Question

[mschmidgall77]

Hi, Mike. I have a couple quick questions on installing a 15kVA transformer with wire-type leads. It’s fed from a 480 volt primary with secondary’s feeding a 120/240 volt panel. The primaries consist of (2) #8 THHN’s and a #10 THHN ground protected by a 2-pole 40 amp breaker. My boss told me that I don’t need a ground rod at the transformer, just to land the #10 on the lug provided. Doesn’t it need a GEC and bonded? He also told me to run #4 THHN’s to a 100 amp panel saying they are protected by the primary OCPD. Don’t the ampacity of the secondary conductors have to equal the rating of the panel board? I am very interested to hear your thoughts. Thanks! Matthew Schmidgall

 

[Dennis Alwon]

I don't know the situation but he is probably using the tap rules stated in art. 240.

You can feed a 200 amp panel with a 100 amp breaker with no issues but you cannot feed a 100 amp panel with a 200 amp breaker.

Normally a transformer must be connected to the grounding electrode in the building. Is this an exterior trany?

Btw, welcome to the forum

 

[augie47]

(Forum questions are answered by your fellow trades people)
The transformer does need a grounding electrode (will discuss further) and a bonding jumper
240/120 secondaries are not considered to be protected by the primary OC protective device. {240.21(C)}
#4s are not adequate for 100 amp in this situation.

 

[Dennis Alwon]

(Forum questions are answered by your fellow trades people)
The transformer does need a grounding electrode (will discuss further) and a bonding jumper
240/120 secondaries are not considered to be protected by the primary OC protective device. {240.21(C)}
#4s are not adequate for 100 amp in this situation.

Thanks Gus I never noticed that a "Y" was not allowed to be protected by the primaries

 

[augie47]

AS to your using the equipment grounding conductor for your GEC there is a Code provision but your #10 would not be adequately sized.

 

[augie47]

Thanks Gus I never noticed that a "Y" was not allowed to be protected by the primaries

"Y" and 3 wire other than Delta-Delts + he is feeding a panel and 480.36 would requires a main.

 

[GoldDigger]

Is the 480V on the primary a service or a feeder? The 2-pole 40A breaker you mention makes it sound like a feeder.
The primary feed includes an EGC (ground) but no neutral. So the transformer case ground lug needs only the EGC.
The remaining question is whether the secondary is being operated as a Separately Derived System.
With no neutral on the primary side and 120/240 on the output side, I do not see you having an option other than SDS. So the neutral on the secondary needs to be grounded other than just by the primary side EGC. The most common connection point is building steel. If you drove a local ground rod, it would still have to be connected to the rest of the building Ground Electrode System (GES).
I will leave the overcurrent protection question for someone else.

PS: My, people jumped on that one fast!