Disconnect Needed for Transformer?

[donallabaugh]

Recently the facility I work in is installing a large number of new CNC based equipment. This equipment requires 200-220Volt 3 Phase power.

The plant I work in has 480 Volt 3-Phase,(supplied through a buss system 18 feet above the floor). A transformer was purchased to be installed with each of the CNC units.

The transformers are (45KW) are bolted to the floor next to the CNC equipment. One Transformer supplies a single piece of equipment. On the first 4 machines a disconnect switch was installed between the 480V buss and the transformer.

Now our divisional engineers want to eliminate the disconnect switches installed between the 480V buss and the transformers. They have stated that there is nothing in the NEC that requires a disconnect, in this case.

I am curious if this type of installation has been directly addressed in the code. I have not been able to find a definitive argument against the policy they want to adopt at this location.

Personally I have stated to the engineers that the disconnects should be installed because I would consider the transformer as part of the equipment since it is needed to derive the correct operating voltage. Of course they want to see where it is required in the code.

Any comments on this type of installation would be appreciated.

 

[petersonra]

Now our divisional engineers want to eliminate the disconnect switches installed between the 480V buss and the transformers. They have stated that there is nothing in the NEC that requires a disconnect, in this case.

I am curious if this type of installation has been directly addressed in the code. I have not been able to find a definitive argument against the policy they want to adopt at this location.

Personally I have stated to the engineers that the disconnects should be installed because I would consider the transformer as part of the equipment since it is needed to derive the correct operating voltage. Of course they want to see where it is required in the code.

Any comments on this type of installation would be appreciated.

IMO, the engineers are right. The transformer is part of the premises wiring system, not part of the machine.

It still needs some means of disconnection, but presumably it has that at its source.

 

[Cavie]

Does your supervisor want to shut down the whole system to replace one transformer?

 

[rdelauter]

I believe the 45 KVA transformers would need 125% protection on the primary side, tabel 450.3(B), with no secondary protection. Would the entire buss system be protected by a OCD that small? Please correct me if I'm wrong about the primary protection.

Rob

 

[Pierre C Belarge]

The conductors on the supply side of the Transformer may need protection, it depends on 240.21(B). And the Transformer primary may need protection as well, 450.3(B).

 

[pfalcon]

...A transformer was purchased to be installed with each of the CNC units...The transformers are (45KW) are bolted to the floor next to the CNC equipment...Now our divisional engineers want to eliminate the disconnect switches...Personally I have stated to the engineers that the disconnects should be installed because I would consider the transformer as part of the equipment since it is needed to derive the correct operating voltage...

When you install a 480/120V transformer to power utility equipment it does not therefore become a part of the toaster. Neither does the 480/230V transformer become part of a CNC simply because it supplies the correct voltage. As installed they are premises devices. A disconnect is typically installed so that the main buss does not have to be shut down for service.

Your engineers did anticipate the need to change fuses right? I bet the interruption for changing a fuse more than pays for installing a disconnect.

 

[donallabaugh]

Thanks for the Replies.

As with many items in the code wording is everything. In this case "premises devices" is key.

For clarification the Transformers 'Are' protected by a fused Buss Plug. They are protected at 125% factor.

One final note. The engineering department has made an additional requirement. If the Transformer supply wiring is hard piped they will not have to have the disconnect, but if flexible buss-drop is used they want a disconnect added. They added this because in many instances flexible buss-drop is used on the factory floor because in our situation equipment can be moved frequently.

Again thanks for your input.

 

[petersonra]

Thanks for the Replies.

As with many items in the code wording is everything. In this case "premises devices" is key.

For clarification the Transformers 'Are' protected by a fused Buss Plug. They are protected at 125% factor.

One final note. The engineering department has made an additional requirement. If the Transformer supply wiring is hard piped they will not have to have the disconnect, but if flexible buss-drop is used they want a disconnect added. They added this because in many instances flexible buss-drop is used on the factory floor because in our situation equipment can be moved frequently.

Again thanks for your input.

It sounds like you are planning to make the drop from a busway.

You have a disconnecting means at the point regardless of whether you have hard conduit or cord. I don't see any need to add a second means of disconnection at all.

I would in fact argue that it might not be code compliant to add a disconnect in line with the cord if you go that way. The code allowing cord to be used for this kind of thing is pretty explicit.

400.1(6) Connection of utilization equipment to facilitate frequent interchange

I would argue that a disconnect switch is not utilization equipment, and thus you cannot run cord to it.