3ph Step-Up Transformer Connection

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Selimekim

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I need to operate a 480V 3ph rack-mount power supply (with no neutral connection) in a building that only has 208V 3ph power. I have a Square D 9T2F transformer (480V delta to 208V wye) which I would like to use as a step-up. I understand the X0 on the 208V side, used as primary, should be left floating. Is it okay to leave the 480V (now secondary) floating also? Or should it be corner grounded? The transformer is dedicated to and wired only to this one piece of equipment.
 
Welcome to the forum.

Must be grounded, derived-grounded, or ground-detected.

Also make sure it's suitable for reverse connection.
 
Thanks Larry!

We have a wall receptacle with 208 3ph (no neutral) and ground. The power supply does have a ground terminal, which I planned to carry through so it is connected to the building ground. But the 480 3ph output side of the transformer is floating, other than being connected to the power supply input. This might be a question for the power supply manufacturer, but I'm not sure how NEC handles floating outputs like this, i.e. do they have to be referenced to something somewhere. In case it matters, I think the transformer output will be hard wired to the power supply (no connectors); both pieces are set up to be wired that way.
 
Yes, it’s advisable to ask the power supply manufacturer if they require a Wye sourced set of AC Mains. The reason being is that it’s common, in electronic equipment, to have a set of surge suppressing devices (like MOVs) on the input mains which are wired in a Wye and grounded at the midpoint. When that midpoint is no longer referenced (like a Wye) to the 3 hot legs, bad outcomes are possible with spikes/transients on the incoming Mains. So a fully floating or corner grounded source can – but not always – be a problem.

If the load was a simple dumb motor, it would not matter at all that your 3 phase supply was floating w/ respect to some “zero point”. But things like VFDs and your power supply can be a different story because they might contain filtering and surge suppression circuitry in the front end.

Also as Larry points out, the NEC is going to want some type of grounding scheme present.

Lastly, the best solution here is (imo) a proper step-up Delta –> Wye transformer. Especially if this is a permanent thing and not something you want to test for 3 hours and then take apart.

tldr: I would vote for the right transformer.
 
I am contacting the manufacturer of the power supply for their advice on this. Thank you all for your help!!
 
MD Automation said:
Yes, it’s advisable to ask the power supply manufacturer if they require a Wye sourced set of AC Mains. The reason being is that it’s common, in electronic equipment, to have a set of surge suppressing devices (like MOVs) on the input mains which are wired in a Wye and grounded at the midpoint. When that midpoint is no longer referenced (like a Wye) to the 3 hot legs, bad outcomes are possible with spikes/transients on the incoming Mains. So a fully floating or corner grounded source can – but not always – be a problem.

If the load was a simple dumb motor, it would not matter at all that your 3 phase supply was floating w/ respect to some “zero point”. But things like VFDs and your power supply can be a different story because they might contain filtering and surge suppression circuitry in the front end.

Also as Larry points out, the NEC is going to want some type of grounding scheme present.

Lastly, the best solution here is (imo) a proper step-up Delta –> Wye transformer. Especially if this is a permanent thing and not something you want to test for 3 hours and then take apart.

tldr: I would vote for the right transformer.
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I'm with you. I would never even consider doing this with a delta secondary. In addition to what you mention, the gear would also have to be rated for corner grounding and you would have 480 to ground which is not ideal in this day and age.
 
The power supply manufacturer tells me it's fine.

Their equipment (power supply) has no neutral connection, so my (abstract) concern was about the common mode voltage range for the floating delta source at the input terminals of the power supply. Everything is grounded and contained in one place; if they have internal TVS protection to ground, that would limit any float voltage. I looked through NEC sec. 450, 250, etc. and didn't find where this might be addressed.
 
250.20 is where the NEC says where you need grounding. It doesn’t specifically say you have to ground the transformer, but the “system” has to be grounded. The step up transformer creates a “separately derived system”, so the rules apply anew.
 
Selimekim said:
The power supply manufacturer tells me it's fine.

Their equipment (power supply) has no neutral connection, so my (abstract) concern was about the common mode voltage range for the floating delta source at the input terminals of the power supply. Everything is grounded and contained in one place; if they have internal TVS protection to ground, that would limit any float voltage. I looked through NEC sec. 450, 250, etc. and didn't find where this might be addressed.
Click to expand...
If it has internal TVS protection and you would happen to corner ground the separately derived system, it will burn out portions of that TVS if only designed for say 300 volts to ground as it would continuously be clamping because 480 to ground is over 300.
 
If I'm reading it right, 250.20B seems to exclude this setup since it doesn't meet any of the conditions. Also (correct me if I'm wrong) what I have seems not to be "premises wiring"; my self-contained transformer will have a line cord on it for the input (208V delta with ground, no neutral), to plug into the wall receptacle, and an in-line receptacle for the output (480V delta with ground, no neutral), where the power supply can plug in. I agree it's probably better to leave the output floating; if the power supply has TVS protection that will limit any common mode voltage. This is in an R&D lab, and I will probably be the only one (occasionally) using it. Thanks for all the insight!
 
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