Matching Transformer Primary Conductors to Primary OCPD

[david luchini]

But I'll repeat from post #10 that if you have a 480D : 208Y/120V transformer, with a 100A 120V L-N load on the secondary, the primary current will be 25A on two of the three line conductors. That's a 4:1 ratio.

The Code doesn't say anything about primary to secondary current ratio.

The primary to secondary voltage ratio of a 480D : 208Y/120V transformer is 480:208. That's 2.3:1

 

[wwhitney]

The Code doesn't say anything about primary to secondary current ratio.

No, but if you think about what's going on in 240.21 for two seconds, it's clearly about current ratios, even though the wording is about voltage ratios. With transformers, the two are inverses of each other.

The primary to secondary voltage ratio of a 480D : 208Y/120V transformer is 480:208. That's 2.3:1

If the wording in 240.21 were "primary circuit to secondary circuit voltage ratio," then per Charlie's Rule and the Article 100 definition I would surrender the point to you, and I would just be commenting that the NEC is wrong here and the ratio for this application should be 4.0:1.

As there are multiple definitions of "voltage" in Article 100, it is equally reasonable to say that the voltage of the secondary is 208Y/120V. And so my comment is that the responsible code user should use the lower secondary voltage of 120V in this application.

Seems like at this point we can agree to disagree.

Cheers, Wayne

 

[jaggedben]

I think the practice of using L-L voltages satisfies the vague code wording, and the requirement is essentially arbitrary anyway. But it does seem wrong to me that if I connect a single phase 4:1 coil (no center tap) that I need a bigger conductor size than if I connect three identical coils to the same primary voltage via a common neutral point.

 

[david luchini]

If the wording in 240.21 were "primary circuit to secondary circuit voltage ratio," then per Charlie's Rule and the Article 100 definition I would surrender the point to you, and I would just be commenting that the NEC is wrong here and the ratio for this application should be 4.0:1.

Adding the word "circuit" does nothing to change the meaning. The NEC isn't wrong here. It's made it's rules about not protecting conductors at the point the they receive their supply. We need only follow their rules, not correct their rules.

(If we were going to correct their rules, the correction would be to remove the allowance of tap conductors or transformer secondary conductors...require all conductors to be protected at their ampacity at the point they receive their supply, and problem solved.)

 

[wwhitney]

Adding the word "circuit" does nothing to change the meaning.

Sure it does. The current wording is plausibly interpretable by either of the following two definitions from Article 100 (2017 NEC for ease of copying). Adding the word "circuit" forces us to use the first definition:

Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.

Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts).

The NEC isn't wrong here. It's made it's rules about not protecting conductors at the point the they receive their supply.

While I agree that the 3:1 ratio in the 25' tap rule is somewhat arbitrary, a reasonable requirement is that once that choice is made, it be applied consistently within the various 25' tap rules. And jaggedben succinctly summarized what is wrong with interpreting the primary-to-secondary voltage ratio of a 480D : 208Y/120V transformer to be 2.3:1 in this context, rather than 4.0:1:

But it does seem wrong to me that if I connect a single phase 4:1 coil (no center tap) that I need a bigger conductor size than if I connect three identical coils to the same primary voltage via a common neutral point.

The presence of those two other single phase transformers in a 480D : 208Y/120V arrangement has zero impact on how well the conductors on one transformer are protected. Saying that if we add transformers connected to only one of our secondary conductors we get to shrink the size of those secondary conductors makes zero sense.

Cheers, Wayne

 

[wwhitney]

(If we were going to correct their rules, the correction would be to remove the allowance of tap conductors or transformer secondary conductors...require all conductors to be protected at their ampacity at the point they receive their supply, and problem solved.)

You can make that proposal if you like.

But that isn't possible in the case of transformers, there would still be secondary conductors from the transformer terminals to the first OCPD. And if you're going to disallow all taps, then you still need to decide if on a 480D : 208Y/120V transformer, those conductors need to have 2.3 times the ampacity of the primary OCPD, or 4.0 times the ampacity of the primary OCPD.

