[SCR]

Can I use a 600/208 autotransformer for distribution? I have 347/600Y supply coming to a machine shop and need to step down to 120/208y for 400 amp 3-phase panel. 150 KVA autotransformer is readily available. Rule CEC26-266 2(a) seems to say I can and i can't. I do have a solidly grounded neutral available from the primary feed. Is this setup acceptable in Canada?

 

[CMP]

It would be helpful if you posted the transformer tag.

 

[frankendodge]

Why not use a 75kva isolation transformer? Are they that hard to get now?

 

[CMP]

He says that he has the autotransformer available. And a neutral for the supply. He also would need a load schedule or calculation to see if it’s suitable.

 

[SCR]

Why not use a 75kva isolation transformer? Are they that hard to get now?

I already have the autotransformer. I know all the pros and cons of isolation versus auto, my main concern is with regard to the CEC, whether this is an acceptable installation or not.

 

[frankendodge]

Gotcha. Thought you meant available, as in at the supplier. Also thought you'd said 200a.

Not sure if there is anything in the cec on it.(I'm without a code book at the moment) But I was always told it's a bad idea to use them as a step down. Faults will run right through them causing high circulating currents. If a winding shorted you could end up with higher than expected voltage on the low side as well.
Only ever wired small ones to step up voltage at a specific piece of machinery.

 

[paulengr]

Gotcha. Thought you meant available, as in at the supplier. Also thought you'd said 200a.

Not sure if there is anything in the cec on it.(I'm without a code book at the moment) But I was always told it's a bad idea to use them as a step down. Faults will run right through them causing high circulating currents. If a winding shorted you could end up with higher than expected voltage on the low side as well.
Only ever wired small ones to step up voltage at a specific piece of machinery.

Step down is fine.

Passing faults is the case with any wye wye. The bigger danger in step down is there is the minuscule potential with a loss of neutral that you will see HV on the “LV” line. It is far more unlikely in a standard transformer because it can only happen with a HV to LV short. The problem is that utility equipment tends to be full of various faults since it is all wye wye and the equipment is built with megavolts of insulation. Imagine what it takes to jump line to line or down an insulator and across a cross arm. That’s not the kind of construction non-utilities use and we don’t have 100+ kV BIL ratings. Our grounds are also vastly superior to any utility ground so with no impedance every utility fault grounds through your transformer and plant system. That’s a nonissue here.

Circulating currents is also true with wye wye. It’s a fact of life. But millions of utility transformers are wye wye. One advantage is unlike deltas it acts like 3 single phase transformers so the propensity to back feed on a single disconnected phase is much less. Not that you would drop only one phase considering the intended use. Juxtapose this small loss against the crazy low %Z and lower losses of an autotranaformer. Just don’t attempt to float the neutral unless you have significant surge protection: we launched parts in a tea stand that way (13.5 kV around 4 MVA) when the oil filled RC filter boiled and misted everything…can you say fuel air bomb? We launched bolts 30 feet away.

 

[SCR]

Step down is fine.

Passing faults is the case with any wye wye. The bigger danger in step down is there is the minuscule potential with a loss of neutral that you will see HV on the “LV” line. It is far more unlikely in a standard transformer because it can only happen with a HV to LV short. The problem is that utility equipment tends to be full of various faults since it is all wye wye and the equipment is built with megavolts of insulation. Imagine what it takes to jump line to line or down an insulator and across a cross arm. That’s not the kind of construction non-utilities use and we don’t have 100+ kV BIL ratings. Our grounds are also vastly superior to any utility ground so with no impedance every utility fault grounds through your transformer and plant system. That’s a nonissue here.

Circulating currents is also true with wye wye. It’s a fact of life. But millions of utility transformers are wye wye. One advantage is unlike deltas it acts like 3 single phase transformers so the propensity to back feed on a single disconnected phase is much less. Not that you would drop only one phase considering the intended use. Juxtapose this small loss against the crazy low %Z and lower losses of an autotranaformer. Just don’t attempt to float the neutral unless you have significant surge protection: we launched parts in a tea stand that way (13.5 kV around 4 MVA) when the oil filled RC filter boiled and misted everything…can you say fuel air bomb? We launched bolts 30 feet away.

So as far as the NEC is concerned, would this autotransformer stepdown be an acceptable installation?

 

[paulengr]

So as far as the NEC is concerned, would this autotransformer stepdown be an acceptable installation?

Yes.

Just be aware of what can go wrong. There is a reason even utilities who are notoriously cheap avoid them,

 

[Breakfasteatre]

So as far as the NEC is concerned, would this autotransformer stepdown be an acceptable installation?

why do you care? You asked about canada, and the NEC is code in the united states

 

[SCR]

why do you care? You asked about canada, and the NEC is code in the united states

Because nobody from Canada could give me a yes or no answer.

 

[Breakfasteatre]

the code rule is 26-264 (2)

and you need to satisfy both (a) and (b) or (a) and (c)

so you have satisfied (a), but do you satisfy either (b) or (c)?

the auto trans is used for starting or controlling an induction motor

or

the auto trans supplies a circuit wholly within the apparatus that contains the auto trans

 

[SCR]

the code rule is 26-264 (2)

and you need to satisfy both (a) and (b) or (a) and (c)

so you have satisfied (a), but do you satisfy either (b) or (c)?

the auto trans is used for starting or controlling an induction motor

or

the auto trans supplies a circuit wholly within the apparatus that contains the auto trans

Where does it say I need to satisfy 2 out of the 3? My interpretation would be that I only need to satisfy a or b or c. Obviously, if this Xformer would be connected to a motor, A and C would not have to be satisfied, so why should A be worried about B or C?

Thanks for your reply though, that's the kind of answer I was hoping to get.

 

[Breakfasteatre]

It's not an interpretation, the only "or" is between (b) and (c)

Where as there is a semicolon after (a) indicated "as well as" or "and"

So it should be read as

(A) and (b) or (c)