240 Volt Transformer Neutral Sizing?

[rosslet]

This has gotten a little silly IMO.

I made the comments in posts #6 and #9 because it sounded like the OP thought he only needed to think about grounding a conductor if he uses the neutral. He needs to think about regardless, first of all, and that was my main point. His previous comments (such as possibly wanting 120V loads in the future) also precluded that he had any exotic reason not to ground the neutral point, or that he would prefer to install ground detectors. Which means the way he will wish to make the system code compliant is to ground the neutral.

I would still say , given the transformer used, that the system can be grounded in accordance with 250.20(B)(1), and therefore must be. In the case of electrofelon's application, his system can't be grounded at 120V because that would be incompatible with the load. So I think that's how he gets out of it. But the OP has no such excuse.

Well that was an interesting discussion!

Anyways I appreciate you mentioning this earlier because, yes, I hadn't really known grounding it was code. I've always done it on the premise of, I didn't see any reason not to...I have to run a compliant ground for chassis so why not the tap too? But I wasn't aware it was absolutely required so thanks for mentioning that!

 

[electrofelon]

This has gotten a little silly IMO.

. Did you click the wrong tab and think you were over at ET?

I would still say , given the transformer used, that the system can be grounded in accordance with 250.20(B)(1), and therefore must be.

I agree . Can the system be grounded so that the voltage to ground doesn't exceed 150 V? It's a yes or no question. I don't see the transformer wiring compartment as being secluded from the outside world by an umbra or penumbra (another supreme Court reference).

In the case of electrofelon's application, his system can't be grounded at 120V because that would be incompatible with the load.

I guess I missed the exception to 250.20 that permits modification of the rules based on the characteristics of the load

 

[wwhitney]

I agree . Can the system be grounded so that the voltage to ground doesn't exceed 150 V? It's a yes or no question. I don't see the transformer wiring compartment as being secluded from the outside world by an umbra or penumbra

Fine, just apply some heat shrink tubing over the unused X0 terminal in the transformer. Now you can no longer ground the system so the voltage to ground doesn't exceed 150V, that terminal is unavailable. (Fake) problem solved.

Cheers, Wayne

 

[wwhitney]

So say I specifically want a 240v "end grounded" system. I actually needed to do this recently to supply a piece of European equipment that needed 240 volts hot to neutral. Pretty much every transformer you will find to do this will be a general purpose type which "can be" grounded in the center. Do we need different code wording to accommodate this?

OK, looking at 250.20 and 250.26 more closely, I will say that what you did was code compliant and we don't need any different code wording to accommodate it.

250.20(B) refers to "alternating current systems". So if you take the position that an unused X0 connection on a 120/240V secondary is part of the "system", then the 250.20(B)(1) applies because the "system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts." [Not sure I buy this part.]

That just tells you that the system needs to be grounded; it doesn't tell you what conductor to ground, or that you must ground it so that the maximum voltage to ground does not exceed 150 volts. For what conductor to ground we need to look at 250.26. That refers to "grounded ac premises wiring systems." If no premises wiring conductor is connected to X0, then you have a "single phase 2-wire" premises wiring system. So 250.26(1) just says to ground one of those two conductors. Which is precisely what you did.

Cheers, Wayne

 

[LarryFine]

To add to the original topic, a 3-wire 1ph 120/240v circuit is, for all practical purposes, two otherwise-independent 2-wire circuits that share a conductor. (A 3ph wye is likewise three 2-wire circuits.)

Because of the opposing polarities, the current on the shared conductor, the neutral, is subtractive, so it only has to carry difference current. That's also why we usually don't consider it to be a CCC.

 

[tortuga]

Here is a hypothetical say this water heater is a 6kw load, and say you need to add another 6kw water heater while also keeping the 480 side as balanced as possible,
Now you could size your 480:240 transformer to 15kw to handle the possible 12kw and hopefully balance the other 480 legs.
If the 2nd water heater is uncertain you'll probably just go for a 10kw and they can swap out the transformer later.

Now tomorrow comes and they do want the 2nd 6kw water heater;
you could add a 2nd 10kw transformer to balance the primary load, but then its not quite balanced and you'll need to upsize the 480 side, or could we make a perfectly balanced primary side with the same materials?

