Can you Buck Down 277/480 volt 3 phase 4 wire down to 120/240 or 208 Volts?

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Trying to bid a job that is sight unseen and I only have pictures to go off of. But what I have going on is a GE panel for 277/480 volt 3 phase 4 wire that is feeding a ton of rows of 277 volt fluorescent fixtures in a parking garage. This customer is wanting to had outlets for trickle chargers which will be rated for 120 volts single phase. I'm not sure if its doable or not but can I feed from a 75 amp breaker to a transformer that bucks down the voltage to install a subpanel that will then feed the required outlets that are needing 120 volts? Hopefully I am making sense...If I need to clarify anything any help on this will be much appreciated. Thanks.
 
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Thanks Bob. The first warehouse I called didn't understand what I am needing? And the the other warehouse has to get back to me because they have to call the manufacturer to see if they even make a transformer that bucks down the voltage and makes it single phase?
 

david luchini

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Thanks Bob. The first warehouse I called didn't understand what I am needing? And the the other warehouse has to get back to me because they have to call the manufacturer to see if they even make a transformer that bucks down the voltage and makes it single phase?
I don't think you're looking for a "bucking" transformer. Just a standard 480-208/120V isolating transformer to supply your subpanel.
 

jim dungar

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I don't think you're looking for a "bucking" transformer. Just a standard 480-208/120V isolating transformer to supply your subpanel.

The word "buck" means to drop the voltage, while "boost" means to raise the voltage.
However in our industry we combine this into a single phrase "buck-boost' which usually means a small transformer with the primary and secondary windings interconnected into an autotransformer which is used to make small, usually <15%, voltage changes.
Rarely, would the 'busk-boost' be used to describe a voltage change of more than 50%, like from 480 down to 208V.

You do not use a transformer to convert from 3-phase to single-phase.
 

__dan

Senior Member
Yep, bucking usually refers to an autotransformer, not seperately derived and has one wire common to both line and load side. From memory I seem to recall they are not usually recommended for large voltage changes, because of what could happen if the windings failed shorted. Autotransformer for bucking are very common with 12/24 or 16/32 secondary windings, which refers to the voltage change (usually applied to the line side). Bucking 277 to 120 the winding secondary would be 157 Volts, probably not something in the catalog.

You are probably thinking of a standard step down transformer, and if so, the supply house will stock that and not have to call the manufacturer for an oddball item.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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Thanks Bob. The first warehouse I called didn't understand what I am needing? And the the other warehouse has to get back to me because they have to call the manufacturer to see if they even make a transformer that bucks down the voltage and makes it single phase?
Reducing the voltage and deriving single phase are two different things. Your 277v lighting is single phase.
 
I don't think you're looking for a "bucking" transformer. Just a standard 480-208/120V isolating transformer to supply your subpanel.
I concur. The only question is whether to go three phase or single phase. Three phase would (most likely) be 480 delta to 120/208y. Single phase would be 480 - 120/240. Not really enough details to say which is better for the situation.
 

Russs57

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Miami, Florida, USA
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Maintenance Engineer
They are giving good advice. Forget buck/boost concept. If the main panel is 480/277 it should be 5 wire but that matter little for now.

What is important is how many 120 volt chargers do they want, what is the KVA/amps/watts of the chargers, and will they be hard wired or cord connected. Also is it a "damp area" where the chargers will be. What about where the transformer/panel will be? These answers will determine the most cost effective solution to the problem.

FWIW I can tell you we have a parking garage with cord connected golf cart chargers at 120 VAC. The outlets are GFI's and those chargers trip them constantly! Major pain in the rear end.
 

drcampbell

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The Motor City, Michigan USA
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Registered Professional Engineer
If you were to buck the voltage, it would be from 277 to 120 volts. There would be one buck transformer per phase, each connected to one hot wire and one grounded wire, and you could use one, two or three phases depending on the load.

First problem: a 277:157-volt buck transformer isn't exactly going to be an off-the-shelf commodity.

Second problem: your bucking voltage would be greater than your output voltage, and you'd need 1.3 kVA of transformer capacity to derive 1.0 kVA of 120-volt power. Not economical, not energy-efficient.

Forget about bucking. Go with a stock 480:120/240 transformer if the load's not very big, or a stock 480:208Y120 if it is.
 

Jraef

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Just to clarify; are you wanting to buck down from individual 277V circuits that are now going to lighting, in order to have an outlet for the trickle chargers? Or are you wanting to add a breaker to the existing 480V panel to feed a transformer to then distribute to 120V outlets?

The reason I ask is because I find it hard to believe that you have a warehouse that does not already have a system of 120V single phase outlets in it, in fact I don’t think that would meet code. So that leaves me to believe that the ask here is to tap into the existing 277V single phase lighting circuits to be able to power the trickle chargers in areas that currently have no 120V outlets.

If thats the case, and its only a couple of outlets, then yes, you can find small single phase 277-120V transformers, one for each circuit. But keep in mind that once you put in an 5-15 receptacle, people are going to want to plug in anything they have to it. So each transformer would have to be rated for 15A if that were the case; 2kVA minimum. If they want more than a couple of those, you should weigh the cost against running new circuits off of your existing 120V panel or adding a new 3 phase 480-208Y120 transformer and panel for them.
 
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