208V single phase through 240/480V transformers

[cyriousn]

We have an application where there is an existing raceway that our contractor would like to re-use for a 208V single phase circuit. They are proposing to use a 240/480 single phase Tx to step up the voltage on one end and then step it back down with 480/240 single phase Tx on the other end. All this to avoid having to upsize a conduit in a difficult location. Would this somehow be a code violation other than possibly a listing issue? My plan is to contact a few of my reps and get their thoughts but figured I'd ask here as well. Thanks in advance.

 

[xptpcrewx]

We have an application where there is an existing raceway that our contractor would like to re-use for a 208V single phase circuit. They are proposing to use a 240/480 single phase Tx to step up the voltage on one end and then step it back down with 480/240 single phase Tx on the other end. All this to avoid having to upsize a conduit in a difficult location. Would this somehow be a code violation other than possibly a listing issue? My plan is to contact a few of my reps and get their thoughts but figured I'd ask here as well. Thanks in advance.

No code violation assuming you do it correctly.

 

[synchro]

We have an application where there is an existing raceway that our contractor would like to re-use for a 208V single phase circuit. They are proposing to use a 240/480 single phase Tx to step up the voltage on one end and then step it back down with 480/240 single phase Tx on the other end.

Using 208V on a 240V winding will have more margin below saturation and so that's technically OK. However, the kVA capability of the transformer will drop by a factor of 208/240 ≈ 0.866 because the voltage has dropped but the allowable current stays the same.

 

[LarryFine]

Are there not 208-480 (or 208-600) units available?

 

[oldsparky52]

Are there not 208-480 (or 208-600) units available?

I'd wager there is not much demand for those in single-phase configurations.

 

[xptpcrewx]

Using 208V on a 240V winding will have more margin below saturation and so that's technically OK. However, the kVA capability of the transformer will drop by a factor of 208/240 ≈ 0.866 because the voltage has dropped but the allowable current stays the same.

Essentially an inefficient use of iron and space along with the added weight. If you can match the 208-V rating (and possibly go with 600-V secondary windings), that would be the best solution unless you already have these 240:480-V transformers lying around.

 

[winnie]

The transformer's required protection and GEC(s) will also play into this.

You might consider autotransformer for the step up and down, which might simplify GEC requirements.

Finally you might investigate if the 208V load is available in a 480V version, eliminating the step down at the far end.

Jon

 

[hillbilly1]

Since it’s single phase, using a 240x480 transformer would work just fine since you are stepping up, then back down. You will not quite have 480 on the run, but would not matter since the voltage at the ends would be the same. Could even go with a 240x600 if you still could not meet your voltage drop requirements. You would still have 208 at both ends minus any voltage drop.

 

[wwhitney]

You might consider autotransformer for the step up and down, which might simplify GEC requirements.

Not NEC compliant for a 208V 2-wire feeder from a 208Y/120V system, but would be OK for a 120/208V 3-wire feeder:

215.11 Circuits Derived from Autotransformers. Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

There are two exceptions but neither would apply.

Cheers, Wayne

 

[winnie]

Not NEC compliant for a 208V 2-wire feeder from a 208Y/120V system, but would be OK for a 120/208V 3-wire feeder:

215.11 Circuits Derived from Autotransformers. Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

There are two exceptions but neither would apply.

Cheers, Wayne

Good point about the continuity of the grounded conductor.

One could do this for a two wire circuit, but all the approaches look pretty messy in terms of phase balance circuit loading.

-Jon