Buck-Boost Converter

[jyl]

Who here knows about buck-boost converters?

My shop has 208v 3-phase. My pottery kiln is for 240v 1-phase. I have an available circuit for 208v 1-phase (2 hots and neutral). I'd like to boost voltage from that circuit to 240v, so the kiln works as it should.

Looks like I can buy a buck-boost converter 208v primary to 240v secondary, that is rated for the appropriate amperage (kiln pulls 45A, circuit is on a 60A breaker and 4 gauge wire). Converter is several hundred $ but converting the kiln elements and controller to 208v will cost more.

[red-beard]

You want a 12 KVA or larger transformer.

[Shaun @ Tru6]

I did this exact job 7 years ago John. I got a BBT for my 240V compressor. Easy to do and works perfectly.

I got this one I am pretty sure.

https://www.grainger.com/product/4LEG1?s_kwcid=AL!2966!3!335651079981!p!!g!!t181052&ef_id=Cj0KCQjw4PKTBhD8ARIsAHChzRIFpjjgVw3Cj64HveWV yVN9341QXfyNjbpgz4MmYRZsMOZeEbpjb1gaAsiBEALw_wcB:G :s&s_kwcid=AL!2966!3!335651079981!p!!g!!t181052&gucid=N:N:PS:Paid:GGL:CSM-2296:AB578S:20500731&gclid=Cj0KCQjw4PKTBhD8ARIsAHChzRIFpjjgVw3Cj64HveWV yVN9341QXfyNjbpgz4MmYRZsMOZeEbpjb1gaAsiBEALw_wcB&gclsrc=aw.ds

[jyl]

https://www.larsonelectronics.com/product/148856/1-phase-buck-boost-step-up-transformer-208v-primary-240v-secondary-at-46-8-amps-50-60hz

"The MT-BBT-208V-240V-46.8A buck and boost transformer is a single phase unit with a 11 KVA rating and a primary voltage of 208 AC using a maximum of 53.1 amps on the primary side. This step-up transformer has a secondary voltage of 240 and provides up to 46.8 amps available on the secondary side."

https://www.larsonelectronics.com/images/product/Specsheet/148856.PDF

https://www.larsonelectronics.com/images/product/WiringDiagram/148856.PDF

This looks like what I need. US made, $530.

[red-beard]

NEC requires you put in breakers and other equipment sized for 125% of the maximum load. 45 Amps --> 56.25 or rounded up to 60 Amps.

[jyl]

Originally Posted by red-beard

NEC requires you put in breakers and other equipment sized for 125% of the maximum load. 45 Amps --> 56.25 or rounded up to 60 Amps.

The breaker is 60A. The circuit is wired with 4 ga in dedicated conduit.

[red-beard]

A 60A breaker on the 208 VAC side is undersized. You need a 70A breaker. The #4 wire is fine.

45A * 240/208 = 51.9A. BUT you must size this up by 125% to meet NEC -> 64.9, round up to 70A

[hcoles]

Originally Posted by red-beard

A 60A breaker on the 208 VAC side is undersized. You need a 70A breaker. The #4 wire is fine.

45A * 240/208 = 51.9A. BUT you must size this up by 125% to meet NEC -> 64.9, round up to 70A

Is not having a 70A breaker a safety issue? Why is it not ok to have a lower rated breaker? This assumes the load will not trip the 60A breaker in normal circumstances.

[jyl]

Originally Posted by hcoles

Is not having a 70A breaker a safety issue? Why is it not ok to have a lower rated breaker? This assumes the load will not trip the 60A breaker in normal circumstances.

I’d appreciate more explanation of this. What I read was that the breaker can be “at most” 125% of the expected amperage. I can certainly swap in a bigger breaker if need be, just trying to learn.

[red-beard]

This is how the National Electrical Code specifies you size a breaker and wiring. Yes, you can have a too small breaker, but you are likely to trip it and thermally burn it out over time.

[red-beard]

[jyl]

Thanks James!

[jyl]

Not directly related to the BBT, but I’ve decided to rebuild that circuit. The three 4 ga wires are in 3/4” conduit, I’m going to replace with larger, for wire fill rules.