Oversight in NEC 240.21(B)(3)?

[wwhitney]

240.21(B)(3) is the 25' tap rule for transformers, which lets the conductors landed on the primary transformer terminals be taps. To paraphrase the 5 requirements listed, they are:

(1) The primary tap conductors must be at least 1/3 the ampacity of the OCPD protecting them from SCGF.
(2) Likewise for the secondary conductors, after you adjust for the transformer current ratio.
(3) One primary conductor plus one secondary conductor must add up to 25' or less.
(4) The primary and secondary conductors must be protected from physical damage.
(5) The secondary conductors terminate in a single OCPD that provides overload protection for them.

Missing from this formulation is any requirement that the secondary OCPD provide overload protection for the primary tap conductors, which seems like a clear oversight.

In other words, what section would the following install violate:

-25 kVA 240V : 240V 2-wire : 2-wire isolation transformer supplying 12 kVA of load
-Primary OCPD is 125A
-Primary conductors are 10' long, 50A ampacity
-Secondary conductors are 10' long, 100A ampacity
-Secondary OCPD is 100A

Cheers, Wayne

 

[Elect117]

I think the primary OCPD would still need to be within the limits of Table 450.3(B) and you would still need 450.14 (disconnect) which is probably why this would rarely be done without a fused disconnect on the primary side at the transformer, but...

25000/240 = 10.4A *2.5 = 26A (can't round past 250%) so 25A OCPD required on the primary.

Arguably, with the 2 wire transformer you might not be required to have secondary protection, and then that could be a fun little thought experiment. But you would still need to figure out what load is being fed that doesn't require an OCPD at it's rating.

 

[wwhitney]

I think the primary OCPD would still need to be within the limits of Table 450.3(B)

It is.

and you would still need 450.14 (disconnect)

The 125A primary OCPD is 10' away.

25000/240 = 10.4A *2.5 = 26A (can't round past 250%) so 25A OCPD required on the primary.

You dropped a zero.

Arguably, with the 2 wire transformer you might not be required to have secondary protection

True, although moot, as we have a feeder tap on the primary side, so the installation needs to comply with one of the 240.21(B) rules.

But you would still need to figure out what load is being fed that doesn't require an OCPD at it's rating.

To make the configuration in my example plausible, let's say we have some European loads that require 240V L-N for whatever reason, hence the isolation transformer to allow us to change the ground reference of our 240V 2-wire circuit. And the 100A secondary OCPD is the main breaker of a panel supplying branch circuits for those loads.

Cheers, Wayne

 

[wwhitney]

To make this thread succinct, the question is:

Why doesn't 240.21(B)(3) require that the secondary OCPD provides overload protection for the primary side tap conductors? Is this an oversight?

Cheers, Wayne

 

[Elect117]

You dropped a zero.

lol gotta love a calculator mistake

 

[Elect117]

To make this thread succinct, the question is:

Why doesn't 240.21(B)(3) require that the secondary OCPD provides overload protection for the primary side tap conductors? Is this an oversight?

Cheers, Wayne

I think it is an intentional decision.

The handbook provides a example that is very similar to yours.

They use #8 with a 110A OCPD on the primary of the transformer. #3 with a 100A OCPD on the secondary. 480- 120/208 XFMR.

So it would appear your interpretation is right and that is their intent.

The conductors on the primary side of the transformer would most likely only be protected against a fault.

They have a note that says "See also 240.21(C)(6) if the primary conductors are protected in accordance with their ampacity." And 240.21(C)(6) is practically identical.


My guess is that the theory is that the secondary of the transformer is adequately protected then the likelihood of the primary ever experiencing overload is null or unlikely. It would have to create enough heat on the secondary side (load side) and not trip that protection and also not get dissipated in the windings at all.


And from my experience, I only ever see the secondary side of a transformer with overheated terminals. I don't think I have found a primary side with melting bushing or melting terminations.

 

[tortuga]

-Secondary OCPD is 100A

Cheers, Wayne

I believe your secondary OCPD would have to be 70A not 100A. 450.3(B) 125% of 50A = 62.5 , then next standard size = 70A.

