Buck-Boost Converter
 

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.

You want a 12 KVA or larger transformer.

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=Cj0KCQjw4PKTBhD8ARIsAHChzRIFpjjgVw3Cj64HveW VyVN9341QXfyNjbpgz4MmYRZsMOZeEbpjb1gaAsiBEALw_wcB: G:s&s_kwcid=AL!2966!3!335651079981!p!!g!!t181052&g ucid=N:N:PS:Paid:GGL:CSM-2296:AB578S:20500731&gclid=Cj0KCQjw4PKTBhD8ARIsAHC hzRIFpjjgVw3Cj64HveWVyVN9341QXfyNjbpgz4MmYRZsMOZeE bpjb1gaAsiBEALw_wcB&gclsrc=aw.ds

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.

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.

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.

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.

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.

Thanks James!

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.