Secondary Neutral Bonding

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babbster

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I am installing a 480V to 120V, 15kVA single phase transformer. I plan on using the secondary lead X1 as the Neutral. I understand that I need to also bond this lead back to the main enclosure's ground point. My question is, what ampacity chart do I need to use in order to size this bonding wire correctly? Should it be sized based off the incoming breaker size (40Amps) or secondary potential (125Amps)?

Thanks!
 
Thank you Benaround...

I read through section 250.30. I am still a bit confused. The code states that it must comply with section 250.28, that says I can follow Table 250.66. Then, Exception 3 under 250.30(A)(1), states "is derived from a transformer... shall not be smaller than the derived phase conductors...".

So, if I follow Table 250.66, then I need 6 AWG. If I follow the Exception, then I need 1/0 (per Table 310.16).

Thanks again for all the help!
 
babbster said:
Should it be sized based off the incoming breaker size (40Amps) or secondary potential (125Amps)?

Thanks!
Click to expand...


I would say neither. I f you look at 250.66 as the others have suggested you will see that it the table lists conductor sizes. You would use the size of the secondary conductors to size a GEC, system bonding jumper and equipment bonding jumper.
 
here is another article,...are you sure 250.30 exception 3 applies???

from the article:
Grounding electrode conductor (GEC)

You must install a GEC to connect the neutral terminal of an SDS to a grounding electrode of a type identified in 250.30(A)(7) [250.30(A)(3)]. You don't have to do this, however, if your SDS is for a Class 1, Class 2, or Class 3 circuit, and is rated 1 kVA or less [250.30(A)(3) Ex No. 1]. Size the GEC per 250.66, based on the area of the secondary conductor.
Click to expand...

http://ecmweb.com/nec/electric_grounding_bonding_separately/


803ecmCBfig1.jpg
 
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Thanks to all...

The diagram is not exactly what I have. I do not have a X0. I am using X1 as a Neutral with 1/0 AWG feeding my load center. Then, I was going to connect a seperate wire from X1, and bring it back to the ground buss where the primary's circuit breaker is located.

I'm not convinced that 250.30 exception 3 applies. The more I read and think this through, I also believe that 250.66 is correct.

Per 250.66; I am basing the 6 AWG bonding conductor from the size of the 1/0 AWG secondary current carrying conductor (125Amps).

Any other ideas/comments?
 
Intention

Intention

My understanding is that the neutral ground bond for the separately derived neutral must be sized to handle the available fault current from a bolted fault: either the secondary overcurrent device or the primary device times the turns ratio.

The equipment grounding conductor on the 480V side must be sized to handle the fault current for a 480V fault and the rating of the 480V OCPD.
 
babbster, So, you have a control transformer? We all could help you better

if you would explain exactly what you are doing. Personally I have not seen a

control transformer with a 125a single pole secondary. I'm willing to help you

out, but, I can't with what info has been given.
 
Thanks to all...

Ok, more info:

I am designing a Main Control Panel & Power Distribution for a production line. I have 480V, 3-ph, 150Amps coming into a main disconnect CB that feeds a buss bar system. On this buss bar system resides a number of 480V breakers that feed multiple loads. There is also a Ground Buss in this main control panel that is fed by the plant. One of the breakers mentioned is a 40Amp that feeds a Square-D 15S1F (http://ecatalog.squared.com/fulldetail.cfm?partnumber=15S1F#docs).

This transformer is wired for 480V in, 120V out. The secondary terminal X3 is jumpered to X1 & X2 is jumpered to X4. This transformer is going to feed a circuit breaker panel that will supply power to numerous 120V receptacles, power supplies, controllers, etc. This circuit breaker panel has an isolated Neutral Bar, and a mounted (non-isolated) Ground Bar.

I planned on bringing X1 to the Neutral Bar, and X4 to a 125Amp Main circuit breaker. These wires will be sized to 1/0Awg.

So, my question remains... I don't know if I have to bring a 1/0Awg cable from X1 to my Main Control Panel's Ground Buss, or if 6Awg will suffice.

I hope this helps clear things up! Thanks again for any suggestions!
 
You'll also have to find a way to split X4 if you want to use all of the spaces. That also means you'll actually be supplying 250a. of OCP.

Anyone know of a single-bus panel?
 
In the cb panel, there is a two-pole 125 cb that is being used as a main. We are only wiring incoming power (X4) to one of the two incoming poles, leaving the second one open. The load sides of this breaker are attached to two bussbars. We are going to bond these two bussbars together, providing the 120V to all breakers.
 
I don't know exactly how the plant electricians do this...

For my new line, I am using drawings of existing lines that my plant provided me. They show what I described above. I had Square-D look into a "bonding kit", but they did not make one. I doubt if the load side of the breaker has any lugs to attach wire (along with clamping onto the buss). I just tried to contact someone who would know, but they left for the day. I will see if I can get him tomorrow AM and ask.

What methods did you come up with?
 
babbster said:
What methods did you come up with?
Click to expand...
1. Find a main-breaker panel with out-feed lugs, and tie them together there.

2. Use a main-lug panel and a back-fed main, and tie the line lugs together.

3. Use a tapping device (fits like a breaker, but it's just for tapping unfused.)

4. Use a 2-pole breaker with the load terminals jumpered (just thought of this one.)
 
I talked to one of the plant electricians. He opened up one of the existing panels. He told me that there is a lug at the top of each buss. They install a #2/0 jumper wire between these two lugs.

The panel that we use is a Square-D, QO142L225G. The main breaker is a QO2125, and there is a main breaker retaining kit; PK4MB2HA.

Hope this helps!
 
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