This is a question I keep going back and forth with trying to determine when indeed transformer secondary protection is required. Now I know when it comes to protection on the secondary of the transformer there are two issues, the first being transformer protection as it relates to 450.3 and the second being secondary cable protection as it relates to 240.21(C)
For 450.3 I have always interpreted this for a transformer with a current of 9A or more as saying that if the primary protective device was 125% or less of rated current then secondary protection is not required. This is ignoring any requirement of 240.21(C) for the time being and just focusing on protection required for the transformer. Now if the primary protective device is sized such that it is sized up to 250% or transformer rating then secondary protetion of the transformer is required and can be applied up to 125% of secondary current. Again this is ignoring for now any requiremnt of 240.21. If a single OCPD is not providing this secondary protection then Note 2 in table 450.3(B) allows a combination of up to 6 protective devices in one locationi to provide this protection if their sum does not exceed 125%.
Now looking at the second issue of cable protection we have to look at 240.21(C). This section requires that for any 3-phase transformer other than a delta-delta transformer the secondary cables cannot be protected by the OCPD on the primary of the transfomer and thus requires an OCPD on the secondary of the transofmer. This secondary OCPD can be locaed at a couple of different locations on the secondary as outlined in 240.21(C). The "The next size" up rule cannot apply to the secondary cables and the secondary OCPD. There is also a provision in 240.21(C)(4) that allows for multiple OCPD's to be grouped in a location in order to provide the secondary protection of the secondary cable. There is also no limit to how many cables and therefore seperate feeder circuits can be connected to the secondary of the transofmrer terminals with each feeder circuit having its own required secondary cable protection.
Do you guys agree that I have stated this correctly?
For 450.3 I have always interpreted this for a transformer with a current of 9A or more as saying that if the primary protective device was 125% or less of rated current then secondary protection is not required. This is ignoring any requirement of 240.21(C) for the time being and just focusing on protection required for the transformer. Now if the primary protective device is sized such that it is sized up to 250% or transformer rating then secondary protetion of the transformer is required and can be applied up to 125% of secondary current. Again this is ignoring for now any requiremnt of 240.21. If a single OCPD is not providing this secondary protection then Note 2 in table 450.3(B) allows a combination of up to 6 protective devices in one locationi to provide this protection if their sum does not exceed 125%.
Now looking at the second issue of cable protection we have to look at 240.21(C). This section requires that for any 3-phase transformer other than a delta-delta transformer the secondary cables cannot be protected by the OCPD on the primary of the transfomer and thus requires an OCPD on the secondary of the transofmer. This secondary OCPD can be locaed at a couple of different locations on the secondary as outlined in 240.21(C). The "The next size" up rule cannot apply to the secondary cables and the secondary OCPD. There is also a provision in 240.21(C)(4) that allows for multiple OCPD's to be grouped in a location in order to provide the secondary protection of the secondary cable. There is also no limit to how many cables and therefore seperate feeder circuits can be connected to the secondary of the transofmrer terminals with each feeder circuit having its own required secondary cable protection.
Do you guys agree that I have stated this correctly?
