# Transformer and NEC OCPD code article 450 and 240



## iwire (Mar 9, 2013)

Can you verify if this is ok?

Total load on the system is approximately 200A

It will be fed by a 3 phase Delta 480V 75KVA transformer to a 208/120 Y electrical panel 225A main breaker.

The transformer is rated at 90A on the primary side and 208A on the secondary side using maths below

75KVA/(1.73x480V) = 90A

75KVA/(1.73X208) = 208A

480V Main Service to 100A Service Disconnect and to transformer wired with ((3) #1 and (1) #8 gnd) and then from transformer 75KVA wired with ((4) 250 KCMIL &amp; (1) #2 gnd) to 225A electrical main breaker. There will be also a 225A rated meter connected from the electrical panel with (4) 250KCmil &amp; (1) #2 gnd.

According to article 450, I should cover my overcurrent protection on the primary and secondary side and also my wire size accordingly to article 240. right? Secondary ground is based on table 250.66


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## Wildsoldier PE (Mar 10, 2013)

iwire said:


> Can you verify if this is ok?
> 
> Total load on the system is approximately 200A
> 
> ...


I dont know how close to 200 amp at 208V 3phase you are but if you use the 75kva transformer you be fine with using 100A in the primary side...however i dont think you need a disconnect on the primary side....a 100A breaker in the panel is fine..this shall protect the primary side. On secondary side 225A main breaker will protect the secondary side of the transformer.

Regarding wiring. I dont know how far is for voltage drop but i would say without counting for voltage drop and using copper wire.

Primary side for 100A, (3)-#3 and (1)-#8 G in 1 inch Emt conduit

Secondary side 225A, (4)-#4/0 and (1)-#4 G in 2 inch Emt conduit

Use #4 cu on your transformer equipment grounding.

If you use aluminum wire then:

Primary side 100A: (3)-#1 and (1)-#6G in 1-1/4 inch EMT conduit.

Secondary side 225A: (4)-#300 &amp; (1)-#2G in 2-1/2 inch EMT conduit

Use #2 AL for your transformer equipment grounding.


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## iwire (Mar 10, 2013)

Hi Wildi

Thanks for the respond. However I have a question on the wiring size. Forgotten to add, the length is around 10ft-15ft, so voltage drop can be disregard.

On the secondary side, based on the table 310.15(B)(16), #4/0 is sufficient to support 225A but because of there are 4 conductors carrying current (neutral) included and rated at 90C

Using 0.8 X 350A = 280A &gt; 260A from table 350.15(B)(16), i should be using 350KCMIL right? in 2.5" EMT conduit


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## Wildsoldier PE (Mar 10, 2013)

Why are you using 90 degree?


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## iwire (Mar 10, 2013)

Wildsoldier PE said:


> Why are you using 90 degree?


I read it somewhere about using 90C and also the area where the equipment be is not ventilated and indoor and has no AC whatsoever. Also, i think the secondary ground should be #2 based on 250.66? right?


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## iwire (Mar 11, 2013)

Another question, do I need to have a 125% capacity on my secondary protection for the branch circuit? If my total load is 208A, my protection should be in 260A, should the next size up should be 275A as main breaker on my electrical panel on the secondary side?


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## ventilator (Mar 11, 2013)

You are allowed to use 90deg C rating (if you cable insulation is 90deg cable) for derating purposes based on ambient and # of conductors in a raceway. However, you are limited by the rating of the termination points in your system which I can almost guarantee are 75deg rated as there are very few devices rated for 90deg at their termination points. This means that you take the ampacity of you wire at 90deg, apply all derating facators and then compare with the 75deg rating. If it is lower than the 75deg rating go to the next cable size up and try again.

As for the 125% on the secondary, code allows you to size the secondary protection on your secondary up to 125%. You don't have to go this high, it is just the max you are allowed to go up to. You just have to make sure that the wiring for the secondary side is sized to match the OCPD you are using. By section 240.6(A) the next standard size up according to code from 250A is 300A.

For grounding, 250.66 is for service grounding. For equipment grounding use 250.122 which is where wildsoldier got teh #4 from.

And not to muddy the waters but if you only have a total of about 10-15ft of wiring you can use the tap rule to reduce the wire size based on 240.21(B)(3). Personally, I never use the tap rule as it is difficult to guarantee the field install with stay within the confines of the rule but just thought I would point it out for informational purposes.


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## iwire (Mar 11, 2013)

thanks. Can you guys take a look at my sketch up? I made changes according to the comments and etc

SKMBT_C45213031110380.pdf


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## Wildsoldier PE (Mar 11, 2013)

Looks like you have 2 continuous loads that can take 100 amp each at the same time...I thought that most of the stuff were outlets etc. that not all are running at the same time.

201.5 x 1.25=251 amps

Use 250A for breaker size on secondary.

Use 110A breaker for primary.

Size wire using the 75Deg. column.

You shall be fine.

I would not add anything else for future to that panel...if you do you probably shall consider going to 112.5 kva transformer.


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## iwire (Mar 11, 2013)

Is because multiple continuous loads, we are using the 125%. However do we still need to use the 125% if the total load for continuous already been accounted for 125% in the load schedule?


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## Wildsoldier PE (Mar 12, 2013)

Does the 100 amp for each load already have the 1.25% on it?


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## iwire (Mar 12, 2013)

Wildsoldier PE said:


> Does the 100 amp for each load already have the 1.25% on it?






Each of the 100A circuit load already accounted with 125% on it.


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## Wildsoldier PE (Mar 12, 2013)

iwire said:


> Wildsoldier PE said:
> 
> 
> > Does the 100 amp for each load already have the 1.25% on it?
> ...


If that's the case then 100 primary and 225 secondary will be fine....I thought you gave full load amps on the first post.


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## iwire (Mar 13, 2013)

thanks!


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## blvengineers (Mar 19, 2013)

Wildsoldier PE said:


> iwire said:
> 
> 
> > Can you verify if this is ok?
> ...


The overcurrent on the primary and secondary are allowed to vary up to 150% of the nominal rating. The wire size can remain per the listed current rating. I have seen fusing on the primary side from 90 amps up to 110 amps. You will not need a primary disconnect if the transformer is in the same room as the 480 volt gear feeding it, Otherwise, use disconnect.


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## iwire (Mar 20, 2013)

blvengineers said:


> Wildsoldier PE said:
> 
> 
> > iwire said:
> ...


thanks..the reason there is disconnect it is an existing disconnect


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