240 Volt Transformer Neutral Sizing?

[rosslet]

Short version: on a single-phase 480VAC to 240Volt center tap transformer, if each of the 120VAC circuits has 50Amps being fed, how much current returns to the transformer on the center tap neutral wire? IE do I need a neutral wire capable of 100A feed or 50A? Assuming all power wires are sufficiently sized etc.

Long version: I have a client that wanted a 240Volt water heater added last minute to a control panel with 480VAC in it so we specified a floor mounted transformer from 480V to 240V (single-phase). This is an industrial application. At the last moment, again, it was decided to bring 120VAC back to the panel for possible future use. So we plan to ground and bring the center tap on the 240V Secondary side back in case they wanted to ever add 120V loads to this panel. So to be clear, we're only adding a single 240VAC load, no 120VAC loads.

I found article 220 (quoted at the end) code seems to imply you must size it at the time of adding loads, which would imply this specific aspect has no affect on me as their are no loads yet or possible ever. So in that case, the size of the neutral need only be as large as the grounding wire (which is somewhere else in code I don't recall off hand) and the ground was 6AWG in this case. The original plan was simply to have three terminal block busses containing the two taps and the neutral. Someone wants to add anything, they wire to these terminal blocks, and balance between the two taps as they add loads.

But it got me thinking, I don't want to leave this circuit easy to screw up for the next guy, so I added a circuit breaker upstream of each power terminal block bus. The idea is, someone will have to bump up this breaker if they want more capacity and in theory they need to take a peak at the capacity as opposed to people just blindly slapping more load on until the larger phase wires are fine but the neutral smokes or something.

So, the 6AWG neutral circuit has components that can take at least 50 Amps, therefore I originally put a 50A breaker to each of the two power terminal block busses.

Which brings us to our question, if both taps are running 50 Amps, is the return load on the neutral to the transformer 50A or 100A?

I got nervous and bumped the breakers down to 20A, which ensures this issues won't be possible, but I cannot for the life of me land on this one and would love some input! Thanks for reading!

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.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor multiplied by 140 percent.
"

oh and little bonus question, does anyone know how one is supposed to refer to the different taps on these split phase systems? I believe they're not two phases in the same way say two of three phases in a 480V delta system. So if they're not really two "phases" what are we supposed to call them? I've started calling them top and bottom tap.

Wikipedia's split phase electric power says this on it which makes sense, but doesn't answer to me what is a sensible way to refer to them.
"Since the two phasors do not define a unique direction of rotation for a revolving magnetic field, a split single-phase is not a two-phase system."

 

[LarryFine]

Welcome to the forum.

Short version: on a single-phase 480VAC to 240Volt center tap transformer, if each of the 120VAC circuits has 50Amps being fed, how much current returns to the transformer on the center tap neutral wire? IE do I need a neutral wire capable of 100A feed or 50A? Assuming all power wires are sufficiently sized etc.

50a, the maximum imbalance.

Which brings us to our question, if both taps are running 50 Amps, is the return load on the neutral to the transformer 50A or 100A?

0a, if both lines are loaded.

oh and little bonus question, does anyone know how one is supposed to refer to the different taps on these split phase systems? I believe they're not two phases in the same way say two of three phases in a 480V delta system. So if they're not really two "phases" what are we supposed to call them? I've started calling them top and bottom tap.

They are of opposite polarity, not "phase."

Simply, the two secondaries are in series.

 

[rosslet]

Welcome to the forum.


50a, the maximum imbalance.


0a, if both lines are loaded.


They are of opposite polarity, not "phase."

Simply, the two secondaries are in series.

Hi Larry!

Thanks for the welcome and response! I think that makes sense if they are opposite polarities. So, if I understand that right, at any point in the AC sine wave the top and bottom taps are opposite. If at a given instant the top was, say, 100 volts the bottom would be -100 volts?

So is the idea that, since the bottom tap is negative, the current flow is in the opposite direction on the neutral? So each circuit in isolation requires 50 Amps on the neutral. Looking at each circuit in isolation is the unbalanced case and that's why 50 Amps on the neutral wire is worst case. But, if both need 50 amps, these unbalanced current flows are opposite direction and that's why we get the zero Amps?

And I did have one other question on this from the code itself. The Exceptions part. The first makes sense, as we've said, the amperage required is simply max unbalanced load. The first thing they mention in the exception is they refer to a 3-wire 2-phase, that's referring to actual phases then? not this?

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.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor multiplied by 140 percent.
"

 

[infinity]

And I did have one other question on this from the code itself. The Exceptions part. The first makes sense, as we've said, the amperage required is simply max unbalanced load. The first thing they mention in the exception is they refer to a 3-wire 2-phase, that's referring to actual phases then? not this?

No a 2-phase system is not what you have. I fact they're so rare you can pretty much assume that you will never see one.

 

[jim dungar]

Simply, the two secondaries are in series.

Yes. This is the most accurate way to describe a center-tapped winding and a reconnectable transformer secondary.

Most confusion seems to be around the use of the center point as a neutral reference. But choosing a reference point does not change the underlying physics.

 

[jaggedben]

... we specified a floor mounted transformer from 480V to 240V (single-phase). This is an industrial application. At the last moment, again, it was decided to bring 120VAC back to the panel for possible future use. So we plan to ground and bring the center tap on the 240V Secondary side back in case they wanted to ever add 120V loads to this panel. ...

You need to ground and bond the center-tap regardless of whether you bring the neutral to the panel or any circuits.

 

[kwired]

You need to ground and bond the center-tap regardless of whether you bring the neutral to the panel or any circuits.

