Transformer Question

Status
Not open for further replies.
Little Bill

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
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?
 
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.


Sent from my iPhone using Tapatalk
 
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.
 
Last edited:
LarryFine said:
Is it possible to switch the primary to avoid idle current?
Click to expand...
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.
 
augie47 said:
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.
Click to expand...
Or a contactor that is so rated.

And don't call me Shirley. :giggle:
 
Little Bill said:
Could one of you girls explain the switch?
Click to expand...
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.
 
  • Like
Reactions: MTW
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
 
LarryFine said:
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.
Click to expand...
augie47 said:
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
Click to expand...
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?
 
Little Bill said:
So from the panel, with the correct size breaker, to the pressure switch, then to the transformer, then to the pump. Correct?
Click to expand...
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?
Click to expand...
Not sure, but start with the one you have in mind. Augie may know.
 
The SqaureD catalog shows their pump switches rated at 480v.
I'm undecided as to this setup providing proper SCGF protection for the pump.
 
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.


Sent from my iPhone using Tapatalk
 
Jraef said:
Inrush of the transformer + inrush of the motor might take that switch out pretty fast...
Click to expand...
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.
 
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.
 
jim dungar said:
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.
Click to expand...
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.
 
Status
Not open for further replies.
Top