This is where a 3-wire two phase system comes into play, using two 10kva 480:240 transformer's, the primary side of at least one would need a center tap. The center-tap of one transformer primary would connect to the end of the 2nd transformer primary (call that the teaser) and your 480 side you see a perfectly balanced ~14 amp load.
There is not a ton of benefits to doing this other than if your 480 side is running maxed out, or you need it very balanced for a generator, or you have a long run form the transformers to the water heaters then you save 1 wire, and possibly a conduit size lower / less de-rating as its one feeder.
Due to the phase relationship the neutral of the two phase side would carry sqrt2 the amps of the outer line wires, but not a big deal as your saving running a 4th wire.

 

[jim dungar]

Here is a hypothetical say this water heater is a 6kw load, and say you need to add another 6kw water heater while also keeping the 480 side as balanced as possible,
Now you could size your 480:240 transformer to 15kw to handle the possible 12kw and hopefully balance the other 480 legs.
If the 2nd water heater is uncertain you'll probably just go for a 10kw and they can swap out the transformer later.

Now tomorrow comes and they do want the 2nd 6kw water heater;
you could add a 2nd 10kw transformer to balance the primary load, but then its not quite balanced and you'll need to upsize the 480 side, or could we make a perfectly balanced primary side with the same materials?

This is where a 3-wire two phase system comes into play, using two 10kva 480:240 transformer's, the primary side of at least one would need a center tap. The center-tap of one transformer primary would connect to the end of the 2nd transformer primary (call that the teaser) and your 480 side you see a perfectly balanced ~14 amp load.
There is not a ton of benefits to doing this other than if your 480 side is running maxed out, or you need it very balanced for a generator, or you have a long run form the transformers to the water heaters then you save 1 wire, and possibly a conduit size lower / less de-rating as its one feeder.
Due to the phase relationship the neutral of the two phase side would carry sqrt2 the amps of the outer line wires, but not a big deal as your saving running a 4th wire.

Sorgel Transformers used to publish a wiring diagram for a "hoppy hookup" which allowed a balanced 120/240V loading on a 480V 3-phase feed although one leg would be about 2X the other two.

Other than the Philadelphia area, I don't think I have ever seen a true 2-phase installation used for general loads.

 

[LarryFine]

Sounds like a variation of a Scott-T.

 

[tortuga]

Yeah I have never seen a 3-wire two phase either, just interested in the theory, I think the 3-wire two phase was a creation of William Stanley, the man credited with the first practical application of the ac system using transformers in Great Barrington, Massachusetts, I think the main reason it still exists is electric rail and foundries. In the foundries its special furnaces that need two phase, possibly chip fabrication.
There were (might still be) two phase 3-wire systems in a foundry near me, there are some old papers on it form OSU in Oregon.
The chip fabricators are pretty tight lipped about their proprietary processes so not easy to get a tour, like you can in Philly.
Also you cant get 120V from it like Philly (5-wire) two phase just so its only useful in industrial.
This is how I *think* it would look in my hypothetical, need to check the math on it:

 

[LarryFine]

 

[jim dungar]

Sounds like a variation of a Scott-T.

It has been many decades since I last saw the 'hoppy hookup' diagram, my recent searches have been unsuccessful.

I could never find a reason to use it.

 

[wwhitney]

This is where a 3-wire two phase system comes into play, using two 10kva 480:240 transformer's, the primary side of at least one would need a center tap. The center-tap of one transformer primary would connect to the end of the 2nd transformer primary (call that the teaser) and your 480 side you see a perfectly balanced ~14 amp load.

That would give you an unbalanced 2-phase secondary. For a balanced 2-phase secondary, your teaser transformer would need to be a 416V:240V transformer, with a sqrt(3):1 turns ratio, not a 480V:240V transformer with a 2:1 turns ratio.

If the primary (480V) legs are A, B, and C, and you create H0 as the midpoint of A and C, then the voltage H0 to B would be 416V. So with both transformers at a 2:1 ratio, you'd get 240V L-L on the secondary connected to A and C, and 208V L-L on the secondary connected to B and H0. These voltages would be at 90 degrees to each other, though.

Cheers, Wayne

 

[Jpflex]

NFPA 70 220.61
"
(A) Basic Calculation
The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.

in order to size the grounded conductor (center tapped /neutral conductor) when speaking of dwellings you would often do an entire calculation. This would include measuring outside dimensions of a house and adding all loads, apply demand percentages to total VA and then using table 250.102c to size your neutral based on the size of line ungrounded wire you determined necessary for the feeder or service.

The size of the hot wire ungrounded conductor and 250.102 c would be based on all the loads you plan to supply power to size your neutral I believe