Cheers, Wayne

 

[jaggedben]

I think it's reasonable on a legalistic level to interpret the existing wording as referring to circuit voltage. I can't blame anyone for understanding it that way. It certainly doesn't make sense to use any of the other Article 100 definitions.

 

[david luchini]

You can make that proposal if you like.

No thanks. The Code is fine the way it is written. It's been fine for decades, no reason to change it now.

 

[wwhitney]

I think it's reasonable on a legalistic level to interpret the existing wording as referring to circuit voltage. I can't blame anyone for understanding it that way.

I don't disagree. It's reasonable given the wording, just not wise.

It certainly doesn't make sense to use any of the other Article 100 definitions.

I obviously disagree on this point. It makes sense to consider the voltage ratio for the case under discussion to be 480V to 208Y/120V. That is after all, how we always describe the transformer.

Which then poses the question, how do you convert that to a single number, should it be 480/208, or should it be 480/120? Which is precisely the point, this situation requires some thought. And if you give it the necessary thought, you see that 480/120 makes more sense in this context.

Cheers, Wayne

 

[wwhitney]

But it does seem wrong to me that if I connect a single phase 4:1 coil (no center tap) that I need a bigger conductor size than if I connect three identical coils to the same primary voltage via a common neutral point.

I illustrated your comment below. If we have three identical single phase transformers, and we just install one of them, labeled P1:S1 in the diagram below, we just get the red circuit (plus the P1 and S1 coils, too hard for me to recolor). The minimum size of the red secondary conductors for 240.21(B)(3) is clearly based on the turns ratio.

Now we add the P2:S2 and P3:S3 transformers. That means the red secondary conductors can now be made smaller, as the L-L secondary voltage is larger than the L-N secondary voltage? Complete nonsense.

Cheers, Wayne

 

[david luchini]

I illustrated your comment below. If we have three identical single phase transformers, and we just install one of them, labeled P1:S1 in the diagram below, we just get the red circuit (plus the P1 and S1 coils, too hard for me to recolor). The minimum size of the red secondary conductors for 240.21(B)(3) is clearly based on the turns ratio.

Now we add the P2:S2 and P3:S3 transformers. That means the red secondary conductors can now be made smaller, as the L-L secondary voltage is larger than the L-N secondary voltage? Complete nonsense.

You just proved his point for him. Well done. (He isn't saying the conductor would be smaller, you are are saying the conductor needs to be larger.)

 

[wwhitney]

(He isn't saying the conductor would be smaller, you are are saying the conductor needs to be larger.)

You have misinterpreted the math.

If the secondary voltage increases, the primary to secondary voltage ratio decreases, meaning the minimum allowable secondary conductor size under 240.21(B)(3) goes down. Which is nonsense.

Perhaps this misunderstanding underlies your comments in this thread?

Cheers, Wayne

 

[david luchini]

You have misinterpreted the math.

Lol....No I haven't. You have misinterpreted the words.

I think the practice of using L-L voltages satisfies the vague code wording, and the requirement is essentially arbitrary anyway. But it does seem wrong to me that if I connect a single phase 4:1 coil (no center tap) that I need a bigger conductor size than if I connect three identical coils to the same primary voltage via a common neutral point.

Now we add the P2:S2 and P3:S3 transformers. That means the red secondary conductors can now be made smaller, as the L-L secondary voltage is larger than the L-N secondary voltage? Complete nonsense.

You literally didn't understand what jaggedben wrote. And then you provided him correct.

 

[wwhitney]

You literally didn't understand what jaggedben wrote. And then you provided him correct.

You're misreading. Y > X is the same as X < Y.

Let me spell out what I understand jaggedben meant in post #43, and what I meant in post #50. The diagram is repeated below for convenience.

Let's say we have (3) identical 480V : 120V single phase transformers, and I need 100A @ 120V. My primary OCPD is 125A, and the primary plus secondary conductors are under 25' in length so I can use 240.21(B)(3).

