# Transformer Cable Size



## micromind (Aug 11, 2007)

750 copper will give about 5 volts of drop. Not bad for a 240/415 volt system.


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## foreverof (Mar 13, 2013)

micromind said:


> 750 copper will give about 5 volts of drop. Not bad for a 240/415 volt system.


Hey micromind, thanks for your suggestions, could you tell me how you come up with this number "750"? Thanks!


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## Hippie (May 12, 2011)

foreverof said:


> Hey micromind, thanks for your suggestions, could you tell me how you come up with this number "750"? Thanks!


750 Kcmils, wire size. I would try to run 2 paralells of smaller wire


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## foreverof (Mar 13, 2013)

How do you guys get the 750 Kcmils from calculation? Please share!


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## micromind (Aug 11, 2007)

foreverof said:


> How do you guys get the 750 Kcmils from calculation? Please share!


It's a standard size, like #12, 2/0, 250, and so on. The standard sizes are listed in the code book; in the back there's a resistance table of all standard sizes.


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## Hippie (May 12, 2011)

I use the voltage drop calculator app from Southwire, but you can do it manually using the ohms per foot of the wire


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## foreverof (Mar 13, 2013)

Hippie said:


> I use the voltage drop calculator app from Southwire, but you can do it manually using the ohms per foot of the wire


Hey Hippie,

I tried the Southwire voltage drop calculator, and I got this result.

2 conductors per phase utilizing a #400 Copper conductor will limit the voltage drop to 2.83% or less when supplying 438.2297 amps for 656 feet on a 415 volt system. 

It is telling me to use 2 conductors per phase, and the conductor is #400. You mention earlier to use 2 instead of one. How come you think 2 parallel is better than one? Thanks.


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## Hippie (May 12, 2011)

foreverof said:


> Hey Hippie,
> 
> I tried the Southwire voltage drop calculator, and I got this result.
> 
> ...


2 runs of smaller wire will be cheaper and easier to install. If the voltage is not so critical that it be within 3% you could go smaller, and unless your actual load is 438 amps you could also figure for less current (smaller wire)

750 copper is super expensive and heavy, 2 smaller runs will be better all around


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## foreverof (Mar 13, 2013)

Thanks Hippie for your suggestions.


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## foreverof (Mar 13, 2013)

I have another questions about voltage drop.

In my transformer spec, it said Rated Voltage output from the secondary winding is 433-250 Volts. Is that mean I can adjust the voltage output to 427.45V, then after the 200m of distance travel and voltage drop of 3%. The voltage will drop to 415V (Which is what I want). You guys think that is possible? Thanks.


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## Hippie (May 12, 2011)

foreverof said:


> I have another questions about voltage drop.
> 
> In my transformer spec, it said Rated Voltage output from the secondary winding is 433-250 Volts. Is that mean I can adjust the voltage output to 427.45V, then after the 200m of distance travel and voltage drop of 3%. The voltage will drop to 415V (Which is what I want). You guys think that is possible? Thanks.


I think what you're seeing is the voltage rating, 430 phase to phase, 250 phase to ground. However if your secondary also has taps to adjust voltage, then yes you can change them to compensate for voltage drop


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## foreverof (Mar 13, 2013)

Guys, I got another questions...really appreciated for your help.
How do I size the neutral cable? Will it be the same as each phase? Thanks.

What I think basically the neutral only for one phase, so should I just size the current for one phase?


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## Hippie (May 12, 2011)

foreverof said:


> Guys, I got another questions...really appreciated for your help.
> How do I size the neutral cable? Will it be the same as each phase? Thanks.
> 
> What I think basically the neutral only for one phase, so should I just size the current for one phase?


That will be determined by the load. What will this circuit be supplying?


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## foreverof (Mar 13, 2013)

Hey Hippie,

Could I assume the load to be the max load that 1 phase can get from the transformer? 

foreverof


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## RUSSIAN (Mar 4, 2008)

foreverof said:


> Hi guys, really need some suggestions.
> 
> I have a Rated output 315KVA 3 phases transformer (delta to wye), step down the voltage from 11000V to 415VAC. Each phase will be 240VAC.
> 
> ...


Hire an electrical contractor....


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## ponyboy (Nov 18, 2012)

RUSSIAN said:


> Hire an electrical contractor....


True. It's not like he's trying to hang a ceiling fan. This job should be engineered damned near


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## micromind (Aug 11, 2007)

foreverof said:


> Hey Hippie,
> 
> Could I assume the load to be the max load that 1 phase can get from the transformer?
> 
> foreverof


You could, but running a full size neutral a long distance costs a lot. Most of the time, the neutral can be reduced one or two sizes. 

Rob

P.S. I agree with the others, two smaller wires are easier to handle than one big one.


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## ElectricBrent (Jan 1, 2013)

really? are you an electrician who never learned to read the CEC? Start with Section 26 for transformers. Voltage Drop calculation is in Table D3. Wire sizing is Table 2.....


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