Secondary Feeder from Step-Dn to Panel

[cforrester97]

I'm having a hard time understanding what the actual requirements are per NEC.

The step down (480-208, 3PH) transformer GEC is sized per NEC 250.102(C)(1) of the secondary, supply side OCPD.

The secondary feed to the 208 panel is sized for GEC per NEC 250.102(C)(1) as well, not EGC as it typically would be sized for.

For example, 175A breaker from 480 panel to transformer. Secondary OCPD (208 panel) is sized at 400A. Ground on transformer is sized #1/0 per NEC 250.102(C)(1). The secondary to the 400A, 208 panel is sized with #1/0 gnd. instead of #3.

If any of this is incorrect, please let me know. This is my understanding currently. I have usually sized secondary per EGC requirement, not GEC, and have also seen it this way on many electrical plans. However, I recently got dinged by the building department for not sizing the secondary feeder ground as GEC and would like a clear answer on why.

Thank you!

 

[don_resqcapt19]

It is a supply side bonding jumper and not an equipment grounding conductor. It is sized based on the ungrounded conductor size and not related to the size of the load end OCPD.
The equipment grounding conductor for a system supplied by a transformer starts on the load side of the first OCPD.

 

[cforrester97]

It is a supply side bonding jumper and not an equipment grounding conductor. It is sized based on the ungrounded conductor size and not related to the size of the load end OCPD.
The equipment grounding conductor for a system supplied by a transformer starts on the load side of the first OCPD.

Thank you for the quick reply. Please excuse my ignorance. I'm trying to wrap my head around this.

The ground in the secondary from the transformer to the 208 panel is the supply side bonding jumper?

Can you also elaborate on the second part on the EGC starting on the load side of the first OCPD?

 

[Elect117]

When you think of a transformer, you should recognize that it is a separately derived system. The secondary conductors are not directly attached to the primary side. They are instead derived from a magnetic field that is coupled.

So the secondary side of the transformer should resemble a service is a few ways. One of those ways is the supply side bonding jumper. Since the system is now separately derived, the bonding connections (supply side bonding jumper from transformer neutral point/SBJ) to the first OCPD must carry the fault current from all the future down stream EGCs back to the source, which is the secondary side of the transformer.

If you try and google it, you might see various terms thrown out about the grounding and bonding of a transformer. The language was changed about 10 years ago. It used to be called equipment bonding jumper at one point and that was probably where confusion started.

I also find that 450.10 is sometimes misunderstood. All of the grounding and bonding wires should land on a single terminal bar in a dry type transformer. So you would have a EGC sized based on the primary side's overcurrent protection, the system bonding jumper from the neutral, the grounding electrode conductor, and the supply side bonding jumper all at one terminal bar. That terminal bar should be directly connected to the frame but not on top of any parts used for ventilation.

 

[don_resqcapt19]

Thank you for the quick reply. Please excuse my ignorance. I'm trying to wrap my head around this.

The ground in the secondary from the transformer to the 208 panel is the supply side bonding jumper?

Can you also elaborate on the second part on the EGC starting on the load side of the first OCPD?

Yes the grounding conductor between the transformer and the first overcurrent device is a supply side bonding jumper. That is because that conductor is on the line side of the first OCPD. The supply side bonding jumper is required by 250.30(A)(2).

Your grounding electrode conductor must be connected at the same location as the system bonding jumper. The system bonding jumper can be either at the transformer or the first OCPD.

As far as the EGCs, they only exist after the first OCDP. They are sized based on the rating of the upstream OCPD, and in the case of a transformer secondary, there is no upstream OCPD. The transformer primary OCPD does not count as the secondary is a new system....separately derived system.

Also note that some of the rules in 240.21(C) set a maximum length on the conductors between the transformer secondary and the first OCPD.

 

[don_resqcapt19]

When you think of a transformer, you should recognize that it is a separately derived system. The secondary conductors are not directly attached to the primary side. They are instead derived from a magnetic field that is coupled.

So the secondary side of the transformer should resemble a service is a few ways. One of those ways is the supply side bonding jumper. Since the system is now separately derived, the bonding connections (supply side bonding jumper from transformer neutral point/SBJ) to the first OCPD must carry the fault current from all the future down stream EGCs back to the source, which is the secondary side of the transformer.

If you try and google it, you might see various terms thrown out about the grounding and bonding of a transformer. The language was changed about 10 years ago. It used to be called equipment bonding jumper at one point and that was probably where confusion started.

I also find that 450.10 is sometimes misunderstood. All of the grounding and bonding wires should land on a single terminal bar in a dry type transformer. So you would have a EGC sized based on the primary side's overcurrent protection, the system bonding jumper from the neutral, the grounding electrode conductor, and the supply side bonding jumper all at one terminal bar. That terminal bar should be directly connected to the frame but not on top of any parts used for ventilation.

There is no requirement that the system bonding jumper and the grounding electrode conductor be at the transformer. They both must be in the same location, but that location can be at the transformer or at the first OCPD enclosure.

 

[Elect117]

There is no requirement that the system bonding jumper and the grounding electrode conductor be at the transformer. They both must be in the same location, but that location can be at the transformer or at the first OCPD enclosure.

Sorry, I meant that if they are installed at the transformer, they should be made at the terminal bar.

 

[cforrester97]

Thank you guys for these explanations, very helpful!