480V to 240V

[Little Bill]

Had a plumber call and leave a message. He said he had a 3Ø 480V water heater. He wants to replace it with a single Ø 240V. I'm just guessing that it is single Ø.
So if a transformer is used, would it need to be 3Ø, or just a single Ø and not use one leg?
Also assuming I would need to know the kW of the heater to size the transformer?
Help/thoughts?

 

[petersonra]

I think if it is three phase it can probably be rewired to single phase.

You don't have enough information to make a reasonable decision.

I am curious why he wants to convert it. The wire size will increase a lot, likely requiring new conduit.

 

[LarryFine]

Why? That's a step in the "wrong" direction.

So if a transformer is used, would it need to be 3Ø, or just a single Ø and not use one leg?

The transformer type is selected to suit the load.

Also assuming I would need to know the kW of the heater to size the transformer?

That's the first thing you need to know, and the voltage.


The customer needs to know they won't get as much hotness.

 

[augie47]

If the water heater is single phase use a single phase transformer fed by two phases of the 3 phase system

 

[Little Bill]

I think some may have missed that the heater is being replaced, so if the voltage matches the heater, there would be no loss of heat. Unless it's me that's not understanding and you mean a 480V heater will get hotter than a 240V.

 

[LarryFine]

Unless it's me that's not understanding and you mean a 480V heater will get hotter than a 240V.

I meant that a 1ph 240v water heater is not going to have the water-heating capacity of a 3ph 480v water heater (unless it is of equal total wattage (of which I am extremely dubious)).

And, as noted above, to even get close will require a new, larger circuit.

 

[tortuga]

As I noted in another thread today the plumber can usually just change the heating elements to match the system voltage. No need for a transformer.
you'll need to check the manufacturers instructions but most thermostats and stuff support 277 and even 480V
Here is a chart from a one we install often for a local Plumbing outfit.

 

[Little Bill]

I called the plumber and he was clueless about what was involved. I think he is just going to order a 3Ø heater for a direct replacement.
At least we both got educated!
Thanks!

 

[ruxton.stanislaw]

I called the plumber and he was clueless about what was involved. I think he is just going to order a 3Ø heater for a direct replacement.
At least we both got educated!
Thanks!

That would make a lot of sense for the situation.

If someone else runs into this and upgrading the conduit is too expensive or otherwise prohibitive, there is something called a Scott-T transformer that is used for splitting a two phase load (in the traditional sense) across three phases. That could be used with an upper and lower element on each phase, for example.

 

[Jraef]

...
And, as noted above, to even get close will require a new, larger circuit.

That's the issue, but then again maybe not. The required transformer would likely be able to re-use 2 (or 3**) of the phases from the existing 480V circuit, then you need a new circuit FROM the transformer secondary anyway. Yes, THAT will be larger, but is that a problem? I think not.

**By the way, just because it's 240V doesn't automatically mean it is single phase. Still though, it needs a transformer no matter what.

That said, I'm with the others in that this entire idea is suspect... I'm willing to bet that the "plumber" is wanting to do this simply because he HAS a 240V water heater that he wants to unload, even though it will COST the end customer more for the transformer, new secondary circuit components, and the added permanent increase in operating costs due to adding the losses in the transformer, as well as HVAC burden if that transformer is in an air conditioned space. Makes no sense.

 

[tortuga]

That said, I'm with the others in that this entire idea is suspect... I'm willing to bet that the "plumber" is wanting to do this simply because he HAS a 240V water heater that he wants to unload,

Yeah the plumber company we work with just stocks 240V units but they also stock the elements in a variety of voltages. Its relatively easy to swap out the elements to the correct voltage at no added cost to the customer.

 

[jim dungar]

That would make a lot of sense for the situation.

If someone else runs into this and upgrading the conduit is too expensive or otherwise prohibitive, there is something called a Scott-T transformer that is used for splitting a two phase load (in the traditional sense) across three phases. That could be used with an upper and lower element on each phase, for example.

A Scott-tee connection goes between two-phase (90°) and three-phase (120°) with balanced loading.
The only reason to do this for an upper and lower element heater would be if both elements were heating at the same time.

 

[Little Bill]

That's the issue, but then again maybe not. The required transformer would likely be able to re-use 2 (or 3**) of the phases from the existing 480V circuit, then you need a new circuit FROM the transformer secondary anyway. Yes, THAT will be larger, but is that a problem? I think not.

