480V Wye stepped down for residential 120/240V 1PH?

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customfab4x4

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Austin, TX
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We've been in the middle of a big 3PH upgrade at a facility and am complete with Phase I. We are up and running with a 480V Wye configuration (400A) into our main building and panel.

Now we need to step it down so we can ultimately turn on the lights and power single phase 120/240V equipment.

I'm not a 3PH commercial electrician so hoping for some help here. Always been on the residential side.

What is the proper way to do this? I keep getting totally different answers from "experts" including transformer suppliers.

Some suggested going with a 480V to 208V step down and then pulling a leg off the 208V for 120V secondary.
Some suggesting a series of transformers to balance the loads correctly (making our utility company happy).

Very much appreciate you former experience and how you would meet these electrical needs. We want to install a 200A residential panel for these purposes.

Thanks.
 
Unless you specifically need 240 V, it makes more sense to use 208/120 V 3-phase delta-wye stepdown transformers and 208/120 V 4-wire panelboards. This will provide all of the 120 V you want. For things requiring more power, 208 V can be specified instead of 240 V. Or just specify them to operate at 480 V or 277 V. Once you are out of residential work, and dealing with 3-phase power systems, 240 V is the exception rather than the rule. I generally tried to avoid it if at all possible.
 
I agree with David. But your question leads me to wonder how familiar you are with 3- phase systems. So please forgive me if you already understand what I am about to say.

You said you have a 480 V WYE. That means between any 2 phases (e.g., A to B or C to A), the voltage is 480 volts. It also means that between any phase and the center of the WYE (i.e., the neutral point), you have 277 volts. It is no accident that dividing those two numbers (480/277) gives you the square root of three (about 1.732).

It works the same for the secondary side of a 480 V to 208Y/120 V transformer. Try dividing 208 by 120, and see what you get. Put another way and correcting something you said, you would not use a "480 V to 208 V step down." Rather, you would use a 480 V (delta configuration) to 208Y/120 V (WYE configuration) step down.

Try an Internet search to get a diagram of that type transformer and its wiring configuration.
 
A simple 480V primary to 240/120V secondary single phase transformer is all you need.

Example:
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Besoeker3 said:
For us, EU, we use residential. It is just simpler - all 230V
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I am well aware, having occasionally visited our daughter, her husband the Geordie, and their two daughters over the past 15 years or so. It never occurred to me to ask, is there a 3-phase system in the UK?
 
retirede said:
A simple 480V primary to 240/120V secondary single phase transformer is all you need.

Example:
Click to expand...
But a single one would unbalance the utility service.

My first design choice would be:
Use 208Y/120 3ph 4w transformer(s) to feed as much load as possible.
Use 3ph 4w panels were possible.
Use 120/208 1ph 3w panels where they make sense (many large residential complexes are done this way).
Add small buck-boost transformers to feed 240V loads that cannot tolerate 208V.
Use 240/120 3ph 4w transformers and panels where significant 240V loads are clustered.

Last resort would be to use at least (3) single phase 120/240V transformers, distributed across all three 480V phases.
 
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customfab4x4 said:
Some suggested going with a 480V to 208V step down and then pulling a leg off the 208V for 120V secondary.
Click to expand...

Not "a" leg, but all 3 legs...


What you are looking for, as mentioned above, is a 480 Delta to 208/120 Wye 3 phase step down transformer. Super common and easy to find. Just need to know the loads on the secondary to size it properly (don't skimp if this is a new facility).

Like David Castor mentioned in post #2, you will pull all three secondary legs and the neutral to feed a 3PH 4W panelboard(s). Then all the 120 VAC circuits, fed from this panel (or panels) are automatically "balanced" across the POCO's 3 incoming phases. All your 208 VAC 3 phase and single phase circuits are also fed from this panelboard.

Unless your 120 VAC loads are really really small - and you don't supply any 208 loads, I'd go with this 3PH step transformer approach.
 
