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XFMR OCP

ROBOJOE77

ROBOJOE77

Member
Location
troutdale oregon
Occupation
manufacturing plant journeyman
2023 NEC handbook, 450.3, blue informational note states: The requirements for OCP for transformer secondaries apply only to the protection of transformers, not the conductors.
What about primary only OCP? Is that also intended only to protect the transformer, not the conductors? It is not specified in the initial statement, but then in the next paragraph states that the OCP for transformers may not provide satisfactory protection for the primary and secondary conductors. Leading me to believe that primary only OCP is to protect the transformer only as well.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
450.3 only deals with protection of the transformer from overloads and removal of a faulted transformer from its source.
Primary and secondary conductors are protected by 240.21.
Secondary panel boards are protected by 408.30

Notice how 450.3 always requires primary side protection. However, it allows the primary protection to be oversized only if secondary protection is provided.
There is actually no NEC requirement to coordinate secondary conductor sizing with the transformer rated output. However in practice, the protection requirements are often all met by a single appropriately sized and located secondary side device.
 
Last edited:
ROBOJOE77

ROBOJOE77

Member
Location
troutdale oregon
Occupation
manufacturing plant journeyman
We have a 3 specific purpose transformers, single phase 480, 380A being fed directly from 600A breakers at the MDP no secondary protection unless you consider the heating elements big fuses (jk). This doesn't seem right to me, but I'm a pretty fresh journeyman too.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
ROBOJOE77 said:
We have a 3 specific purpose transformers, single phase 480, 380A being fed directly from 600A breakers at the MDP no secondary protection unless you consider the heating elements big fuses (jk). This doesn't seem right to me, but I'm a pretty fresh journeyman too.
Click to expand...
How old is this installation?
What is the secondary winding configuration?
Is this an 'supervised industrial installation?

Many decades ago this was probably a compliant installation.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
ROBOJOE77 said:
2023 NEC handbook, 450.3, blue informational note states: The requirements for OCP for transformer secondaries apply only to the protection of transformers, not the conductors.
What about primary only OCP? Is that also intended only to protect the transformer, not the conductors? It is not specified in the initial statement, but then in the next paragraph states that the OCP for transformers may not provide satisfactory protection for the primary and secondary conductors. Leading me to believe that primary only OCP is to protect the transformer only as well.
Click to expand...
See 240.21(C)(1). The handbook is correct the rules in 450 only apply to the the protection of the transformer itself, and not the primary or secondary conductors for that transformer. The rules in 240.21(C) apply to the protection of the secondary conductors, and there are no special rules for the primary conductor protection. The rule in 240.21(C)(1) sets out specific cases where the primary protection can protect the primary conductors, the primary and secondary windings of the transformer, and the secondary conductors. Note that these are very specific rules and do not apply to most installations.
 
ROBOJOE77

ROBOJOE77

Member
Location
troutdale oregon
Occupation
manufacturing plant journeyman
jim dungar said:
How old is this installation?
What is the secondary winding configuration?
Is this an 'supervised industrial installation?

Many decades ago this was probably a compliant installation.
Click to expand...
It is an older installation, not sure of the age. I've only been here for 3 years and playing catch up. It is a 'supervised industrial installation.
 
ROBOJOE77

ROBOJOE77

Member
Location
troutdale oregon
Occupation
manufacturing plant journeyman
don_resqcapt19 said:
See 240.21(C)(1). The handbook is correct the rules in 450 only apply to the the protection of the transformer itself, and not the primary or secondary conductors for that transformer. The rules in 240.21(C) apply to the protection of the secondary conductors, and there are no special rules for the primary conductor protection. The rule in 240.21(C)(1) sets out specific cases where the primary protection can protect the primary conductors, the primary and secondary windings of the transformer, and the secondary conductors. Note that these are very specific rules and do not apply to most installations.
Click to expand...
So, what I'm gathering is that if the only circuit protection is a primary breaker, it should be calculated to protect the transformer, so it should be rated at 500A and not 600A. Primary only protection puts it at 1.25 x 380 = 475?
 
E

Elect117

Senior Member
Location
California
Occupation
Engineer E.E. P.E.
Your installation sounds more complicated than a simple answer.

Single phase transformer with a single phase secondary side, can have the secondary side protected on the primary side of the transformer. 240.21(C)(1). It is a rare occasion.

You are using the FLA of the transformer when you could have a transformer with a variable rating or an overloading factor. Those could have been used to size the primary side protection. The conductors would most likely be oversized on the primary side. You might be able to verify that.
 
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