**By the way, just because it's 240V doesn't automatically mean it is single phase. Still though, it needs a transformer no matter what.

That said, I'm with the others in that this entire idea is suspect... I'm willing to bet that the "plumber" is wanting to do this simply because he HAS a 240V water heater that he wants to unload, even though it will COST the end customer more for the transformer, new secondary circuit components, and the added permanent increase in operating costs due to adding the losses in the transformer, as well as HVAC burden if that transformer is in an air conditioned space. Makes no sense.

He is ordering a direct replacement 3Ø WH.

 

[ruxton.stanislaw]

A Scott-tee connection goes between two-phase (90°) and three-phase (120°) with balanced loading.
The only reason to do this for an upper and lower element heater would be if both elements were heating at the same time.

The three phase load would present as unbalanced with only one of the two secondary phases under load, however, would not exceed the circuit ampacity. The premise for doing this is that some form of a 480 - 240v step down transformer would be required anyhow and that using two of the three existing current carrying conductors would not allow for enough power.

 

[tortuga]

There are many cool uses for a scott-t and two phase in modern applications for a large 120/240 split phase load.

 

[jim dungar]

There are many cool uses for a scott-t and two phase in modern applications for a large 120/240 split phase load.

There is little advantage when you are using a lot of 120/240V loads as the 90° phase angle would not be used. It might be helpful for lots of 240V loading but, there is a reason these are not normally found in transformer connection charts.

 

[wwhitney]

There are many cool uses for a scott-t and two phase in modern applications for a large 120/240 split phase load.

If you have a balanced 3 phase supply, and have only 120/240V single phase loads, you can't do any better than using (3) single phase transformers with 120/240V secondaries, with the primaries in a delta configuration. The secondaries will have all their neutrals grounded, so jointly they create "hexaphase."

Cheers, Wayne

 

[tortuga]

If you have a balanced 3 phase supply, and have only 120/240V single phase loads, you can't do any better than using (3) single phase transformers with 120/240V secondaries, with the primaries in a delta configuration. The secondaries will have all their neutrals grounded, so jointly they create "hexaphase."

Cheers, Wayne

I have seen a few scott-T's but never that.
I would be interested in a example.

 

[wwhitney]

I have seen a few scott-T's but never that.
I would be interested in a example.

Well, this is an example from the theoretical side, I have no idea if anyone does this. @winnie has described this idea before:

Say you have a 150 kW 3-phase supply, and all your loads are 120/240V (so 120V 2-wire, 240V 2-wire, and 120V/240V 3-wire). If you're OK with 120/208V "single phase" 3-wire on the secondary, you can just use a 150 kW 3-phase transformer with a 208Y/120V secondary (or maybe your supply is already 208Y/120V and you don't need a transformer). But maybe for some reason you insist on 240V L-L on the secondary, not 208V L-L.

Then you can use (3) 50 kW 2-wire primary, 120/240V 3 wire secondary transformers, with the primaries arranged A-B, B-C, and C-A. You get (3) different 50 kW 120/240V supplies you can distribute your loads amongst. Because the neutral point of each secondary is grounded, the voltage phasor diagram of all 3 secondaries together looks like the 6 lines from the center of a hexagon to its 6 vertices. Hence the name hexaphase. But if any given load is only supplied by one individual transformer, that relationship between the transformer secondaries is academic, the loads don't care.

I took your earlier comment to suggest you could instead use a Scott-T transformer to get 240V/120V 2-phase. That can't possibly use the 3 phase supply as efficiently. While one of the secondary L-L voltages is in phase with one of the primary L-L voltages, the other secondary L-L voltage is at 90 degrees to that first secondary L-L voltage. So I believe the current on this second secondary L-L leg will end up split between the two other primary L-L voltages. Then, for example, if the load is resistive, the current will be out of phase with both of those two other primary L-L voltages, and your available Watts on the secondary will be less than the VA of the transformer. [I need to look more closely at the Scott-T transformer diagram, but I believe this should be similar to supplying a 208V load L-N on the a 240V high leg delta secondary.]

Cheers, Wayne

 

[tortuga]

I took your earlier comment to suggest you could instead use a Scott-T transformer to get 240V/120V 2-phase. That can't possibly use the 3 phase supply as efficiently.

My understanding is if the 2 phase loads are balanced it does produce a balanced 3 phase load.