Matching Transformer Primary Conductors to Primary OCPD

[Strathead]

The secondary voltage of the transformer isn't 120V. It's 208V. It's a 480-208/120V transformer.

I would dispute this statement Each coil is a transformer. The coil ratio is 4 to 1 which is 480 volts to 120 volts.

 

[wwhitney]

And strangely enough, the (3) single phase transformers connected together in a delta-wye, with the same secondary rated current, will have (approximately) the same minimum required conductor ampacity of 67A. Imagine that.

Yet for a given a primary size OCPD ratio (125% or 250%, depending on your choice with respect to Table 450.3(B)), the primary OCPD size will go up by a factor of sqrt(3) with the delta-wye configuration. I see zero reason to expect the minimum secondary conductor size to remain unchanged with the primary OCPD size increases.

This is where we differ--I expect an equal level of protection of the secondary conductors, which requires the minimum secondary conductor size to scale with the primary rated current or OCPD size. You seem to expect it to scale with the secondary rated current.

Cheers, Wayne

 

[david luchini]

I think this question from post #67 sums up our difference of opinion quite simply: "Why do you think that we should be allowed to have a larger primary OCPD, without increasing the minimum secondary conductor size?"

I don't know, maybe because that's literally what the Code says.

Also, the primary OCPD size increase because the primary rated current of the transformer increased. In both cases the primary OCPD remains at 250% of the primary rated current.

 

[david luchini]

I would dispute this statement Each coil is a transformer. The coil ratio is 4 to 1 which is 480 volts to 120 volts.

See post #61

 

[wwhitney]

I don't know, maybe because that's literally what the Code says.

This comment is content-free, as whether the code says that or not is based on how you interpret the ratio of 480V : 208Y/120V, as 480:208 or 480:120. Which is what we are debating.

See post #61

I'm only seeing that language in 450.3, so that would not to apply to 240.21. Is there some broader language elsewhere?

Cheers, Wayne

 

[david luchini]

This comment is content-free,

Read the rest of the comment.

 

[wwhitney]

Also, the primary OCPD size increase because the primary rated current of the transformer increased. In both cases the primary OCPD remains at 250% of the primary rated current.

I see you added that after I responded. I agree that is why the primary OCPD size increased. I disagree that means we get a pass on accordingly adjusting the minimum secondary conductor size.

When a conductor doesn't have an OCPD sized to 240.4 at its source of supply, we should expect the minimum conductor size to go up with increasing size of the upstream OCPD that is partially protecting it. Pretty simple.

Cheers, Wayne

 

[david luchini]

I see you added that after I responded. I agree that is why the primary OCPD size increased. I disagree that means we get a pass on accordingly adjusting the minimum secondary conductor size.

When a conductor doesn't have an OCPD sized to 240.4 at its source of supply, we should expect the minimum conductor size to go up with increasing size of the upstream OCPD that is partially protecting it.

Cheers, Wayne

And when transformer have the same rated secondary current, and each are protected at the max allowable OCPD, we should expect the required minimum conductor ampacity on the secondary to be the same.

 

[wwhitney]

And when transformer have the same rated secondary current, and each are protected at the max allowable OCPD, we should expect the required minimum conductor ampacity on the secondary to be the same.

Both expectations can not be met at the same time, since as we've seen, comparing a delta-wye transformer topology to a single phase topology means the primary rated current and primary OCPD goes up by a factor of sqrt(3), while the secondary rated current is unchanged.

The expectation I stated is about safety and protecting the conductor. The expectation you stated is just about convenience for the designer, as far as I can see. So as they are in opposition, safety takes precedence.

Cheers, Wayne

 

[david luchini]

And of course there's the fact that pretty much

The expectation I stated is about safety and protecting the conductor. The expectation you stated is just about convenience for the designer, as far as I can see. So as they are in opposition, safety takes precedence.

Cheers, Wayne

The 2.3:1 transformer ratio provides safety and protects the conductor per the NEC requirements. It has nothing to do with convenience.

Again, the fact that you have to come up with the most ridiculous scenarios to prove your point basically disproves your point. But I get it. You like to be contrary.

 

[wwhitney]

Again, the fact that you have to come up with the most ridiculous scenarios to prove your point basically disproves your point.

There is nothing unusual about delta-wye transformers, which have been the primary focus of the discussion.

Cheers, Wayne

 

[david luchini]

There is nothing unusual about delta-wye transformers, which have been the primary focus of the discussion.

Cheers, Wayne

Okay. How unusual is a 240V : 24V/240V single phase transformer? How unusual is needing a transformer for 100A, 120V load and using a 25kVA transformer (208A rated secondary) instead of a 15kVA transformer (125A rated secondary) because using the smaller transformer doesn't fit with your argument? Seems unusual to me.

How unusual is thinking that the Code would never allow 500A on a 100A secondary conductor before the primary OCPD sees it's rated current, but thinking that 1000A on a 100A tap conductor before the upstream OCPD sees it's rated current is no problem. Seems unusual to me.

 

[david luchini]

 

[wwhitney]

Okay. How unusual is a 240V : 24V/240V single phase transformer?

Pretty unusual, as I stated in the example.

How unusual is needing a transformer for 100A, 120V load and using a 25kVA transformer (208A rated secondary) instead of a 15kVA transformer (125A rated secondary) because using the smaller transformer doesn't fit with your argument? Seems unusual to me.

That example spoke about leaving room for extra expansion, and you could have multiple runs of secondary conductors.