Although they manufacture a 480 to 120/240 3 phase transformer there are a number of reasons you need to avoid them.
The most common approach, as mentioned above, is to use a 480 to 208Y120 3 phase transformer.
IF you find you have to have 240v, you can use a single phase 240/120 as shown min Post 5 and if you need more than one you can feed them in a manner to somewhat balance the load on the 480 system./
A lot what depend on such details as the propose load on the 480 system overall and the proposed 240v load.
 
customfab4x4 said:
turn on the lights and power single phase 120/240V equipment.
Click to expand...
Whats the calculated load for the 120/240 equipment?
augie47 said:
Although they manufacture a 480 to 120/240 3 phase transformer there are a number of reasons you need to avoid them.
The most common approach, as mentioned above, is to use a 480 to 208Y120 3 phase transformer.
IF you find you have to have 240v, you can use a single phase 240/120 as shown min Post 5 and if you need more than one you can feed them in a manner to somewhat balance the load on the 480 system./
A lot what depend on such details as the propose load on the 480 system overall and the proposed 240v load.
Click to expand...
Agreed if I needed to keep the 480 system(service) balanced and wanted the maximum 120/240 single phase load I could squeeze out of the 480 system, I'd use a scott-t transformer and derive two single phase panels.
 
David Castor said:
Unless you specifically need 240 V, it makes more sense to use 208/120 V 3-phase delta-wye stepdown transformers and 208/120 V 4-wire panelboards. This will provide all of the 120 V you want. For things requiring more power, 208 V can be specified instead of 240 V. Or just specify them to operate at 480 V or 277 V. Once you are out of residential work, and dealing with 3-phase power systems, 240 V is the exception rather than the rule. I generally tried to avoid it if at all possible.
Click to expand...
I mean, it varies by your area / town / neighborhood and how the PoCo set it up when the community was initially built.
Comparatively speaking, 120/208Y is the newest system.

120/240 has been universally available for single family homes.
120v is point of reference for a lot of things. The secondary for PTs used in distribution is 120v at full scale rather than 100, 200 and such.

Physics dictate that you can't get 1:2 ratio from a wye, and since 120v is what things are designed around, it's the other number that gets the odd voltage rather than have 139/240Y and 120/240 1ph3W. "208 to 230" motors are compromise and as you get to closer to the upper boundary of lowest voltage motors, you'll have 200v motors and 230v motors.

Some older light industrial facilities made use of corner ground delta 240v which did have its advantage of only needing two pole switch gear.
high-leg full delta was useful for primarily motor facilities while open delta allowed provision of 3 phase service to primarily single phase small customers that needed to use 3 phase equipment.
 
charlie b said:
I am well aware, having occasionally visited our daughter, her husband the Geordie, and their two daughters over the past 15 years or so. It never occurred to me to ask, is there a 3-phase system in the UK?
Click to expand...
Much of Europe is standardized around 400Y/230, so like bigger bretheren to our 208Y/120. In some EU countries, they get all three poles for things like tankless water heaters while others are like 1PH3W network power where each home gets 208/120 3W 1PH.

Who knows, maybe newest development of single family homes will get 208Y/120 with all 3 poles as EVs become more common.
 
hillbilly1 said:
Unless you have a fair amount of equipment requiring 240 volts, the three phase 120/208 is a much better choice.
Click to expand...
A 480 V Δ to 208Y/120 V transformer is probably the most reasonable way to go here. Find out what the loads are going to be. If there are items that would operate better at 240 V, you may want to adjust the transformer taps to obtain a nominal voltage of 220Y/127 V or use a buck boost transformer for that equipment.
 
ruxton.stanislaw said:
A 480 V Δ to 208Y/120 V transformer is probably the most reasonable way to go here. Find out what the loads are going to be. If there are items that would operate better at 240 V, you may want to adjust the transformer taps to obtain a nominal voltage of 220Y/127 V or use a buck boost transformer for that equipment.
Click to expand...
Here in the states that is a rare voltage.
 
jim dungar said:
But a single one would unbalance the utility service.

My first design choice would be:
Use 208Y/120 3ph 4w transformer(s) to feed as much load as possible.
Use 3ph 4w panels were possible.
Use 120/208 1ph 3w panels where they make sense (many large residential complexes are done this way).
Add small buck-boost transformers to feed 240V loads that cannot tolerate 208V.
Use 240/120 3ph 4w transformers and panels where significant 240V loads are clustered.

