XFMR protection 450

[don_resqcapt19]

I thought the OCPD on the secondary must be sized so that current in excess of the the transformers FLC x 125% will not be exceeded.

If the primary OCPD has a rating that is 125% or less than the primary rated current, there is no requirement any overload protection for the secondary winding.

I do understand the 240.21 rules. I think 240.4 (F) overrules 240.21 (C) (2-6).

How does 240.4(F) over rule the rules in 240.21(C)? 240.4(F) and 240.21(C)(1) state the conditions where the secondary conductors can be protected by the OCPD on the primary side of the transformer. The rules in 240.21(C)(2) -(6) tell us how to protect the secondary conductors when you can't use the provisions of 240.4(F) or 240.21(C)(1)

I just dont agree with the thought of putting a 125 amp OCPD on the secondary conductors of a transformer with a FLC of 36 amps.

In this case the transformer secondary winding is protected by the primary OCPD. As long as the secondary conductors have an ampacity of at least 125 amps, there is no code or safety issue with protecting them at 125 amps.

 

[tryinghard]

I thought the OCPD on the secondary must be sized so that current in excess of the the transformers FLC x 125% will not be exceeded. I do understand the 240.21 rules. I think 240.4 (F) overrules 240.21 (C) (2-6). I just dont agree with the thought of putting a 125 amp OCPD on the secondary conductors of a transformer with a FLC of 36 amps.

Keep in mind secondary does not always go to panelboards.

 

[yired29]

In this case the transformer secondary winding is protected by the primary OCPD. As long as the secondary conductors have an ampacity of at least 125 amps, there is no code or safety issue with protecting them at 125 amps

.

This is a 480volt primary 120/240 secondary.
1:2 ratio
20 amps primary 40 amp secondary
If we have over 40 amps on a balanced secondary the primary would trip.
With a neutral present we could have a line to neutral load
1:4 ratio
20 amp primary 80 amp secondary
It would take over 80 amps before primary would trip.

This means we would need to pull 80 amps through the secondary conductors and windings in order to trip the 20 amp OCPD on the primary side.
Thats 80 amps through windings that are sized for 36 amp FLC.
This is what 240.4 (F) is stating.
Does this help

 

[yired29]

This is why I believe OCPD on the secondary needs to be sized based on table 450.3 (B). And should in no case exceed 45 amps with this size transformer and voltage configuration. (delta 3 wire to delta 4 wire dual voltage transformer)

 

[don_resqcapt19]

.

This is a 480volt primary 120/240 secondary.
1:2 ratio
20 amps primary 40 amp secondary
If we have over 40 amps on a balanced secondary the primary would trip.
With a neutral present we could have a line to neutral load
1:4 ratio
20 amp primary 80 amp secondary
It would take over 80 amps before primary would trip.

This means we would need to pull 80 amps through the secondary conductors and windings in order to trip the 20 amp OCPD on the primary side.
Thats 80 amps through windings that are sized for 36 amp FLC.
This is what 240.4 (F) is stating.
Does this help

I fully understand how a transformer works. This debate is about what protection is required by the NEC for this transformer. The code does not require secondary overcurrent protection for this transformer. 240.4(F) does not say that the secondary of the transformer needs overcurrent protection.

 

[don_resqcapt19]

This is why I believe OCPD on the secondary needs to be sized based on table 450.3 (B). And should in no case exceed 45 amps with this size transformer and voltage configuration. (delta 3 wire to delta 4 wire dual voltage transformer)

The protection for this transformer is sized per Table 450.3(B) and that table does not require secondary overcurrent protection when the primary protection is sized at 125% of less of the rated primary current. Nothing is Article 240 changes this. In general, Article 240 only covers the overcurrent protection of conductors, not equipment.

240.3 Other Articles.
Equipment shall be protected against overcurrent in accordance with the article in this Code that covers the type of equipment specified in Table 240.3.

 

[yired29]

don_resqcapt19;1107633]The protection for this transformer is sized per Table 450.3(B) and that table does not require secondary overcurrent protection when the primary protection is sized at 125% of less of the rated primary current.

Earlier in this thread it was said you could land these secondary conductors on any size OCPD as long as they were sized to be equal to or greater than said OCPD. ( Not in those words exatly)

As I said in my last post if this is a multi volt secondary the line to neutral currents pose a problem for the primary OCPD to protect the secondary windings and conductors.

1:4 voltage ratio and 4:1 current ratio with the line to neutral load....
It would take 80 amps or greater on a line to neutral load to trip primary OCPD.

The secondary of this transformer is 15 kVA 120/240.
150000 / (240 x 1.73) = 36.12 FLC x 1.25 = 45.15
So at 36 amps at 240 volts we have 18 amps on the secondary.
The primary of this transformer is 15 kVA 480
15000 / (480 x 1.73) = 18.06

This is a 1:2 voltage ratio primary to secondary.
But this is a multivolt secondary
1:4 voltage ratio with 120 volt load

That means if there is a unbalance it would take more than the rated FLC of the secondary to trip the primary OCPD by 2 fold.

80 amp 120 volt load to trip the 20 primary OCPD
With anything larger than a 45 amp OCPD on the secondary the transformer is not properly protected.

The math does not lie
As long as my math is correct

Once again I am not trying to have a pissing contest bit this is the way I understand the rules...It's all about the math. That is what 240.4 (F) and 240.21 (C) (1) is stating.

