Establishing 240/120V service with 480V Split Phase

S

Sean.Day72

Member
Location
Florida
I have a situation where there are existing 480V/ 240V split phase transformer along a roadway. They are being using for 480V roadway lighting circuits.
I need to drop a 240V/120V service, I'm assuming the only way to do this is to provide a 480P/ 240S V buck/boost transformer
I was wondering if I could just bring over 240V from the utility transformer from just brining over half of the split phase, but I don't see how I could get 120V from that.
 
1) Buck/boost transformer typically means an autotransformer arrangement, where primary and secondary share coils. You probably don't mean an autotransformer (though it is possible), you probably mean using a single phase 480V : 120/240V transformer. That would be the standard way to do the job.

2) You could just bring 240V from the existing service, and then use a single phase 240V : 120/240V transformer. This would double the primary current, and probably give you higher voltage drop. Since many transformers can be connected either 240V or 480V on the primary side, you'd likely use exactly the same transformer as in option 1.

3) The 240V from the existing service is 240V hot to ground. You can't change it to 120V hot to ground/ 240V hot to hot without using a transformer.
 
Sean.Day72 said:
I have a situation where there are existing 480V/ 240V split phase transformer along a roadway. They are being using for 480V roadway lighting circuits.
I need to drop a 240V/120V service, I'm assuming the only way to do this is to provide a 480P/ 240S V buck/boost transformer
I was wondering if I could just bring over 240V from the utility transformer from just brining over half of the split phase, but I don't see how I could get 120V from that.
Click to expand...
That’s a big leap for a book/boost, you just need a step down transformer.
 
A related question:

Suppose we were custom winding a transformer for this application, rather than sticking with what's available off the shelf. For a given 120/240V load rating, which would use less aluminum/steel, a 480V autotransformer with quarter point connections, or a 480V : 120V/240V isolation transformer?

Cheers, Wayne
 
Sean.Day72 said:
I have a situation where there are existing 480V/ 240V split phase transformer along a roadway. They are being using for 480V roadway lighting circuits.
I need to drop a 240V/120V service
Click to expand...

Another side question are you sure the load really requires 120V?
If this by chance for a camera system or some DOT roadway electronics / telcom gear it might have a 100-240v power supply or even 100-277V its pretty common for DOT spec or telcom huts to accept '240V' as they are converting it to DC anyway.
 
wwhitney said:
A related question:

Suppose we were custom winding a transformer for this application, rather than sticking with what's available off the shelf. For a given 120/240V load rating, which would use less aluminum/steel, a 480V autotransformer with quarter point connections, or a 480V : 120V/240V isolation transformer?

Cheers, Wayne
Click to expand...
Though popular overseas, they are not popular in the US.
 
wwhitney said:
A related question:

Suppose we were custom winding a transformer for this application, rather than sticking with what's available off the shelf. For a given 120/240V load rating, which would use less aluminum/steel, a 480V autotransformer with quarter point connections, or a 480V : 120V/240V isolation transformer?

Cheers, Wayne
Click to expand...

I would expect the autotransformer to use less aluminium/steel.

Say you have a 4 coil 120/240 : 120/240 isolation transformer; I'd expect about double the kVA capacity if wired as a 240/480:120/240 autotransformer.

-Jonathan
 
The service will be powering dynamic signange on the side of the road which needs 120V.

I'm looking at an Eaton EP general purpose fully encapsulated transformer to mount to a rack. (10 KVA). The one thing that is not in the literature is if I can do a split phase if I use a 240V or if I just procure 120V . It will connect to a small 12 pole load center to feed a handful of circuits.
 
Eight out ten times I am asked for something like this for some outdoor camera / sign / radio / wifi / IoT /thingamajig that works fine on just about any voltage 90 to 264VAC.. The other two times its not compatible with 277 and I do need a autotransformer..
it turns out to be unnecessary and a waste of time due to somebody not reading spec's right.
Last time it was one of these 48VDC power supplies, they insisted it needed 120 VAC when really it says right on the unit
Input Voltage Range 90 to 264VAC and even 127 to 370VDC, the reseller did not include that info just said 120VAC.

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wwhitney said:
Shouldn't we be calling this a 240/480 : 240/480 isolation transformer? Because we need the two primary coils to be rated for 480V when wired in series.

Cheers, Wayne
Click to expand...

