Transformer Calculations

[DLaHaye]

I have current two situation at some our facilities on a existing transformer installation . I am trying to correct the issues with limited cost and impact to system.

Scenario #1 : 15 KVA Single Phase 480 Primary /240-120 Secondary
All equipment is mounted outside , transformer is less than 10ft. from panel
Primary has a 40 amp OCPD with #8 wire
Secondary - is supplying a 200 amp main breaker in panel on 3/0 wire

Question: I know the secondary panel is over sized for the capacity of transformer but the secondary conductors are protected. The way I understand it I am not required to have Secondary OCP for the transformer given it is outside and less than 10ft. from panel as well as Primary OCPD is sized @125%. According to 240.21(C) (2)&(4) also , Table 450.3(B). Would that be correct and can I leave installation alone?


Scenario #2 : 25 KVA Single Phase 480 Primary /240-120 Secondary
All equipment is mounted outside
Primary has a 70 amp OCPD with #4 wire
Secondary - is supplying a 200 amp main breaker in panel on #2 wire (which I know is not correct)

Question : My thoughts to correct this would be to change the 200 amp MB to 110 amp to protect my secondary conductors. Would this be a proper solution? ( other than pulling 3/0 wire)

 

[lauraj]

According to 450.3(B) I don't see how you can get away with this installation. That type of transformer requires both primary and secondary protection according to 240.21(C)(1) so the secondary OCPD cannot be greater than 125% of the FLA. I think you have to change both secondary OCPDs to comply with the NEC.

 

[DLaHaye]

According to 450.3(B) I don't see how you can get away with this installation. That type of transformer requires both primary and secondary protection according to 240.21(C)(1) so the secondary OCPD cannot be greater than 125% of the FLA. I think you have to change both secondary OCPDs to comply with the NEC.[/QUOT

What about 240.21(C)(2) and (4) . Also 450.3(B) read that if sized at 125%max on primary I am not required to have secondary protection . Please help me understand what I am missing. Thank you in advance

 

[augie47]

On the 1st install you are covered as far as transformer protection is concerned {450.3(b)}
since you have OCP @ 125% of the primary. You do need secondary protection per 240.21(C) since your secondary is 3 wire. Since you have a 3/0 on a 200 amp breaker, you have that covered.

IN the second situation, again, since you have a 3 wire secondary you must protect the secondary conductors with a OCP device, The 110 amp breaker will provide that protection.

 

[DLaHaye]

Augie47,
Thank you for reply sounds like the route I will go.

You stated that, You do need secondary protection per 240.21(C) since your secondary is 3 wire. Please explain, I read 240.21(C)(1) About the two wire system but, I also read 240.21(C)(2) and (4) about under 10ft. and outside installations. I just want to be clear for future clarity and possible testing purposes.

 

[don_resqcapt19]

Lets be careful here. There are two sets of rules involved. The ones in Article 450 are for the protection of the transformer itself. As long as the device on the primary side is rated at 125% or less of the primary rated current, you are done with the protection of the transformer.

Then you look to 240.21(C) for the protection of the secondary conductors. With the exception of a 2 wire to 2 wire transformer or a delta to delta transformer, the secondary conductors always require protection.

 

[augie47]

Augie47,
Thank you for reply sounds like the route I will go.

You stated that, You do need secondary protection per 240.21(C) since your secondary is 3 wire. Please explain, I read 240.21(C)(1) About the two wire system but, I also read 240.21(C)(2) and (4) about under 10ft. and outside installations. I just want to be clear for future clarity and possible testing purposes.

As don points out 240.21(C)(1) will assist you in making a determination of the need for secondary OCP, (C)(2) and (C)(4) address the length of conductors before termination and do not alter the requirement for OCP..

 

[lauraj]

Augie47,
Thank you for reply sounds like the route I will go.

You stated that, You do need secondary protection per 240.21(C) since your secondary is 3 wire. Please explain, I read 240.21(C)(1) About the two wire system but, I also read 240.21(C)(2) and (4) about under 10ft. and outside installations. I just want to be clear for future clarity and possible testing purposes.

Secondary is 240/120V so it is 4-wire, there for both primary and secondary protection is required, limiting the secondary protection to 125%.

 

[dinkelja]

Secondary is 240/120V so it is 4-wire.

Incorrect. 480-120/240V, single phase transformer is a 3-wire system, the EGC isn't counted.

 

[Smart $]

Secondary is 240/120V so it is 4-wire, there for both primary and secondary protection is required, limiting the secondary protection to 125%.

Transformers are 1Ø by the description provided. Therefore 3-wire secondary is the correct assessment.

As Don pointed out above, 450.3 does not require primary and secondary protection of the transformer provided the maximum primary only OCP rating is not exceeded.

The requirement for secondary conductor protection when other than 2-wire 1Ø or 3-wire 3Ø is a completely separate issue.

 

[lauraj]

Oh, I missed that it was single phase 120/240V.

So to expand on what Don was saying and to make sure I understand the 240.21(C)(1) requirement. Since this transformer isn't a 2-wire to 2-wire single phase transformer, the secondary conductors are not considered protected by the primary OCPD. Therefore we require a secondary OCPD. If we are installing a secondary OCPD, it cannot be sized in excess of 125%. Am I understanding this correctly?

 

[david luchini]

Oh, I missed that it was single phase 120/240V.

So to expand on what Don was saying and to make sure I understand the 240.21(C)(1) requirement. Since this transformer isn't a 2-wire to 2-wire single phase transformer, the secondary conductors are not considered protected by the primary OCPD. Therefore we require a secondary OCPD. If we are installing a secondary OCPD, it cannot be sized in excess of 125%. Am I understanding this correctly?

The secondary OCPD can be greater than 125% if the transformer primary OCPD is sized per the "primary only protection" method in T450.3(B).

The transformer secondary conductors cannot have an ampacity that is lower than the secondary OCPD.

 

[lauraj]

The secondary OCPD can be greater than 125% if the transformer primary OCPD is sized per the "primary only protection" method in T450.3(B).

The transformer secondary conductors cannot have an ampacity that is lower than the secondary OCPD.

Ok, I think I'm finally getting it. Here's a follow up question now that is nagging at me: When using 240.21(C)(1) and I have a delta-delta 3-wire which states that my secondary conductors are now protected by the primary OCPD, do I still need to follow the requirements of the 10 or 25 foot rules for the ampacity limitations of the conductors on the secondary side?

 

[Smart $]

Ok, I think I'm finally getting it. Here's a follow up question now that is nagging at me: When using 240.21(C)(1) and I have a delta-delta 3-wire which states that my secondary conductors are now protected by the primary OCPD, do I still need to follow the requirements of the 10 or 25 foot rules for the ampacity limitations of the conductors on the secondary side?

No. The minimum secondary conductor ampacity is the primary OCP rating times the primary-to-secondary voltage ratio. If you want to use a lower ampacity conductor, then you have to use one of the other rules.