The main difference is single voltage secondary or multi voltage secondary.

 

[yired29]

I fully understand how a transformer works. This debate is about what protection is required by the NEC for this transformer. The code does not require secondary overcurrent protection for this transformer. 240.4(F) does not say that the secondary of the transformer needs overcurrent protection.

I disagree no where does 240.4 (F) say that the primary OCPD can provide protection on a multi voltage secondary therefore it is necessary. It states the opposite only on single voltage secondary does the primary offer any protection.

 

[don_resqcapt19]

I disagree no where does 240.4 (F) say that the primary OCPD can provide protection on a multi voltage secondary therefore it is necessary. It states the opposite only on single voltage secondary does the primary offer any protection.

The point that you are missing is the fact that the rules in Article 240 have nothing to do with the required protection of the transformer itself.

 

[don_resqcapt19]

Earlier in this thread it was said you could land these secondary conductors on any size OCPD as long as they were sized to be equal to or greater than said OCPD. ( Not in those words exatly)

I did say that and it is code compliant for the transformer in this thread as the transformer itself does not require protection for the secondary windings...the only required protection on the secondary side of this transformer is for the secondary conductors. The only code rule is that these conductors be protected at or below their ampacity.

As I said in my last post if this is a multi volt secondary the line to neutral currents pose a problem for the primary OCPD to protect the secondary windings and conductors.

1:4 voltage ratio and 4:1 current ratio with the line to neutral load....
It would take 80 amps or greater on a line to neutral load to trip primary OCPD.

The secondary of this transformer is 15 kVA 120/240.
150000 / (240 x 1.73) = 36.12 FLC x 1.25 = 45.15
So at 36 amps at 240 volts we have 18 amps on the secondary.
The primary of this transformer is 15 kVA 480
15000 / (480 x 1.73) = 18.06

This is a 1:2 voltage ratio primary to secondary.
But this is a multivolt secondary
1:4 voltage ratio with 120 volt load

That means if there is a unbalance it would take more than the rated FLC of the secondary to trip the primary OCPD by 2 fold.

80 amp 120 volt load to trip the 20 primary OCPD
With anything larger than a 45 amp OCPD on the secondary the transformer is not properly protected.

The math does not lie
As long as my math is correct

Once again I am not trying to have a pissing contest bit this is the way I understand the rules...It's all about the math. That is what 240.4 (F) and 240.21 (C) (1) is stating.

The main difference is single voltage secondary or multi voltage secondary.

Your math and electrical theory is correct, but the issue here is the rules found in the NEC. The rules you keep citing only apply to the required protection of the secondary conductors...those rules have absolutely nothing to do with the protection of the transformer itself.
The mixing of the rules in Article 450 and those in Article 240 is a very common code mistake.

 

[yired29]

The point that you are missing is the fact that the rules in Article 240 have nothing to do with the required protection of the transformer itself.

What about the overload on a line to neutral load that you are relying on the primary OCPD to take care of.

Is my math correct?
80 amps on the secondary of a transformer rated at 36 FLC to trip the primary OCPD???

What if the line to neutral load was 70 amps the primary would never trip, could the secondary windings handle that?

Is that protecting the transformer?

I don't believe so.

Our responsibility is to protect persons and property. Thats 90.1

That transformer is property.

I'll ask some of the inspectors with the IAEI Dupage County chapter soon on how they see this..

 

[don_resqcapt19]

What about the overload on a line to neutral load that you are relying on the primary OCPD to take care of.

Is my math correct?
80 amps on the secondary of a transformer rated at 36 FLC to trip the primary OCPD???

What if the line to neutral load was 70 amps the primary would never trip, could the secondary windings handle that?

Is that protecting the transformer?

I don't believe so.

Our responsibility is to protect persons and property. Thats 90.1

That transformer is property.

I'll ask some of the inspectors with the IAEI Dupage County chapter soon on how they see this..

Please show me the words in the code that say the two sections that you keep citing have anything to do with the protection of the transformer. The two sections specifically apply to the protection of conductors.

 

[iwire]

I'll ask some of the inspectors with the IAEI Dupage County chapter soon on how they see this..

You can do that and all you will have is another persons opinion.

Read the words in the NEC for yourself and understand they do not always say what we expect them to say.

Do me a favor and take a look at 230.90(A) Exception 3 and tell us what protects the conductors that exception is talking about?

I know that section has nothing to do with transformer but it is proof positive that sometimes the NEC relies only on Article 220 load calculations to protect conductors.

 

[tryinghard]

.

This is a 480volt primary 120/240 secondary.
1:2 ratio
20 amps primary 40 amp secondary
If we have over 40 amps on a balanced secondary the primary would trip.
With a neutral present we could have a line to neutral load
1:4 ratio
20 amp primary 80 amp secondary
It would take over 80 amps before primary would trip.

This means we would need to pull 80 amps through the secondary conductors and windings in order to trip the 20 amp OCPD on the primary side.
Thats 80 amps through windings that are sized for 36 amp FLC.
This is what 240.4 (F) is stating.
Does this help

How can you get more than 36a neutral load on this secondary grounded conductor?

 

[tryinghard]

Earlier in this thread it was said you could land these secondary conductors on any size OCPD as long as they were sized to be equal to or greater than said OCPD. ( Not in those words exatly)...

408.36 (exact words), panelboard OCP.