Fusing Transformers

[ThomasT2025]

Hey Mike! I have a funny question. Our salesmen messed up when ordering a RTU. We have a 208v feed that needs to be 480v. They decided on getting a transformer to put up on the roof. When fusing this properly, do we need to have a fused disconnect on the primary side or the primary side along with the secondary side? Thanks!

 

[augie47]

The devil is in the details but, generally speaking, both primary and secondary require OCP. You need to follow Art 450 and 240.21

 

[Eddie702]

I would think where the 208 feed comes from (source) would protect the transformer. Then a fused disconnect on the secondary at the AC.

Why not call the mfg and see if you can swap the unit? The freight charge going back will be cheaper than the transformer.

I had a job with 4 of them that our dummy salesmen ordered the wrong voltage. I refused to wire them with transformers and sent them back

 

[SceneryDriver]

The transformer will work, but you'll be paying the no-load losses on the transformer forever. Better to get the right equipment that's set up for the right voltage.


SceneryDriver

 

[SceneryDriver]

When you order the replacement RTU, ask if they'll throw in a replacement salesman.


SceneryDriver

 

[Deltaforce]

I would think where the 208 feed comes from (source) would protect the transformer.

Built-on protection not need?

 

[augie47]

If the primary protection (source) device sized properly and in-sight or lockable then no other primary protection is needed. Likewise, if the RTU OCP meets the protetion requirement for the transformer and the 240.21(C) requirement it can serve as the secondary protection.
Again, the specifics of the job are important. My earlier comment was simply to state most transformer installs need primary and secondary protection. Where is dependent on details.