Transformer Questions

[cppoly]

I'm having some difficulty understanding the difference between transformer protection on the primary and secondary and feeder protection on the secondary and primary. Is there a difference?

I attached a sketch. Can someone help?

 

[don_resqcapt19]

The feeder conductors are normally connected to the load side of the primary (feeder) overcurrent protective device.

There is no specified location for the primary OCPD. If there is not a disconnect within sight of the transformer, the transformer must be marked with the location of the nearest means of disconnect. The disconnect is normally the same device that provides the feeder and primary overcurrent protection.

We need more information about secondary ties to answer the question.


While many call the transformer secondary conductors taps, the NEC does not. They are transformer secondary conductors and the overcurrent protection of these conductors is covered by 240.21(C). There are many similarities between the tap conductor rules in 240.21(B) and the transformer secondary conductor rules in 240.21(C) and as a result many call both conductors "taps".

 

[augie47]

In addition:
Transformer protection rules are found in Art 450.
Feeder overcurrent protection rules are found in Art 240.21(B)
Transformer secondary overcurrent rules are found in 240.21(C). They are similar to the "tap" rules above, but transformer secondary conductors are not taps (see the definition of taps in 240.2)

 

[cppoly]

Thanks. I think my questions are even more basic though.

I literally don't know the distinction between transformer OCPD AND feeder OCPD. Depending on where the OCPD goes and what is considered feeder protection and what is considered transformer protection on the one line diagram.

I attached a markup.

Is OCPD #1 considered BOTH feeder primary protection AND transformer primary protection?
Is OCPD #2 considered BOTH feeder secondary protection AND transformer secondary protection?

 

[jim dungar]

Yes.

A properly sized device can serve both as transformer protection and as feeder protection.
If proper sizing is not feasible then separate devices may be used.

 

[don_resqcapt19]

In my experience, the feeder OCPD almost always serves as the primary OCPD, but remember the feeder has to be sized to the OCPD, and not to the full load current of the transformer. The OCPD on the secondary is normally sized to protect the secondary conductors per the rules in 240.21(C), but this will almost always provide the transformer secondary protection that is required by Table 450.3. The location of the secondary OCPD is based on the rules in 240.21(C). The rules in 450, do not specify a location.

 

[augie47]

It's a bit confusing as the devices can wear more than one hat.
Check Art 100 definitions for Branch Circuits and Feeders.
In you Post 4, OCPD1 would be a Branch Circuit OCP device, IMO, as it is the last OCP ahead of the outlet (transformer). It is protecting the branch circuit conductors to the transformer.
It is also a transformer primary protective device per Art 450.

Your OCPD2 is a transformer secondary protective device per Art 450.
It could also be feeder OCP if it feeds more than one branch circuit OCP device downstream, or it could be a branch circuit OCPD if it is the final OCP prior to outlet(s).

 

[cppoly]

In my experience, the feeder OCPD almost always serves as the primary OCPD, but remember the feeder has to be sized to the OCPD, and not to the full load current of the transformer. The OCPD on the secondary is normally sized to protect the secondary conductors per the rules in 240.21(C), but this will almost always provide the transformer secondary protection that is required by Table 450.3. The location of the secondary OCPD is based on the rules in 240.21(C). The rules in 450, do not specify a location.

This requires some reading to fully understand so I will have to read all those sections =)

 

[cppoly]

It's a bit confusing as the devices can wear more than one hat.
Check Art 100 definitions for Branch Circuits and Feeders.
In you Post 4, OCPD1 would be a Branch Circuit OCP device, IMO, as it is the last OCP ahead of the outlet (transformer). It is protecting the branch circuit conductors to the transformer.
It is also a transformer primary protective device per Art 450.

Your OCPD2 is a transformer secondary protective device per Art 450.
It could also be feeder OCP if it feeds more than one branch circuit OCP device downstream, or it could be a branch circuit OCPD if it is the final OCP prior to outlet(s).

So both OCPDs act as both: feeder & transformer protection. If the percentages happen to match up with with article 450, then no additional OCPD is required to protect the transformer to meet the primary & secondary percentages as listed in the table 450.3(B). If they don't meet table 450.3(B)'s requirements, then additional OCPD is required at the transformer?

 

[brian john]

I found that Ryan J summed it up fairly well in this two part post.

http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=1371

http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=1487

 

[cppoly]

Read both the articles. They are a good read.