 

[wwhitney]

I believe your secondary OCPD would have to be 70A not 100A. 450.3(B) 125% of 50A = 62.5 , then next standard size = 70A.

Huh? 25 kVA 240V : 240V, so the rated load current is 104A on both the primary and secondary.

Cheers, Wayne

 

[Tulsa Electrician]

To make this thread succinct, the question is:

Why doesn't 240.21(B)(3) require that the secondary OCPD provides overload protection for the primary side tap conductors? Is this an oversight?

Cheers, Wayne

Read thru a couple times.
The way I see it the it does. How would there be an overload between the secondary OCPD and the primary OCPD if the second OCPD is operating correctly.
Any issue between the two effecting current beyond load va and secondary OCPD rating on the tapped primary conductor would be a fault.

The transformer kva does not exceed the tap conductors ampacity. If it did than I would think there could be an issue.
25000/480=52.083 amps.

Edit:
If anything adding verbage, it would reflect kva to tap conductors ampacity.
The tap conductors on the primary must equal or exceed KVA rating in ampacity unless the tap conductor has protection at its rating in ampacity.

 

[wwhitney]

I think it is an intentional decision.

The handbook provides a example that is very similar to yours.

They use #8 with a 110A OCPD on the primary of the transformer. #3 with a 100A OCPD on the secondary. 480- 120/208 XFMR.

So it would appear your interpretation is right and that is their intent.

Not following why you think this is similar to my example.

If the 208Y/120V side of the transformer is loaded to 100A on each line conductor, then the primary line current will be 100A * 208/480 = 43A. And #8 on the primary has an ampacity of 50A. So the 100A secondary OCPD is providing overload protection for the primary side #8.

My question is what is the violation if we change that #8 to a #10, with a 30A ampacity? I don't see it as violating 240.21(B)(3), as 240.21(B)(3)(5) only speaks to the secondary OCPD providing overload protection for the secondary conductors. It has no language about doing the 100A * 208/480 computation I just did to check if the primary tap conductors are protected against overload.

And to avoid a 215.2 violation, I'll specify that the transformer has a calculated load that draws less than 30A on the primary.

Cheers, Wayne

 

[Pinnie]

To make this thread succinct, the question is:

Why doesn't 240.21(B)(3) require that the secondary OCPD provides overload protection for the primary side tap conductors? Is this an oversight?

Cheers, Wayne

This was trade hog ais response.

“The reason 240.21(B)(3) does not require the secondary OCPD to provide overload protection for the primary side tap conductors is that the primary side OCPD is intended to protect the primary conductors and the transformer itself. The secondary conductors are protected by the conditions outlined in 240.21(B)(3), which include limitations on the length of the secondary conductors, the location of the secondary OCPD, and the ampacity of the secondary conductors relative to the load.
This is not an oversight but a deliberate allowance in the NEC to provide flexibility in certain transformer applications. The NEC provides these allowances because the primary OCPD, in conjunction with the specific installation conditions, can adequately protect the conductors from overcurrent conditions. The conditions specified in 240.21(B)(3) ensure that the risk of overcurrent on the secondary conductors is minimized, even without direct overload protection from a secondary OCPD.
For example, one of the conditions is that the secondary conductors must terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the conductors. This ensures that the secondary conductors are not overloaded, even though they are not directly protected by an OCPD on the primary side.
In summary, NEC 240.21(B)(3) provides a structured approach to protecting transformer secondary conductors without requiring direct overload protection from a secondary OCPD, as long as specific conditions are met to ensure safety.”

 

[Elect117]

Not following why you think this is similar to my example.

If the 208Y/120V side of the transformer is loaded to 100A on each line conductor, then the primary line current will be 100A * 208/480 = 43A. And #8 on the primary has an ampacity of 50A. So the 100A secondary OCPD is providing overload protection for the primary side #8.

My question is what is the violation if we change that #8 to a #10, with a 30A ampacity? I don't see it as violating 240.21(B)(3), as 240.21(B)(3)(5) only speaks to the secondary OCPD providing overload protection for the secondary conductors. It has no language about doing the 100A * 208/480 computation I just did to check if the primary tap conductors are protected against overload.

