160 has a wire or earth neutral return or whether there is an EGC as part of the run. Nor have you detailed whether there are, as I assume there would be, adequate (under the NEC) ground electrodes at each load site.

At low (<600) voltages a simple earth connection, no matter how well the ground rods are driven, cannot be counted on to clear a line to ground fault. That is the job of the EGC which has a lower, known and measured impedance back to the source. If the secondary OCPD is in your opinion inadequate, you may need (make that "want") to run a larger EGC.
The utility is allowed to create a situation where a line to ground fault on the secondary side, within the service wiring itself, will not be effectively protected by any OCPD. That is their problem and their choice to do. (One reason that service wiring is allowed inside the building only to limited extent.)
Not providing an EGC in your case will only make the situation worse. Bonding the neutral to the EGC and/or local ground electrodes should be done at only one place; in a standard installation it would in the vicinity of the service point. Bonding the neutral to the EGC for an SDS should also be done at only one place, typically at the secondary neutral tap of the transformer.

 

[IRAQ TIGER]

Numerous Errors

Numerous Errors

1 - The persons responsible for the transformer do have the secondary XO bonded and grounded as it would be in the states.

2 - The typical install consists of a MDP adjacent to the transformer that does have a main installed, this then feeds other structures.

3 - The MDP typically has a EGC installed from the transformer XO to the EGC bar of the MDP and is not bonded again to the neutral bar of the panel. Which according to NEC 250.30 (A) is correct. But my superiors believe that it SHOULD BE bonded again in the MDP.:rant:

4 - Does Article 450 apply to utility transformers?:?

5 - My superiors are incorrect in that stating that I cannot use Article 230.90 (A) (2) to use a 500 Kcmil for a 400A service because Article 450 must be used for the conductors attached to this transformer.

6 - Lets say for example a 400A service lateral installed between a utility transformer and an exterior MDP with a #3 AWG. This #3 AWG is sized for the LOAD side of the service and intended to handle the fault current available based on the 400A breaker. But if a ground fault occurs between the transformer and the MDP the #3 AWG EGC will not be able to handle the excessive fault current. But if the MDP was only supplied with 3 phase conductors and a grounded conductor that is bonded inside the MDP the fault current should have sufficient mass to survive the event.

 

[augie47]

There may be a difference of opinions on this, but I can't say your superiors are completely out of line in their thinking.
When you look at most everything associated with the term "service", we are speaking of a UTILITY distribution system, so, from the beginning you need to determine if the supply is UTILITY. It sounds like the whole system may be set up as a SDS and Art 230 would not apply in any way.

Unless you employ an exception, a system bonding jumper would be required from your SDS to your MDP {'11 Code 250.30(A)(2)} however it would be sized per 250.66, not 250.122

Likewise, if this is deemed a SDS, you could not use 230.90.

 

[IRAQ TIGER]

RFI

RFI

Sir,


Please for my clarity define SDS.

Thanks

 

[JDBrown]

Please for my clarity define SDS.

Separately Derived System.

 

[augie47]

Separately Derived System. A premises wiring system
whose power is derived from a source of electric energy or
equipment other than a service. Such systems have no direct
electrical connection, including a solidly connected
grounded circuit conductor, to supply conductors originating
in another system.


Transformers are often separately derived systems. Take the same transformer and allow the Utilty to feed it and you have a "service" on the secondary regulated by Art 230. Install the same one on a premises wiring system (no meter, no wiring (primary or secondary) controlled by the utility) and it becomes a SDS and Articles 240, 450, etc apply.
The NEC treats them differently. In you case, Art 240.21(C) and 250.30 as well as others would probably be the pertinent Articles over Art 230.

 

[GoldDigger]

Separately Derived System.

A few simple examples of an SDS are a generator or other power source other than the Utility, or a customer owned transformer where the secondary side is managed by the customer and has its own grounding system which does not have to match that used by POCO. There will be no direct connection between any SDS wiring and POCO wiring, other than perhaps the ground. The neutral will be separate from the POCO neutral unless deliberately connected.