# Correctly Sizing Feeder ?



## patrickw1961 (Oct 8, 2010)

What is the correct way and formula for sizing a 480 volt feeder that will be feeding a transformer that will supply power to a 200 amp 208 3 phase panel ?


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## Bob Badger (Apr 19, 2009)

First thing

What size is the transformer?

I will guess it is a 75 kVA which means the 480 volt feeder should be about 125 to 150 amps. 

You might be able to sneak by with 100 amps, but that 100 amp breaker might not handle the inrush of the transformer and if that happens you will be redoing it all.


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## Magnettica (Jan 23, 2007)

Ohm's Law

200 amps x (208x 1.732) = 72 kva
72kva/ (480 x 1.732) = 87 amps


215.2(A) 

87 amps x 125% = 108 amps


Table 310.16, using the the 75º column for copper, #2 copper is needed for your primary feeders.


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## Bob Badger (Apr 19, 2009)

Mag your math is right but there are no 72 kva trans availabe. 

The primary side should be 125 to 250 percent of the trans size.


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## bobelectric (Feb 24, 2007)

I would use 3 #2 cu. thhn protected @ 90 amps for the primary.


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## Bob Badger (Apr 19, 2009)

bobelectric said:


> I would use 3 #2 cu. thhn protected @ 90 amps for the primary.



Yeah that would be NEC compliant and it might work fine.

Or when you go to energize the unloaded transformer the breaker may go from off to 'trip' all at once.

The more energy efficient they make transformers and motors the higher the inrush these items have.


Now lets say you do decide to use 2 AWG with a 100 amp breaker and it keeps tripping when you try to energizer the transformer, now what do you do?

Personally I have successfully used a 100 as the primary side breaker with a 75 Kva but all the jobs I have done that have been designed by engineers will use a 125 to 150 amp breaker for a 75 kva transformer as that is the recommended practice to prevent inrush trips.

There is a reason Table 450.3(B) of the NEC allows the breaker on the primary side to be up to 250% of the rated current and inrush tripping is the reason.


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## kbsparky (Sep 20, 2007)

Bob Badger said:


> Mag your math is right but there are no 72 kva trans availabe.
> 
> The primary side should be 125 to 250 percent of the trans size.


We recently installed a 200 Amp 480 Volt service with a connected load of 80 kVA. 

The utility installed a 45 kVA transformer to supply this service. :blink:


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## Bob Badger (Apr 19, 2009)

kbsparky said:


> We recently installed a 200 Amp 480 Volt service with a connected load of 80 kVA.
> 
> The utility installed a 45 kVA transformer to supply this service. :blink:


That is pretty much perfect.

The utility knows that the NEC load calculation run very high.


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## Magnettica (Jan 23, 2007)

You know I appreciate all the feedback, but wasn't the original question by the OP about sizing feeders? :yes:


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## Bob Badger (Apr 19, 2009)

Magnettica said:


> You know I appreciate all the feedback, but wasn't the original question by the OP about sizing feeders? :yes:


I think so, and the feeder is to a transformer so the feeder has to be sized to the size of the OCPD.

Step one figure out what size OCPD you want to use to supply the transformer.

Step two select conductors that are rated at least that much.


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## NolaTigaBait (Oct 19, 2008)

So for a 75kva fully loaded, you would need #2 protected by a 125 amp breaker...

75000/480*1.73=90

90*1.25=112.5...so #2 would be good for 115 amps @75 and the next size up is 125 amps....did i get this right?


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## NolaTigaBait (Oct 19, 2008)

Bob Badger said:


> Mag your math is right but there are no 72 kva trans availabe.
> 
> The primary side should be 125 to 250 percent of the trans size.


Where did you get 250% from...the chart says over 9 amps use 125% and the next size up if it doesn't correspond with 240.6...Am I missing something?


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## Bob Badger (Apr 19, 2009)

NolaTigaBait said:


> Where did you get 250% from...the chart says over 9 amps use 125% and the next size up if it doesn't correspond with 240.6...Am I missing something?


Table 450.3(B) 

For the type of transformer he is installing there will be secondary protection.


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## Magnettica (Jan 23, 2007)

Bob Badger said:


> Table 450.3(B)
> 
> For the type of transformer he is installing there will be secondary protection.


So the feeders would need to be larger that #2 copper?


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## Magnettica (Jan 23, 2007)

Bob could you explain note 3 in 450.3(B)?


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## NolaTigaBait (Oct 19, 2008)

Magnettica said:


> So the feeders would need to be larger that #2 copper?


Thats the way I understand it now...If you were to fuse it at say 150 amps then you would need 1/0 copper on the primary side...then it says 125% on the secondary side...


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## Magnettica (Jan 23, 2007)

NolaTigaBait said:


> Thats the way I understand it now...If you were to fuse it at say 150 amps then you would need 1/0 copper on the primary side...then it says 125% on the secondary side...


Primary and secondary protection is at 250% though.


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## NolaTigaBait (Oct 19, 2008)

Magnettica said:


> Primary and secondary protection is at 250% though.


It says 250% primary and on the secondary its up to 125%


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## bobelectric (Feb 24, 2007)

What would Joe say? Good Night!


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## Magnettica (Jan 23, 2007)

NolaTigaBait said:


> It says 250% primary and on the secondary its up to 125%


Table 450.3(B)

Currents of 9 Amperes or More

Primary and Secondary Protection 250% (See Note 3)


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