And to avoid a 215.2 violation, I'll specify that the transformer has a calculated load that draws less than 30A on the primary.

Cheers, Wayne

I guess the example isn't similar. I thought it was similar in that their interpretation lines up with yours.

They aren't concerned with the primary conductors' protection against overload so long as the ratio holds.

In your scenario the primary conductor is arguably compromised but the code doesn't care about it. I agree with you. Your scenario is legal as far as I can tell.

 

[tortuga]

Huh? 25 kVA 240V : 240V, so the rated load current is 104A on both the primary and secondary.

Cheers, Wayne

Woops I read too fast and thought the

12 kVA of load

was a 12kva transformer.

-Primary OCPD is 125A

The way I understand it is if your tapping a feeder to a transformer using the tap rule for transformers in 240.21(B)(3) then you would not have a primary OCPD as thats what the rule is allowing you to omit, just a secondary OCPD. A tap ends at any single OCPD so if you have a primary OCPD its just a different kind of tap.

 

[wwhitney]

The way I understand it is if your tapping a feeder to a transformer using the tap rule for transformers in 240.21(B)(3) then you would not have a primary OCPD

Unless the transformer is supplied by a service, there will always be a primary side OCPD. 240.21(B)(3) just lets you size a portion of the primary side conductors smaller that the minimum size ordinarily allowed to be protected by that primary side OCPD.

Hence in my example, the primary side OCPD is 125A, but the primary side conductors only have 50A amapcity.

Cheers, Wayne

 

[wwhitney]

Your scenario is legal as far as I can tell.

Do you agree it shouldn't be, and 240.21(B)(3) should have an additional requirement to do the computation for minimum ampacity primary side tap conductor, which was 100A * 208/408 in your example? If so, I'll write up a proposed 2029 PI.

Cheers, Wayne

 

[tortuga]

Unless the transformer is supplied by a service, there will always be a primary side OCPD. 240.21(B)(3) just lets you size a portion of the primary side conductors smaller that the minimum size ordinarily allowed to be protected by that primary side OCPD.

Hence in my example, the primary side OCPD is 125A, but the primary side conductors only have 50A amapcity.

Cheers, Wayne

Is this what your describing in your example??

 

[wwhitney]

Is this what your describing in your example?

Delete the "to other loads", those aren't required and just muddy the situation. Make the #6 conductor a #8 75C conductor, so its ampacity matches the 50A I specified. The #8 can originate from the 125A OCPD if you like. Add the 100A secondary OCPD. And specify that the #8 is 10', and the #3 from the transformer to the secondary OCPD is 10' long. Then yes.

Also, your computation in the "1/3 of feeder size" box is wrong. 25' tap rules are based on the ampacity--the ampacity of the of the tap conductor has to be at least 1/3 of the OCPD providing SC/GF protection. Or for motor conductors under 430.28, 1/3 the ampacity of the feeder conductor being tapped (since the OCPD maybe greater than that conductor's ampacity).

Cheers, Wayne

 

[Tulsa Electrician]

I missed the 1 to 1. Thought it was 480 to 240,Sorry.
I see what you saying now.

 

[Tulsa Electrician]

deleted

 

[Elect117]

Do you agree it shouldn't be, and 240.21(B)(3) should have an additional requirement to do the computation for minimum ampacity primary side tap conductor, which was 100A * 208/408 in your example? If so, I'll write up a proposed 2029 PI.

Cheers, Wayne

I do agree.

I genuinely can not think of a reason why the side connected by #8 would not end up with heat damaged lugs / terminations on the the windings or at the point of their tap.

Because in this case where the transformer is 1 to 1, theoretically it is the same risk to have the #8 on the secondary side protected by 100A OCPD which the code already addresses and does not allow.

Is it only in the 1 to 1 ratio that this could happen?

Could we say a 100A primary with #8 primary side tap. 480:240 - ration is 2:1, so 30%*100 OCPD, so the minimum on the secondary side is 60A? We could have 3/0 off the transformer no? Woudn't that put us in a similar situation?

200/2 = 100A possibly seen on the primary side? So any thing between 50A and 100A wouldn't trip the primary side OCPD.