So I install just the red conductors in the diagram along with just one transformer, P1:S1. I use 50A primary conductors (50 > 125/3), and I use 100A secondary conductors, as that's the load I need to supply. The inspector says "wait, that violates 240.21(B)(3), as 125A (primary OCPD) * 480V/120V / 3 = 167A, so your conductors need to be a minimum 167A ampacity."

To which if I am to follow your interpration, I could respond "OK, we'll need a full delta-wye configuration in the future anyway, so I'll just install that now." I add the black conductors, and the two transformers P2:S2 and P3:S3. Then I say, "Now the computation is 125A (primary OCPD) * 480V/208V / 3 = 96A, so my 100A conductors comply with 240.21(B)(3)."

That's a nonsensical result, as these extra transformers and conductors make zero difference in what happens with the red circuit. And that's what jaggedben was saying in post #43 seems "wrong" with the interpretation that the primary to secondary voltage ratio for a 480D : 208Y/120V transformer is 2.3.

Cheers, Wayne

 

[jaggedben]

For the record, Wayne understood my words correctly. I agree with him on the finer points. I just don't believe that it's an important life safety issue that requires revising what is evidently standard industry interpretation of this code section. I agree with what david said in post #48.

My example was a thought experiment. Fortunately for people who want to get 120V from a single 480V phase, they can use the common 120/240 split phase secondary. And then my common industry practice they can have conductors half the ampacity of what would be required under Wayne's interpretation. ;-)

 

[david luchini]

For the record, Wayne understood my words correctly. I agree with him on the finer points. I just don't believe that it's an important life safety issue that requires revising what is evidently standard industry interpretation of this code section. I agree with what david said in post #48.

Where did you suggest that connecting three identical single phase 4:1 ratio transformers in a delta-wye would give you smaller secondary conductors than connecting one single phase 4:1 ratio transformer. I don't see that in your posts.

 

[david luchini]

Let's say we have (3) identical 480V : 120V single phase transformers, and I need 100A @ 120V. My primary OCPD is 125A, and the primary plus secondary conductors are under 25' in length so I can use 240.21(B)(3).

So I install just the red conductors in the diagram along with just one transformer, P1:S1. I use 50A primary conductors (50 > 125/3), and I use 100A secondary conductors, as that's the load I need to supply. The inspector says "wait, that violates 240.21(B)(3), as 125A (primary OCPD) * 480V/120V / 3 = 167A, so your conductors need to be a minimum 167A ampacity."

To which if I am to follow your interpration, I could respond "OK, we'll need a full delta-wye configuration in the future anyway, so I'll just install that now." I add the black conductors, and the two transformers P2:S2 and P3:S3. Then I say, "Now the computation is 125A (primary OCPD) * 480V/208V / 3 = 96A, so my 100A conductors comply with 240.21(B)(3)."

I think if I'm the inspector, I am going to say..."You can't protect a 15kVA, 480-120V, single phase transformer with a 125A primary OCPD."

 

[wwhitney]

I think if I'm the inspector, I am going to say..."You can't protect a 15kVA, 480-120V, single phase transformer with a 125A primary OCPD."

Notice I never specified the transformer size, as it's immaterial to this question. We can make it 25 kVA if you like.

Cheers, Wayne

 

[jaggedben]

Where did you suggest that connecting three identical single phase 4:1 ratio transformers in a delta-wye would give you smaller secondary conductors than connecting one single phase 4:1 ratio transformer. I don't see that in your posts.

You quoted it all in post #53. I said the single phase secondary conductors were required to be bigger than the wye secondary conductors. Wayne said the wye secondary conductors were allowed to be smaller. Same thing.

 

[jaggedben]

I think if I'm the inspector, I am going to say..."You can't protect a 15kVA, 480-120V, single phase transformer with a 125A primary OCPD."

We're discussing the minimum ampacity requirements for secondary conductors. That's all.