Matching Transformer Primary Conductors to Primary OCPD

[Jim_SWFL]

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?

 

[augie47]

The overcurrent device protects the conductors (also) so they need to be sized to match the OCP.

 

[Jim_SWFL]

The overcurrent device protects the conductors (also) so they need to be sized to match the OCP.

How does this explanation fit a motor circuit where the OCPD is sized from 430.52? In that case, the conductors would be sized for 125% of FLC, but the overcurrent could be 250% of FLC. I believe in that case, we are relying on the thermal overload at the motor to protect the conductors from anything other than a short or ground fault.

Wouldn't the secondary OCPD prevent overload on the primary conductors in a similar way?

Either way, I don't know of a code-based justification to size the primary conductors smaller than the OCPD. And, since there isn't, I am wondering whether there is something in this scenario that I am overlooking?

 

[wwhitney]

Wouldn't the secondary OCPD prevent overload on the primary conductors in a similar way?

Either way, I don't know of a code-based justification to size the primary conductors smaller than the OCPD. And, since there isn't, I am wondering whether there is something in this scenario that I am overlooking?

See 240.21(B)(3) for a limited allowance in this regard.

Note that when 240.21 uses the phrase "primary-to-secondary voltage ratio" my understanding based on the physics is that it means (or should mean) the transformer turns ratio. So for the case of a delta-wye transformer, or any secondary with a center-tapped coil, that would be the ratio of the primary L-L voltage to the secondary L-N voltage.

Cheers, Wayne

 

[david luchini]

Note that when 240.21 uses the phrase "primary-to-secondary voltage ratio" my understanding based on the physics is that it means (or should mean) the transformer turns ratio. So for the case of a delta-wye transformer, or any secondary with a center-tapped coil, that would be the ratio of the primary L-L voltage to the secondary L-N voltage.

That's not correct. The "Primary-to-secondary voltage ratio" is the ratio of the Primary L-L voltage to the Secondary L-L voltage.

 

[electrofelon]

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?

The overcurrent device protects the conductors (also) so they need to be sized to match the OCP.

There is the provision in 240.21(B)(3) which would allow for taps / reduced size conductors on the primary side of a transformer.

Edit:. Sorry, didn't catch Wayne had already mentioned that.

 

[electrofelon]

Note that when 240.21 uses the phrase "primary-to-secondary voltage ratio" my understanding based on the physics is that it means (or should mean) the transformer turns ratio. So for the case of a delta-wye transformer, or any secondary with a center-tapped coil, that would be the ratio of the primary L-L voltage to the secondary L-N voltage.

Cheers, Wayne

Wayne, can you explain why you believe that is the more logical computation?

 

[wwhitney]

That's not correct. The "Primary-to-secondary voltage ratio" is the ratio of the Primary L-L voltage to the Secondary L-L voltage.

The physics does not support that interpretation. The requirement is in regard to ampacity, so the value should work out to be the worst case current ratio.

As a simple example, consider a single phase 240V : 120/240V transformer. For an L-L secondary load, the ratio of secondary to primary current is 1:1. While for a L-N secondary load (only), the ratio of secondary to primary current is 2:1.

So if we interpret the phrase "primary-to-secondary voltage ratio" in this case to be 1:1, we would be allowing a conductor on the secondary to see 6 times its ampacity before the supply-side OCPD sees its rated current. There's no non-transformer version of the 25' tap rule that would allow this 6:1 ratio; the limit is 3:1.

Thus in interpreting the "primary-to-secondary voltage ratio" in 240.21, we should be use the worst case voltage ratio, which is not necessarily the L-L voltage ratio.

See https://forums.mikeholt.com/threads/transformer-tap-rule.2578067/ for a lengthy debate on this question.

Cheers, Wayne

 

[david luchini]

The physics does not support that interpretation. The requirement is in regard to ampacity, so the value should work out to be the worst case current ratio.

The physics say that if I have a 36kVA, 208V, 3ph load on the secondary of a 480-208/120V transformer, the secondary current will be 100A, and the primary current will be 43.3A. The Secondary-to-Primary current ratio of the transformer is 2.3. The Primary-to-Secondary voltage ratio of the transformer is 2.3.

 

[wwhitney]

The physics say that if I have a 36kVA, 208V, 3ph load on the secondary of a 480-208/120V transformer, the secondary current will be 100A, and the primary current will be 43.3A. The Secondary-to-Primary current ratio of the transformer is 2.3. The Primary-to-Secondary voltage ratio of the transformer is 2.3.

That's correct. But another way that the secondary conductors can see 100A is via a 12kVA ,120V, 1ph L-N secondary load. In which case the primary current will only be 25A, and the current ratio will be 4.0. Which is the ratio of the primary voltage 480V to the secondary voltage 120V.