I don't think you would be required to bond the center tap in such application, but if you don't then you would have a 240 volt two wire system and would need to ground one of those two conductors. This would make it unacceptable to use the common 120/240 volt circuit breakers that are relatively inexpensive compared to ones that can be used on 240 volts to ground systems.

 

[electrofelon]

I don't think you would be required to bond the center tap in such application, but if you don't then you would have a 240 volt two wire system and would need to ground one of those two conductors. This would make it unacceptable to use the common 120/240 volt circuit breakers that are relatively inexpensive compared to ones that can be used on 240 volts to ground systems.

I think you would have to ground the center tap
From 250.20(B):


Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts.

I guess it might depend on one's philosophy of what "can be grounded" means, specifically if it's just what conductors are brought out of the transformer. We have the same situation arise for secondary conductors protected by primary overcurrent device on a two-wire transformer.

 

[jaggedben]

It needs to be grounded and the EGC needs to be bonded to the center tap to have an effective ground fault return path.

 

[electrofelon]

It needs to be grounded and the EGC needs to be bonded to the center tap to have an effective ground fault return path.

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?

 

[wwhitney]

You need to ground and bond the center-tap regardless of whether you bring the neutral to the panel or any circuits.

I will disagree. 250.20(B) refers to the grounding the "system". If I have a 120/240V center tapped transformer secondary, and I only bring 2 wires out of the enclosure with 240V between them, then that system as a whole is 240V 2-wire. Therefore 250.20(B)(1) does not apply.

Note that 250.20(A) refers directly to transformers, so the avoidance of the word transformer in 250.20(B) appears deliberate to me, and implies the above interpretation.

Cheers, Wayne

 

[electrofelon]

I will disagree. 250.20(B) refers to the grounding the "system". If I have a 120/240V center tapped transformer secondary, and I only bring 2 wires out of the enclosure with 240V between them, then that system as a whole is 240V 2-wire. Therefore 250.20(B)(1) does not apply.

Note that 250.20(A) refers directly to transformers, so the avoidance of the word transformer in 250.20(B) appears deliberate to me, and implies the above interpretation.

Cheers, Wayne

I don't think I agree with you wayne, got to be Clearance Thomas here, and ask where are you getting this verbiage about the conductors inside of the transformer are not to be considered as part of the system? I see nothing whatsoever stating that.

 

[wwhitney]

I don't think I agree with you wayne

Really? So for the 240V 2-wire grounded system you mentioned, you went to the trouble of getting a transformer with only two secondary connections, rather than using one with a more common split secondary?

Would you prefer that 250.20(B)(1) be reworded to use the term circuit conductor, just like 250.20(B)(2) and (3) do? I.e. "Where the system a circuit conductor can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts."

Personally, I don't think that changes the meaning of 250.20(B)(1) any, but it if you think it's clearer, it would be a good PI.

Cheers, Wayne

 

[kwired]

Really? So for the 240V 2-wire grounded system you mentioned, you went to the trouble of getting a transformer with only two secondary connections, rather than using one with a more common split secondary?

Would you prefer that 250.20(B)(1) be reworded to use the term circuit conductor, just like 250.20(B)(2) and (3) do? I.e. "Where the system a circuit conductor can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts."

Personally, I don't think that changes the meaning of 250.20(B)(1) any, but it if you think it's clearer, it would be a good PI.

Cheers, Wayne

I'm with you. It is a 240 volt two wire secondary if you don't connect anything to the center tap.

Not many are going to do this way other than if powering say some European made equipment that was designed to operate on two wire 240 volts. Otherwise still more desirable to ground the center tap even if you don't have any neutral loads.

 

[LarryFine]

Understanding the Neutral Conductor

Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics: Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts...

forums.mikeholt.com

 

[tortuga]

In parts of Europe they still use 230V ungrounded wye, so I doubt anything from Europe would need a grounded 240v conductor.
But good discussion.

No a 2-phase system is not what you have. I fact they're so rare you can pretty much assume that you will never see one.

And 3-wire two phase is a rare bird I have never seen, I have herd that was only in New England.
4-wire and 5-wire two phase I have seen much more than other rarity's like corner grounded especially in its home territory Ohio, PA & NJ, and there are emerging new uses for it, like balanced single phase loads off 480 systems.

 

[wwhitney]

And 3-wire two phase is a rare bird I have never seen, I have herd that was only in New England.

So, two phase motors have two sets of windings, while three phase motors have three? And with a two phase motor and 3-wire 2 phase, you just wire one winding L1-N and the other winding L3-N, which are 90 degrees apart, and the motor doesn't care that two of the 4 connections are at the same voltage?

Cheers, Wayne

 

[electrofelon]

Really? So for the 240V 2-wire grounded system you mentioned, you went to the trouble of getting a transformer with only two secondary connections, rather than using one with a more common split secondary?

No I did not. Such a thing is rather hard to find. I just capped off the center tap and ignored it, even those I see it as technically a violation of 250.20.

Would you prefer that 250.20(B)(1) be reworded to use the term circuit conductor, just like 250.20(B)(2) and (3) do? I.e. "Where the system a circuit conductor can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts."

Personally, I don't think that changes the meaning of 250.20(B)(1) any, but it if you think it's clearer, it would be a good PI.

I don't think I like that wording either, doesn't seem any less ambiguous to me

 

[electrofelon]

I'm with you. It is a 240 volt two wire secondary if you don't connect anything to the center tap.

And I do not agree. Again, I think you are inventing a different classification for conductors that do not exit the transformer enclosure with no basis in the code for doing such.

 

[jaggedben]

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.