Last resort would be to use at least (3) single phase 120/240V transformers, distributed across all three 480V phases.
Click to expand...

The utility service to our plant was so large (medium voltage) that adding a few kva single phase load wasn’t noticeable.

But for a smaller, commercial-size service, you’re absolutely correct.
 
How much single phase 120 or 240v load anyways? An assortment of load that amounts to a single household on a 480v service likely won't matter on facility level balance and may not need to be evened out.

There are single phase loads like 277v lights, single phase condenser fan motors and such that are not on all the time that all act to cancel things out.
 
Wow! Such great responses and seems to be a debate. That has been my experience but also my frustration as I speak with half a dozen people and seem to get half a dozen responses. Some much better than others.

Charlie B, thanks for your helpful insight. I'm not a 3PH guy and my language or terminology may not be as technically accurate as it should be. I apologize for that and really do appreciate the clarification for future discussions.

"It works the same for the secondary side of a 480 V to 208Y/120 V transformer. Try dividing 208 by 120, and see what you get. Put another way and correcting something you said, you would not use a "480 V to 208 V step down." Rather, you would use a 480 V (delta configuration) to 208Y/120 V (WYE configuration) step down." - I stand corrected, Thanks!

So in summary, it seems there are two approaches being debated. I actually have a 3PH expert coming for a consultation with me later today so I will discuss these options with him. In my discussions yesterday with him, he seems to be suggesting Option 2 but didn't mention concerns about unbalanced loads.

Option 1) 480 Delta to 208/120 Wye 3 phase step down transformer

Option 2) A simple 480V primary to 240/120V secondary single phase transformer is all you need

Phase I of the upgrades is today. Thing is, we also have Phase II plans that also involve 120/240 needs. But current state needs are: (1) 120V LED warehouse lights & exterior lights (maybe up to 40A), three 240V 1PH motors for hydraulic pumps (less than 3HP each, maybe a total of 40A at 220/240), 220/240V 50A RV outlet, two 208/230V 1.5-2 ton split ductless (~40A total), and various 120V electrical outlets. I had sized a 50kVA transformer for up to 200A of needs but likely only really need 100-140A at any single point in time.

As Phase II of the project (2025), we will be erecting a 10,000 SF building. Most my 1PH needs remains associated more with residential type loads. Power tools, lights, etc. All our high load stuff is 3PH 240V and 3PH 480V. This building is for a millwork, woodworking shop. But I can address these power needs at a different time, if needed. Just an input to consideration in terms of looking across all our power needs.

For Option 1:

I like this suggestion: "A 480 V Δ to 208Y/120 V transformer is probably the most reasonable way to go here. Find out what the loads are going to be. If there are items that would operate better at 240 V, you may want to adjust the transformer taps to obtain a nominal voltage of 220Y/127 V or use a buck boost transformer for that equipment."

I need to go check our motors driving the hydraulic pumps for their ratings. Chinese so may actually be rated as 208/230. Seems all the minisplits that I'm looking at are 208/230. So not running anything super sensitive that would demand 220/240 ratings.

Thoughts here, considering above?

As suggested, I can supplement buck boost transformers specific to any equipment that requires higher 240V. And I don't see any immediate needs for that right now.


For Option 2:

Tortuga mentioned a solution that has not been suggested to me. "Agreed if I needed to keep the 480 system(service) balanced and wanted the maximum 120/240 single phase load I could squeeze out of the 480 system, I'd use a scott-t transformer and derive two single phase panels."

Thoughts on this approach? In the meantime, I will do my best to research "scott-t transformers". I must say that I do prefer the notion of having 220/240 always available but that's in an ideal world.
 
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Just to give you a few items to consider...
You might look at supplying your warehouse lighting load (40a) at 277 rather than 120 (Some newer lights will operated on either)
The 208Y/120- is still best for balancing so if you have limited equipment that requires 240 you could look at the possibility of a small buck-n-boost transformer (or a small 240/120 transformer) just for that equipment
 
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