Sizing conductors between transformer primary OCPD and transformer.

[Tip DS]

Given a transformer that has primary and secondary OCPD, with the primary using 250% rated transformer current for sizing. Does the wiring between the primary OCPD and the transformer get sized based on the OCPD, or the rated current?

I ask, because the service to the transformer is going to have conductors sized for the rated current of the transformer. It seems like using heavier conductors after the primary OCPD would be a mismatch for the service conductors.

 

[infinity]

The primary conductors are sized according to the OCPD protecting them. Are these actually service conductors?

 

[electrofelon]

Given a transformer that has primary and secondary OCPD, with the primary using 250% rated transformer current for sizing. Does the wiring between the primary OCPD and the transformer get sized based on the OCPD, or the rated current?

I ask, because the service to the transformer is going to have conductors sized for the rated current of the transformer. It seems like using heavier conductors after the primary OCPD would be a mismatch for the service conductors.

This is a common misunderstanding. The 250% primary OCPD sizing is for the transformer protection, and does not give you permission to size the conductors smaller than the OCPD (except next size up).

 

[infinity]

Choosing the primary OCPD is a design issue. It needs to be large enough so that it doesn't trip from the inrush current when you energize the transformer. Personally I see 250% as overkill.

 

[don_resqcapt19]

Choosing the primary OCPD is a design issue. It needs to be large enough so that it doesn't trip from the inrush current when you energize the transformer. Personally I see 250% as overkill.

A PE friend of mine will even go under 125% a lot of the times, especially where the 125% would require the next frame size breaker or the larger disconnect. For example he is likely to use a 100 amp OCPD for the 480 volt primary of a 3 phase 75 kVA transformer. 125% of the primary current would be 113 amps.

 

[infinity]

A PE friend of mine will even go under 125% a lot of the times, especially where the 125% would require the next frame size breaker or the larger disconnect. For example he is likely to use a 100 amp OCPD for the 480 volt primary of a 3 phase 75 kVA transformer. 125% of the primary current would be 113 amps.

We typically see 125% as the starting point with a rounding up. From your example the 113 amps would get a 125 amp OCPD and conductors but nothing nearing 250%.

 

[electrofelon]

we had a recent thread on this. Someone claimed to have investigated the typical trip curves and found 125% should not be an issue for typical three phase transformers. I have never had a tripping problem for three phase transformers at 125%, However single phase transformers seem to be more "inrushy" and I had one recently that would trip about 1 out of every 4 times with a 120% primary OCPD. That was a breaker, I think it would not be an issue with time delay fuses.

 

[infinity]

we had a recent thread on this. Someone claimed to have investigated the typical trip curves and found 125% should not be an issue for typical three phase transformers. I have never had a tripping problem for three phase transformers at 125%, However single phase transformers seem to be more "inrushy" and I had one recently that would trip about 1 out of every 4 times with a 120% primary OCPD. That was a breaker, I think it would not be an issue with time delay fuses.

I've never seen a problem at 125% either with 3 phase.

 

[don_resqcapt19]

We typically see 125% as the starting point with a rounding up. From your example the 113 amps would get a 125 amp OCPD and conductors but nothing nearing 250%.

I also, have always used the 125% as a starting point, but this engineer is not doing that and has not had inrush tripping issues. He will use 125% when that does not trigger an need for a larger disconnect, or larger frame size breaker, but in my example the 125% will require a 200 amp disconnect and using the 100 amp at 111% has worked for me and that is what he specs. He is looking at the cost difference in the primary conductor and the larger size disconnect.

 

[infinity]

I also, have always used the 125% as a starting point, but this engineer is not doing that and has not had inrush tripping issues. He will use 125% when that does not trigger an need for a larger disconnect, or larger frame size breaker, but in my example the 125% will require a 200 amp disconnect and using the 100 amp at 111% has worked for me and that is what he specs. He is looking at the cost difference in the primary conductor and the larger size disconnect.

I can see that design as an easy way to save money.

 

[Tip DS]

The primary conductors are sized according to the OCPD protecting them...

That's what I have been doing...sizing conductors based on OCPD rating. What seems funny (humorous) to me is that I am putting the primary and secondary OCPD in a transformer protection panel with a disconnect. That panel shows the feed at the rated voltage and current of the transformer. So, for example... a 30kVA 575/480 transformer would have a panel with 70A primary OCPD and 50A secondary OCPD. That would mean the conductors between the primary OCPD and the transformer primary would be #6 (THHN). The panel would get a tag on it, that would reflect 31 FLA, so it would be hooked up with #10 (THHN) or #8 (60° wire). Seems like a disconnect in the code to allow #10 on one side of an OCPD and require #6 on the other side.

...Are these actually service conductors?

Truthfully, I throw that term around, but I don't know. We refer to OUR connection as the service connection, but I don't imagine that's accurate in most situations. We typically get our "service" from a mill's electrical system - normally from a breaker they install in their panel to support equipment they purchase from us.