1 Transformer 3 machines

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mmiller

Member
I have a question for you guys. I have 480v supply 3 phase and I need to use a transformer to get 240v 3 phase for 3 machines. IF I use a 75kva transformer I have enough power for the three machines. If I run a OCPD on the primary side do I need any on the secondary side ? http://forums.mikeholt.com/images/smilies/confused.gif
Each machine has a main breaker as the main power hookup.

These are plastic injection molding machines.

Thanks
 

pete m.

Senior Member
Location
Ohio
As long as you protect the transformer in accordance with 450.3(B) and your secondary conductors are protected in accordance with 240.21(C)(1) I think you would have a code compliant installation.

Pete
 

raider1

Senior Member
Staff member
Location
Logan, Utah
The secondary conductors of the transformer must be protected from overcurrent in accordance with one of the methods provided in 240.21(C).

Chris
 

erickench

Senior Member
Location
Brooklyn, NY
Take a look at NEC 240.4(F). If the transformer is a delta-delta then the secondary conductors are permitted to be protected by the primary OCPD if in accordance with the rules of NEC 450.3.
 

mmiller

Member
OKay conductor size then....

OKay conductor size then....

So my runs are not any more than 25ft. Looking at 240.21(B)(2) each tap must be of 1/3 amperage of the feed conductor ampacity.

So If the secondary of my 75kva can make 180A then each set of my taps must be at least 60A or better and the main CB on the piece of equipment I think is 75A Then I should be able to run say #6 THHN or would #4 be better ?

or am I even reading this correctly ?

Thanks
Mike Miller :-?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
First look at 240.4(F). If I understand your situation, you have a 480 delta to 240 delta transformer. If this is the case, per 240.4(F) your secondary conductors are considered to be protected by the transformer primary OCPD if their ampacity meets the ratio rule (in this case 2:1)
If it is not a delta/delta transformer or the conductors don't meet the ratio rule, then you would need to meet the applicable rules in 240.21(C) (not B) as Chris advised in post #3. From what you say, probably 240.21(C)(3).

Be sure to also look at 250.30, and, if you do have delta-delta possibly 250.21(B)
 

mmiller

Member
Makes sense, but want to be 100% sure

Makes sense, but want to be 100% sure

So, to be properly protected the secondary conductors would have to be 100A capable or better according to 240.21(c)(3) So I would be running #3 to each unit does this sound correct ? But the total name plate load of the machines would be around 225A and the measured is only around 30A when all machines would be running.

Can I add the 3 sets of the conductors together ? So if I have three sets of #4 THHN I would have more than the 100A of the primary OCPD, Or does each set need to be rated for the 100A ?

I appreciate all the help and guideance you guys have offered me, I really enjoy learning and doing the job correctly & safely

Regards,
Mike Miller

I don't know why the message has some crazy tabs setup or something strange going on
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
So, to be properly protected the secondary conductors would have to be 100A capable or better according to 240.21(c)(3) So I would be running #3 to each unit does this sound correct ? But the total name plate load of the machines would be around 225A and the measured is only around 30A when all machines would be running.

Mike, I'm not sure how you arrived at 100A, per 240.21(C)(3). Maybe you can elaborate.

In any case, I don't think you can apply 240.21(C)(3) to the installation that you described. 240.21(C)(3) says you have to comply with ALL of the conditions 1-4. Condition 3 says that all the secondary OPCDs are "grouped." You have described a 3 sets of secondary going to 3 machines with a main c/b at each machine. The OCPD's are not "grouped" in your case, so 240.21(C)(3) cannot apply.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Mike,
Lets go back a step.
Is the transformer a 480/240 3 wire delta ?
What size OCPD do you have on the primary ?
 

mmiller

Member
In can eloaborate

In can eloaborate

I have secondary hand book that stated if 240.21(C)(3) applied then secondary conductors would have to be at least equal to the oh....I read it wrong I think......The tap conductors must be at least equal to the secondary rating of the transformer, the give an example in the book of a tap feeding 8 x 100a fused switches. That means on my 75kva delta delta 480/240 transformer I have 180a available on the secondary and then I would need an awfully big wire (to me anyhow) Feeding each machine. It does terminate onto a 75A breaker on each machine.

But you are right I am talking about using 3 taps off the transformer one set for each machine......would this then not be applicable ?

The book is the 2005 McGraw Hill NEC Hand book

Mike
 
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