No. I was saying that a transformer designed to take 120V or 240V (isolated primary in series or parallel) to 120V or 240V or 120/240V (isolated secondary in various configurations of 2 coils) would supply double its rated kVA if it were instead connected as a 4 coil autotransformer to converter 240/480V to 120/240V.

The proposed transformer could not be used as an isolating transformer to go from 480V to 120/240V.

I was presenting this example to show that the autotransformer configuration would result in 2x transformer rated kVA delivered to the load.

-Jonathan
 
Sean.Day72 said:
The service will be powering dynamic signange on the side of the road which needs 120V.

I'm looking at an Eaton EP general purpose fully encapsulated transformer to mount to a rack. (10 KVA). The one thing that is not in the literature is if I can do a split phase if I use a 240V or if I just procure 120V . It will connect to a small 12 pole load center to feed a handful of circuits.
Click to expand...

Can you provide the part number for the transformer?

Jonathan
 
wwhitney said:
A related question:

Suppose we were custom winding a transformer for this application, rather than sticking with what's available off the shelf. For a given 120/240V load rating, which would use less aluminum/steel, a 480V autotransformer with quarter point connections, or a 480V : 120V/240V isolation transformer?

Cheers, Wayne
Click to expand...
Which conductor do you ground if you have a transformer with quarter point connections? If you know only knowledgeable people would ever work on it maybe you do ground one the quarter points so there is 120/240 across the grounded conductor and only use the other end of the winding for 480 volt loads. Best bet for less confusion and overall safety is probably to use 480 x 120/240 isolation transformer.

It is possible to connect straight 240 volt loads to the 240/480 existing supply, you can not protect them with 120/240 rated circuit breakers though, and you remember the neutral of that 240/480 is a grounded conductor and should be treated like any other grounded conductor....separate neutral and grounds beyond service equipment, identified with white or gray, no overcurrent device in that conductor unless it simultaneously is disconnected with the ungrounded conductor(s)...
 
kwired said:
Which conductor do you ground if you have a transformer with quarter point connections? If you know only knowledgeable people would ever work on it maybe you do ground one the quarter points so there is 120/240 across the grounded conductor and only use the other end of the winding for 480 volt loads. Best bet for less confusion and overall safety is probably to use 480 x 120/240 isolation transformer.
Click to expand...

A transformer with four 120V coils in series as an autotransformer would not be regrounded.

The grounded conductor would come from the supply and continue to the load.

If you grounded one of the quarter points you would have a 120V ground fault.

But this is tangential to the OP. The OP needs 120/240V and doesn't need the headache of an autotransformer, even though it means less copper and iron in the box
 
winnie said:
A transformer with four 120V coils in series as an autotransformer would not be regrounded. The grounded conductor would come from the supply and continue to the load.
Click to expand...
Right, because 210.9 / 215.11 requires the grounded conductor for the load be connected to the grounded conductor from the supply. And if you connect that common grounded conductor to the autotransformer, you could have the circulating current problem.

Slightly confused, though, on the resulting currents if we have just a 10A 120V load. Starting at the grounded conductor on the supply, it carries 10A to the load, through the load, and then the 120V line conductor carries 10A back to one of the transformer quarter points. How do we determine how that 10A gets split up between the 480V L1/L2 conductors?

Thanks,
Wayne
 
wwhitney said:
How do we determine how that 10A gets split up between the 480V L1/L2 conductors?
Click to expand...
I guess ampere-turn considerations mean the ampere turns in the two distinct sections of the coil should be equal (or with the proper sign convention, the sum should be zero). That means if L1 is the line that is closer to the 120V system line conductor, L1 sees 7.5A, and L2 sees 2.5A (or maybe we should call that -2.5A, since the currents are adding in the neutral.) Power balance wise, we have 7.5A * 240V = 1800W coming out of L1/N, 1200W going to the load, and 600W going back into L2/N.

So it seems to me that for this example of a split voltage 120/240V secondary, the autotransformer only has 4/3 the VA rating as the isolation transformer.

Cheers, Wayne
 
tortuga said:
What about Kirchhoff's current law? You'd just see 2.5 A on each of the two 480 legs no?
Click to expand...
No, that doesn't comply with KCL. The 120V ungrounded current is 10A (on one line only) so the sum of the 480V ungrounded currents must be 10A. Remember that the autotransformer has no connection to the grounded conductor.

Cheers, Wayne
 
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