OCP for transformer

[transman2]

I have a temp trailer pulled into our facility just outside our building and need to run power to. We have single phase 480v just inside the building and I have found a 50 kva single phase 480-120/240 transformer to loan the EC to step down with. My question is should the EC install a disconnect on the primary and secondary of the transformer or just the secondary. There are 2-100 amp panels in the trailer FYI. Thanks!!

 

[templdl]

I have a temp trailer pulled into our facility just outside our building and need to run power to. We have single phase 480v just inside the building and I have found a 50 kva single phase 480-120/240 transformer to loan the EC to step down with. My question is should the EC install a disconnect on the primary and secondary of the transformer or just the secondary. There are 2-100 amp panels in the trailer FYI. Thanks!!

Of course you have a copy of the current NEC and can reference art 450.3 over current protection regarding transformers table 450.3(a) which also references back to art 240. That's the mother load.
If you review those code sections you should be good to go.

 

[kwired]

The 120/240 secondary will require overcurrent protection regardless of what protection is on the primary.

Primary will have a disconnect but doesn't necessarily need to be within sight of the transformer, see 450.14.

Primary overcurrent protection level can generally be up to 250% rated current if there is secondary protection, see 450.3(B)

 

[transman2]

The 120/240 secondary will require overcurrent protection regardless of what protection is on the primary.

Primary will have a disconnect but doesn't necessarily need to be within sight of the transformer, see 450.14.

Primary overcurrent protection level can generally be up to 250% rated current if there is secondary protection, see 450.3(B)

Does that secondary protection have to be located at the transformer or does the mains in the trailer count as this protection.

 

[templdl]

Does that secondary protection have to be located at the transformer or does the mains in the trailer count as this protection.

Does the "mains" mean more than one. Being more specific is the a single breaker feeding the transformer asnd how far away is it located fro the transformer primary? Does it fall within the "tap rule" requirements?

 

[templdl]

Does that secondary protection have to be located at the transformer or does the mains in the trailer count as this protection.

Does the "mains" mean more than one. Being more specific is the a single breaker feeding the transformer asnd how far away is it located fro the transformer primary? Does it fall within the "tap rule" requirements per the NEC?

 

[transman2]

Does the "mains" mean more than one. Being more specific is the a single breaker feeding the transformer asnd how far away is it located fro the transformer primary? Does it fall within the "tap rule" requirements per the NEC?

There are 2 -100 amp panels in trailer. 1 being 85 ft away and the other being 120 ft away. Right now we have a disconnect for the primary at the outdoor transformer but nothing on the secondary. The secondary has no disconnect and feeds into the trailers 2 panels with each having a 100amp main.

 

[augie47]

The transformer itself could be protected by the primary overcurrent device if properly sized per 450.3.
Your secondary conductors would need to follow the 240.21(C) tap rules.
Off hand I would say if your transformer is inside, it would need to follow the 10 or 25 ft tap rule accordingly. If you transformer is outside then the 240.21(C)(4) rule might allow your trailer disconnects to apply. You would need to review the particulars in 240.21(C)

 

[templdl]

There are 2 -100 amp panels in trailer. 1 being 85 ft away and the other being 120 ft away. Right now we have a disconnect for the primary at the outdoor transformer but nothing on the secondary. The secondary has no disconnect and feeds into the trailers 2 panels with each having a 100amp main.

Knowing this I think that it very important that you look at and understand the tap rules as Augie pointed out. If you straighten that out I think that you will have a good install.

 

[transman2]

Got it!!! Thanks!!!

 

[kwired]

The transformer itself could be protected by the primary overcurrent device if properly sized per 450.3.
Your secondary conductors would need to follow the 240.21(C) tap rules.
Off hand I would say if your transformer is inside, it would need to follow the 10 or 25 ft tap rule accordingly. If you transformer is outside then the 240.21(C)(4) rule might allow your trailer disconnects to apply. You would need to review the particulars in 240.21(C)

You can only protect the secondary via the primary overcurrent protection if the secondary is two wire single phase or three wire three phase, all other "multiwire" systems need secondary protection. This is in 240.21(C)(1).

 

[augie47]

You can only protect the secondary via the primary overcurrent protection if the secondary is two wire single phase or three wire three phase, all other "multiwire" systems need secondary protection. This is in 240.21(C)(1).

I don't know that I ever indicated otherwise. The 240.21 tap rules I referenced are for OCP. The only thing that changes is the location of that protection.

 

[kwired]

I don't know that I ever indicated otherwise. The 240.21 tap rules I referenced are for OCP. The only thing that changes is the location of that protection.

Maybe I didn't understand:

Originally Posted by augie47
The transformer itself could be protected by the primary overcurrent device if properly sized per 450.3

If you have a three wire secondary (on a single phase unit) you can protect the primary, but that will not prevent possibly overloading one half of the secondary. Put 10 kVA of load on one 120 volt half of a 10 kVA transformer the primary protection don't know it is all on one side of the secondary.