Cheers, Wayne

 

[david luchini]

That's correct. But another way that the secondary conductors can see 100A is via a 12kVA ,120V, 1ph L-N secondary load. In which case the primary current will only be 25A, and the current ratio will be 4.0. Which is the ratio of the primary voltage 480V to the secondary voltage 120V.

Cheers, Wayne

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

 

[wwhitney]

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

Well, I'd say it's both, it's a multi-voltage secondary.

But more to the point, do you disagree with the computation that shows a 4.0 current ratio is possible? If so, let's discuss the physics.

If not, do you really think the intent of the 240.21(B)(3) 25' tap rule is to allow a 100A conductor to see 500A before the upstream OCPD see its rated current? That's what you'd get with a 125A primary side OCPD, which would be allowed if you interpret the voltage ratio to be 2.3, rather than 4.0.

Cheers, Wayne

 

[wwhitney]

To take this to an absurd level, I could get a custom wound 240V : 24V/240V single phase transformer, where the secondary has a tap 10% along the length of the secondary. [Or maybe I could use an off-the-shelf 240V:240V isolation transformer that has +5% and -5% taps, reverse fed so the taps are on the secondary, and connect to both taps.]

Then if we interpret that transformer as having a 1:1 primary-to-secondary voltage ratio, 240.21(B)(3) would allow secondary 100A conductors with a 300A primary-side OCPD. Yet 2 out of 3 of the 100A conductors could see 3000A without overloading the primary-side OCPD.

Clearly we need to interpret the phrase "primary-to-secondary voltage ratio" for the worst case.

Cheers, Wayne

 

[david luchini]

Well, I'd say it's both, it's a multi-voltage secondary.

But more to the point, do you disagree with the computation that shows a 4.0 current ratio is possible? If so, let's discuss the physics.

If not, do you really think the intent of the 240.21(B)(3) 25' tap rule is to allow a 100A conductor to see 500A before the upstream OCPD see its rated current? That's what you'd get with a 125A primary side OCPD, which would be allowed if you interpret the voltage ratio to be 2.3, rather than 4.0.

Cheers, Wayne

Did you miss 240.21(B)(3)(5)? How is the 100A conductor going to see 500A when it terminates in a 100A OCPD?

 

[wwhitney]

Did you miss 240.21(B)((3)(5)? How is the 100A conductor going to see 500A when it terminates in a 100A OCPD?

By a fault on the conductor between the transformer and the secondary OCPD. Which is a risk that all tap conductors are subject to, the are protected against load-end overload, but not against short circuit or ground fault.

My understanding of the ampacity limits of 3:1 on the 25' tap rules and 10:1 on the 10' tap rules is to limit that risk. The tap conductor is (at least partially) protected by the supply-side OCPD, but with a transformer, the primary side OCPD's trip curve is shifted by the current transformation ratio.

Heck, we can pair my absurd example with an available fault current of under 3000A, and now the primary OCPD will never clear.

Cheers, Wayne

 

[david luchini]

By a fault on the conductor between the transformer and the secondary OCPD. Which is a risk that all tap conductors are subject to, the are protected against load-end overload, but not against short circuit or ground fault.

My understanding of the ampacity limits of 3:1 on the 25' tap rules and 10:1 on the 10' tap rules is to limit that risk. The tap conductor is (at least partially) protected by the supply-side OCPD, but with a transformer, the primary side OCPD's trip curve is shifted by the current transformation ratio.

Heck, we can pair my absurd example with an available fault current of under 3000A, and now the primary OCPD will never clear.

Cheers, Wayne

Great. See 110.10.

 

[wwhitney]

Great. See 110.10.

OK, so the 1/3 ratio in 240.21(B)(2) is to provide a level of protection against "extensive damage to the electrical equipment of the circuit,"
as required by 110.10. Then if 240.21(B)(3) is to provide the same level of protection, the phrase "primary-to-secondary voltage ratio" needs to be interpreted as the worst case voltage ratio, per the physics demonstrated.

Cheers, Wayne

 

[david luchini]

Editing

Cheers, Wayne

What's the impedance of this 45kVA transformer that you're going to see a 500A fault on the secondary? 25% impedance?

 

[wwhitney]

What's the impedance of this 45kVA transformer that you're going to see a 500A fault on the secondary? 25% impedance?

If it's a bolted fault, then the total impedance would presumably be low enough to trip the primary OCPD. But because of the 5:1 ratio, exceeding the maximum 3:1 ratio in the other 25' tap rules, the opening time may be slower than with those rules, causing greater damage.

If the fault has a higher impedance, the difference could be more dramatic. The point is the whole trip curve protecting the tap conductor is shifted over by a factor of 5, rather than 3.

Cheers, Wayne

 

[wwhitney]

To reiterate post #17 is a simpler way: if the limits in 240.21(B)(3) are supposed to protect tap conductors as well as 240.21(B)(2) does, then the voltage ratio needs to be the worst case voltage ratio. That's a more succinct version of my original post.

Cheers, Wayne