Transformer Question

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
Had a guy call me and ask about power for a well pump. Apparently he already had the pump but found out the only power there was 480V.
I told him he would have to get a transformer but I don't know much about them. So I have a few questions.
Would the secondary side need OCP?
The load doesn't require a neutral but they might want some 120V things, what kind of transformer would get you from 480V to 240V and also have a neutral?
 

Shaneyj

Senior Member
Location
Katy, Texas
Occupation
Project Engineer
If the pump is single phase, I’d use a single phase 480 to 120/240 - appropriate kva. That would give you the same distribution you’d see in a typical residence.


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augie47

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Tennessee
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State Electrical Inspector (Retired)
Agree... If you are going to have to install a transformer go ahead and install a 120/240 so you can have receptacles and lighting when desired.
It will need a panel with a main based on the secondary current of the transformer.
You will also need a grounding electrode system.
Most commonly I've done a 2kva (ACMET279742S) or 3kva (Acme T279743S) but it will depend on your pump HP.

If the budget is tight, you can do a 480/240 (no neutral) and omit secondary protection.
 
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augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
Is it possible to switch the primary to avoid idle current?
That's a good idea. I've done a lot of well pumps and never thought of that.
I don't know if the "standard" well pump switch is rated 480v but surely one is available.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
That's a good idea. I've done a lot of well pumps and never thought of that.
I don't know if the "standard" well pump switch is rated 480v but surely one is available.
Or a contactor that is so rated.

And don't call me Shirley. :giggle:
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Could one of you girls explain the switch?
The pressure switch that controls the pump. My question was about using it to switch the primary.

Sort of seeing the transformer and the pump as a single load, so it's only on when the pump runs.
 
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augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
If you want to supply just the pump.install a OCP device for the transformer primary, feed it thru your pump pressure switch to the transformer and feed the pump from the transformer secondary... 2 wire-2 wire
 

Little Bill

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Location
Tennessee NEC:2017
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Semi-Retired Electrician
The pressure switch that controls the pump. My question was about using it to switch the primary.

Sort of seeing the transformer and the pump as a single load, so it's only on when the pump runs.
If you want to supply just the pump.install a OCP device for the transformer primary, feed it thru your pump pressure switch to the transformer and feed the pump from the transformer secondary... 2 wire-2 wire
So from the panel, with the correct size breaker, to the pressure switch, then to the transformer, then to the pump. Correct?
So the pressure switches are rated for up to 480V or have to get one rated for 480V?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
So from the panel, with the correct size breaker, to the pressure switch, then to the transformer, then to the pump. Correct?
Yeppers, unless you also need a disconnect near the load.

So the pressure switches are rated for up to 480V or have to get one rated for 480V?
Not sure, but start with the one you have in mind. Augie may know.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The SqaureD catalog shows their pump switches rated at 480v.
I'm undecided as to this setup providing proper SCGF protection for the pump.
 

Shaneyj

Senior Member
Location
Katy, Texas
Occupation
Project Engineer
The sq d pressure switches common on compressors, well pumps, sump pumps and the like have different hp rating at different voltages.
I just looked at one rated 1 hp at 480V.
If the load exceeds 10 amps, I’d use the pressure switch to control a contactor and pass the pump/transformer load through a heavier duty contact.


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jim dungar

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Location
Wisconsin
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Retired Electrical Engineer - Power Systems
Inrush of the transformer + inrush of the motor might take that switch out pretty fast...
You will not have both maximum inrushes at the same time. The motor will not see current until the transformer action occurs after the core is magnetized. The switch contacts, while not simultaneously seeing both inrush currents will likely experience 'inrush' current for a longer time period than it may have been designed for.
 
Location
Arkansas
Occupation
Laboratory Engineer
I am a big fan of soft starters for applications like this. I have never tried one before a transformer between the load, but in theory, it would work.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Idle current would probably not be that bad, I have two 45 kva single phase transformers, one steps up, the other steps down. The total idle current on the source end is <1 amp.
 

Jraef

Moderator, OTD
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Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
You will not have both maximum inrushes at the same time. The motor will not see current until the transformer action occurs after the core is magnetized. The switch contacts, while not simultaneously seeing both inrush currents will likely experience 'inrush' current for a longer time period than it may have been designed for.
Yes, that’s what I was meaning, didn’t say it well. One after the other, extending the time that the circuit components will experience higher than normal current.
 
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