How unusual is thinking that the Code would never allow 500A on a 100A secondary conductor before the primary OCPD sees it's rated current, but thinking that 1000A on a 100A tap conductor before the upstream OCPD sees it's rated current is no problem. Seems unusual to me.

That's comparing your interpretation of a 25' tap rule to a 10' tap rule. Apples to oranges.

Perhaps you'd care to comment on the substance of my argument, rather than the rhetoric.

Cheers, Wayne

 

[wwhitney]

[Schematic of 37.5 kVA 480D : 208Y/120V transformer and OCPD/conductor sizing under 240.21(B)(3).]

That image is clearly on point for this discussion, what's the source?

The image certainly supports the idea that your interpretation is industry standard. Which I don't believe I've expressed an opinion on; my position has simply been that this interpretation, industry standard or not, does not provide the same level of protection as 240.21(B)(2), which makes it a poor interpretation, in my opinion.

This image does bring to mind another way to illustrate the problem I see. Let's replace the transformer with a 240D : 415Y/240V transformer, also 37.5 kVA. That's a little exotic, but something you might find in a data center that was converted from a prior use which already had an adequately sized 240V delta service.

Then the rated primary current is 90A, and the rated secondary current is 52A. So we could use a 225A primary OCPD, 90A primary conductors, and 60A secondary conductors to a 60A main breaker panelboard. If the primary-to-secondary voltage ratio in 240.21(B)(3) is 240/415, that complies: 225 * (240/415) / 3 = 43.4A.

Now consider the case that the secondary panelboard just supplies L-N 2-wire loads, and only loads on one phase are powered on (the others are off). As a thought experiment, we could just delete the transformer and refeed that particular L-N 2-wire circuit from two of the formerly primary L-L conductors. If we do that, none of the currents in the system change at all (lots of implementation details change, as the circuit no longer has a grounded conductor). And if the tap rules are consistent, the new arrangement should satisfy 240.21(B)(2). But it doesn't: 225/3 = 75A, while we have only 60A conductors.

This discrepancy arises because it is more appropriate to use 240:240 as the primary-to-secondary voltage ratio on this 240D : 415Y/240V transformer. That makes 240.21(B)(3) exactly equivalent to 240.21(B)(2).

Cheers, Wayne

 

[milmat1]

If I have both primary and secondary protection, I can go up to 250% on the primary to avoid a trip when I energize the transformer.

Why should I have conductors that exceed the FLA of the transformer by matching this 250% OCPD?

I think everyone is over complicating this. The simple question he asked was about the conductors needing to be oversized for the FLA of the XFMR.
And my thought is because the OCPD is always to protect conductors. So the conductors couldn't be smaller the the OCPD.

 

[electrofelon]

I think everyone is over complicating this. The simple question he asked was about the conductors needing to be oversized for the FLA of the XFMR.
And my thought is because the OCPD is always to protect conductors. So the conductors couldn't be smaller the the OCPD.

But we are allowed a larger OCPD for motors, to accommodate for inrush, transformers also have inrush issues so why doesn't the code allow a similar thing for transformers?

 

[Jpflex]

Why should I have conductors that exceed the FLA of the transformer by matching this 250% OCPD?

Inrush current upon initial start before counter electromotive force can limit current is the reason primary transforner OCPD is sized higher than thransforner primary running current

 

[topgone]

Inrush current upon initial start before counter electromotive force can limit current is the reason primary transforner OCPD is sized higher than thransforner primary running current

OCPDs have lots of settings. One takes care of overload protection of the equipment, another setting for starting/ inrush currents, and another for protecting the equipment from short circuits, unintentional grounding, etc. For the common overload, the code specifies 125% of the full-load amps.

 

[Jim_SWFL]

So the conductors couldn't be smaller the the OCPD

This isn't always true though. 240.4(B) often allows the OCPD to exceed the conductor ratings in table 350.16 (2020 NEC), though not by a lot.

Motor circuits allow for inrush without requiring the conductors to be sized for it.

Inrush current upon initial start before counter electromotive force can limit current is the reason primary transforner OCPD is sized higher than thransforner primary running current

I understand, I am asking why the conductors need to match the OCPD (sized for inrush) instead of the FLA, when, in the example I provided, there is primary and secondary protection to ensure that an overload condition won't jeopardize the conductors. An overload condition can only occur based on current flow in the secondary circuit. As long as that is limited by an OCPD on the secondary, the transformer and all conductors should be adequately protected.

An extreme event like a short circuit or ground fault should be arrested by one of the circuit protective devices depending on the location of the fault.

OCPDs have lots of settings. One takes care of overload protection of the equipment, another setting for starting/ inrush currents, and another for protecting the equipment from short circuits, unintentional grounding, etc. For the common overload, the code specifies 125% of the full-load amps.

I think I posted that I had decided to use a primary OCPD with adjustable settings so I could keep the breaker closer to the FLA of the transformer.

It is wasteful to size conductors for something that happens rarely and lasts for .1 seconds.

My opinion is that the OCPD contemplated in article 450 are to protect the transformer, not the conductors, and that it is the responsibility of the circuit designer to make sure the conductors have adequate protection from an overload, with the direction given in article 240.21(B) and (C).

In the 2017 Handbook, exhibit 240.9 shows a 110A OCPD, feeding a set of conductors rated at 50A on the primary side. The secondary side of this transformer has protection not exceeding 125% of the transformer current rating, so the primary transformer protection is permitted to be no more than 250%. The 110A fuse shown is 244% of the FLA of 45A.

This would seem to indicate that the primary conductors can have an ampacity lower than the primary protection.