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Comments on 208VAC triple-phase to single-phase conversion

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208VAC triple-phase to single-phase conversion

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The question:

Is there a way to convert from triple-phase 208VAC to single-phase 208VAC?

Background:

Hi everyone, I am stuck on a project I'm working on. I'd like to convert 208VAC (three phase, NEMA L5-20R) to 230VAC (single phase, NEMA 5-20R) for use with a piece of 230VAC equipment that plugs into the 5-20R.

The wall outlet I am limited to using is the aforementioned 208VAC outlet in wye arrangement. Only two of the three 120V hots are accessible, as well as the grounded neutral. 208V is taken by the difference between the two phase-shifted 120V hots. See the diagram below:

Image alt text

The goal is to get a single phase voltage much like how you get 120VAC single phase at a typical NEMA 5-15R outlet you see in a North American residence/business. But instead of 120V, it needs to be 230VAC for the equipment I want to attach. So like this: Image alt text

Not being too experienced in this field, I got a single-phase T-1-81051 autotransformer from Grainger but realized too late that I couldn't take the two three-phase hots from the wall and use it as a pseudo single-phase. I recognized this after looking more at the wiring diagram for it (Fig. H) and realizing that I would be shorting the connections if I made the attachments as shown. Image alt text Image alt text

Everything else matches what I need for the equipment load, but the input not being single-phase is a problem. My question is if there's a way to convert triple-phase to single-phase, maintaining a neutral/ground connection. I want to convert the power from the wall from 208V triple-phase to 208V single-phase before feeding that single-phase into the transformer I got.

I searched a bit and learned about phase converters, but it seemed like they only go the opposite way around, converting from single-phase to triple-phase only.

Any help is appreciated! I know others have much more experience and might be aware of other options out there. I'd be happy to answer any questions! Thanks in advance.

Edit: This shows the voltages I measured using a digital multimeter at the NEMA L5-20 outlet. The voltages agree with a standard three-phase system: Image alt text

transformer
posted almost 3 years ago
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cosined‭
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1 comment thread

Why do you say convert triple phase to single phase when this is contradicted by only having two of t... (2 comments)
Why do you say convert triple phase to single phase when this is contradicted by only having two of t...
Andy aka‭ wrote almost 3 years ago
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Why do you say convert triple phase to single phase when this is contradicted by only having two of the three hots available. Please fix this. It's a big deal.

cosined‭ wrote almost 3 years ago
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Hi Andy aka, first off, I gotta say that I am a big fan of yours. I've come across literally hundreds of your responses on StackExchange over the years as a silent reader. You've helped me a lot. Thank you for your help and willingness to share your expertise!

As for why I say triple phase in the question, it's because that is how the outlet is wired and how the voltages are derived. The electricians for the building did not expose the third hot for whatever reason, but the two available hots (each 120V from grounded neutral, presumably phase-shifted by 120deg from each other to give a 208V difference across them) and grounded neutral indicate that it is a three phase outlet. The outlet has a NEMA L5-20 receptacle, which of course has only three sockets as you'd expect. The two upper sockets are hot and the remaining one is neutral which is grounded at the breaker. I think. I spoke to the electricians and that's how they explained it to me.