Temp Scenario

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horsegoer said:
My confusion was I didn’t think it was possible or “electrically” correct to splice/tap a 3 phase conductor cable to (3) cables each with 2 phases.
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As long as the tap rules for ampacity and OCPD are followed, taking only two phases (2 hot one neutral) in different combinations to each disconnect would be compliant.
 
Think about it this way. A two pole breaker on a three phase buss.
Each two pole breaker installed On phase AB,CA,BC.

Now following the tap rule conductor sizing it can be done in a j- box with tap conductors feeding each single phase disc.
 
240.21C (4) 1 requires that the outdoor secondary of a transformer of unlimited length, hit ONE circuit breaker of set of fuses. So you can not splice to each building before having a OCPD. I would also pull all 3 phases and divide is up at your splice box (example building 1 a/b, building 2 b/c and building 3 c/a) If you choose to only take 2 hot legs to the splice box, no reduction in neutral size is allowed. The OCPD size would still be the same amperage if it were 3 phase, 175 amps.
 
When your foreman stated that. It may have been based on one of your statements.
A sample would be; run three wires from transformer to j box then tap off cables to trailer disc. That would tell me two hots netural nad EGC.
Then his statement makes since.
Other than the word balance.

Not to add confusing to the thread however important related to balancing. Your single phase will have an unbalanced netural load no matter the load, line to grounded balance. This is due to single phase from a three phase system.
No harm no foul just be aware that the netural of the single phase will carry darn near the same current as the phase conductors no matter what on the 120 volt loads.
So you will have three current carrying conductors for your calcs. Again no big deal.

With you SDS at the MDP you will have to or should factor in voltage drop to the source and size you cable from that.
You have mentioned 100' difference in primary distance that now is added to the secondary.
Then you feeder length to trailer has increased by 100'.

The majority of your load is line to netural. Since it is single phase from three phase you have to consider the voltage drop on the netural and use connected at 120 versus 208 for you netural VD calculation.

Last time I brought this up you did not have time to learn and understand.
This one reason to run the primary further ( the extra 100') because your using smaller conductors and three cables for them now.
That extra.100' using VD at 120 will require a larger cable for the load.
You can not balance out that netural load due to stated above.

Use 80% of the 125 for your netural load voltage drop and size cable from that.
Now when you do that your EGC must be sized from the ratio difference.
That is if you doing three cables from secondary side of the trans being located by MDP.

I would recommend you design with a one line and let foreman review. This will keep a lot of confusion from taking place as the one line can be easy revised.
If the formen won't budge on three cables then design with disc right after trans and run 3- p 4- wire feeder to j-box.
Size by voltage drops from that.
By doing that you will have less voltage drop on the netural due to bigger cable with out increasing the EGC. Based on a 200 amp feeder to j- box.

I seen that you planned on using smaller fuses in The 200 amp secondary OCPD. I would size based on VD for the load. You may want a 150 to 200 amp feeder to the j- box. Then size you secondary based on this. If you plan and size 200 you should be fine for the 100' change in voltage. Run the numbers.

As others said it's the owners money. This is one reason we do not let owner make these choices. Tell us what you need and let us do our job.
He is going to complain big time when he gets the bill due to all the money spent with no results no including lost time. Or. Not.

So for quick re-cap.
Step one simple one line. If Both agree.

Step two do calcs.
Sub step start with minimum than adjust for VD etc.

Step three final size conductors.

Step four size OCPD protection.

Step five add all conductor and OCPD to one line.
Done.
Get approval.
Order material
Install.

All other steps should fall in line in the field based on site conditions by a competent foreman.
 
Two one minute drawing for step 1
 

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infinity said:
You are using all three phases.
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He said we needed to use mc cable with THREE phase conductors NOT TWO because we had to balance out the phases.
How would we be using all three phases on the xfmr if we were to run a cable with o ly TWO phase conductors
 
horsegoer said:
He said we needed to use mc cable with THREE phase conductors NOT TWO because we had to balance out the phases.
How would we be using all three phases on the xfmr if we were to run a cable with o ly TWO phase conductors
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Answered in Posts 7,12 & 23.
You run 3 phases to your j box (A-B-C) then single phase to each trailer (A-B, B-C, A-C)
 
horsegoer said:
He said we needed to use mc cable with THREE phase conductors NOT TWO because we had to balance out the phases.
How would we be using all three phases on the xfmr if we were to run a cable with o ly TWO phase conductors
Click to expand...
He obviously confused if he's referring to the cable between the disconnect and the panel.
 
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