Neutral awg size for Auto transformer

I have a transformer for my new Doosan lathe. Ellison sold me an autotransformer to get the 3 phase 240 delta I have down to 220.

My speedio uses an isolation transformer and the X0 had to be connected to neutral to get the 3 phases to be equal.

Correct my if I'm wrong, but I'd think this one would need X0 connected to neutral as well (Y030CECF3L0U):

https://documents.hammondpowersolut...e/Specialty/HPS-Autotransformers-Brochure.pdf

Manual says machine needs 6 awg. If the transformer gets a neutral, what awg does it need?

Thanks

You say you have delta power? Why would you run a neutral? Or are you treating the output of the autotransformer as wye power? I.e., 3 supply wires to the autotransformer and 3+1 supply and neutral from the autotransformer?

Never mind, to answer your question directly, the neutral should be sized the same as the supply conductors. There might be some exception if you could guarantee line-to-line balance, but if your Speedio needed help evening out the lines such exception would not apply.

If its a 2 transformer open delta auto transformer connection then just make it the same size and dont think about it.

However the neutral for the primary of the transformer need only carry 1/10th the current, as.. You can open the box yourself and see those are 10 or 12 gauge wires.

sfriedberg said:

You say you have delta power? Why would you run a neutral? Or are you treating the output of the autotransformer as wye power?

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Yes, output is 208 Wye. Input is delta. The neutral went to the center of the output to get them all to be 208 to ground. Without, they were a few volts off and the Speedio could tell.

sfriedberg said:

.. to answer your question directly, the neutral should be sized the same as the supply conductors.

Click to expand...

Thanks. Makes sense. I'll roll with that.

johansen said:

for the primary of the transformer need only carry 1/10th the current, as.. You can open the box yourself and see those are 10 or 12 gauge wires.

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I will open it up and see what they are using. Thanks for the help.

#10 works for ALL neutrals in the real world.

CalG said:

#10 works for ALL neutrals in the real world.

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Well,certainly not for ALL neutrals, not really, not for long.

For "MOST", or at least "MANY", 3 phase neutrals, sure, but that does not mean you are allowed to use #10, even so.

10AWG certainly doesn't work for single-phase neutrals!

CalG said:

#10 works for ALL neutrals in the real world.

Click to expand...

IF.. your idea of "real world".. is confined to "within the on-board controls cabinet of the subset of machine-tools that require a Wye service AT ALL"? Probably.

"Real world" ??

Seems to be less WORK to simply follow NFPA-70 and go a size or so UP, not DOWN?

Says "66 Amps" right on the label of my EGS/Hevi-Duty Delta-Wye transformer. #6 could do. But I had #4 handy also. So I used #4.

#10 would be a bad choice.

A 15A, 20 A , 30 A breaker to a 3-P branch off the Square-D 3-Phase load center? EACH .. ampacity .. gets what NFPA-70 wants for that size. One size UP actually. USACE habit.

I'm far too lazy to try to engineer around "the code" after all the work put into it by so many experienced folks over so many years .. when I can benefit from all that so very-damned cheaply.

Safely, even.

A 3Φ autotranformer is not the correct type for your application, if the lathe is a CNC type.
You really need an Delta/Wye isolation type like you used on the speedio.
If you have a 240V center tapped delta supply system, with the wild leg (one phase higher voltage to ground).

A Wye type autotransformer has all three legs equidistant from the center wye point, with symmetrical voltages.
A center tapped delta does not have an equidistant common (neutral) or symmetrical voltages on each leg.

If you connect it to a delta neutral, as you plan, you will have large circulating currents between the two systems, as they have different and unequal neutral grounding location points.

If you do not connect the supply delta neutral to the autotransformer center, then your output to the machine must remain un-grounded, not good for your safety or the machine health. The machine requires a wye input, grounded system.

I would talk to the technical support people at Ellison, and have them confirm the salespersons mistake.

Hmm, Well Ellison sold me the auto transformer with the machine. This is a popular model so I'm thinking they've done this before. I called HPS before wiring it and they said do not connect the center to neutral (just passes through and grounds the can). The machine does not require neutral. It gets 3 hots and ground. Considering the ground is just for safety (not carrying current) I suppose it doesn't care what the phase to ground is. The others are all 220 to each other.

What's annoying is HPS tech support said not to connect neutral to the center on the isolation transformer. I informed them that they told me to do exactly that about 4 years ago on the same tech support line.. silence.

if you have a center tapped delta service, then you cannot connect the neutral of a 3 phase WYE auto transformer to the neutral of your 120/240/208Y service.



you may try and run the machine without the neutral connection and pray the lathe has no significant line to line imbalance.. which it may not, given that it has no neutral connection.

or you can create a neutral connection by finding yourself a 2:1 transfomer and connecting it as an auto transformer to the high leg of your delta service and your neutral. the true "neutral" is located 69 volts "up" from the neutral, 120v down from the high leg.. in the middle of the triangle. you will need to use a 240v:120v transformer and reconnect it as an auto transformer. a 120v:60v transformer will be saturated because 120v+60V is less than 208.

here is a diagram: https://upload.wikimedia.org/wikipe...gh_leg_delta.svg/320px-High_leg_delta.svg.png
what i'm saying is you can get to that imaginary center, with a 2:1 auto transformer connected from neutral to high leg.

but i believe you can get away without it in this case.

can't edit my post anymore...
correction:

the neutral is located 69 volts away from the mains "neutral" and 138 volts away from the "high leg"