# 3 Phase Transformer bonding question



## Shandley (Aug 18, 2010)

Greetings,

I am hoping for a cut and dry answer here.

I have a 75KVA transformer. 480V Delta Primary and 208/120V Wye Secondary.

In order to avoid having a floating neutral, I need to bond the neutral on the secondary to ground.

*What size conductor do I need for this?*


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## raider1 (Jan 22, 2007)

What size secondary phase conductors are you running?

The system bonding jumper is sized based on the size of the secondary phase conductors.

I am basing this on the NEC and section 250.30(A)(1).

I am not sure what the canadian electrical code may require.

Chris


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## Shandley (Aug 18, 2010)

Since the Canadian Electrical Code is similar in nearly every way to the NEC except for indexing, I will go with whatever you tell me it is in the NEC 

The secondary conductors I am using are 4/0


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## jwjrw (Jan 14, 2010)

Shandley said:


> Since the Canadian Electrical Code is similar in nearly every way to the NEC except for indexing, I will go with whatever you tell me it is in the NEC
> 
> The secondary conductors I am using are 4/0


 

I believe a #2 sized off 250.66


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## Shandley (Aug 18, 2010)

Much thanks. That's what I was going to use


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## HAND (Jul 15, 2010)

Shandley said:


> Since the Canadian Electrical Code is similar in nearly every way to the NEC except for indexing, I will go with whatever you tell me it is in the NEC
> 
> The secondary conductors I am using are 4/0


You need to go with 250, and the ocpd has to be 250.
240.21c2
#2cu is ok for the gec

Or 225 with the 4/0, #2 gec.
I didn't mean to confuse but there will be 25 unused amps in your case.


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## nitro71 (Sep 17, 2009)

Without a load calculation I'm pretty sure you are going to have to have the neutral the same size as the phase conductors. Without a calc you don't know how much you can undersize it.


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## Introyble (Jul 10, 2010)

Shandley said:


> Greetings,
> 
> I am hoping for a cut and dry answer here.
> 
> ...


its kva divided by 208 times the square root of 3

then you take that answer and multiply it by 1.25

Interesting note, the nec 240.4(b) size up rule does not apply in this situation. Secondary conductors may not be smaller than the overcurrent protective device nec 110.14(c)


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## Introyble (Jul 10, 2010)

Actually, I need to correct myself.

NEC 250.30(a)(1)

Bonding XO terminals according to table 250.66


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## jwjrw (Jan 14, 2010)

Introyble said:


> Actually, I need to correct myself.
> 
> NEC 250.30(a)(1)
> 
> Bonding XO terminals according to table 250.66


 

Which I told the op in post #4......:whistling2:


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## Introyble (Jul 10, 2010)

jwjrw said:


> Which I told the op in post #4......:whistling2:


yep I see it now :laughing:


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