Where to start?

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mepengineer

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New York, NY
i dont know where to start, i am guessing that i need to find the transformer size from there i can find the primary/secondary
feeders and size of mcb
 

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charlie b

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Location
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I would start by informing the person who handed me this assignment that not enough information was provided. You can certainly assume that each panel will carry its entire 150 amp load (i.e., all of it running at the same time). But in real life, we get to apply demand factors to certain loads, so that the transformer need not be sized to handle 300 amps. You can also assume that all 300 amps worth of connected load will be running for periods of 3 hours or longer. That would require us to treat it as continuous loads, so that we need to apply a factor of 125%. But in real life, not all of the loads on a panel will run for 3 hour periods, so that the transformer need not be sized to handle 375 amps.

But if you want to get to the answer that the question's author is looking for, then I would start with a total load of 300 amps. From that, you derive the transformer rating. Then you size the primary and seconday conductors. Let us know what your results are, and we can help from there.
 

GoldDigger

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Placerville, CA, USA
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And I think there is either a poorly set part or a trap in the description of using TAP conductors. You will have to identify correctly the exact points on the diagram where the conductors sized by the tap rule will run.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
I don't think it's a poor question. It is designed to see what the person knows, and what would be reasonable assumptions.

Simply state assumptions and proceed; I would do part b. first. You want to use all standard equipment.
 

Smart $

Esteemed Member
Location
Ohio
I would go with 400A (standard equip)
2 x 200A panels
112.5KVA is to small, go with 150KVA

Size accordingly.
Depends on assumptions...

(150A?2)?(120V?3?)=108kVA

112.5kVA?(480V?sqrt3)=135.3... x250%=338.3A.... round down to 300A primary OCPD.
400AF/300AT disconnect, 300kcmil primary conductors.

112.5kVA?(120V?3?)=312.5A.... ?125%=390.6A.... round up to 400A secondary OCPD.
400AF/400AT disconnect, 600kcmil secondary conductors (also post-fuse because already using it, but could be downsized to 500kcmil, economics factor into decision).

(2) 200A MCB panels, 3/0 feeder taps

All ratings, sizes based on 75?C terminations, 75?C or better Cu conductors , no adjustment and correction required.

I leave grounding conductors as an exercise for the OP'er.... :D
 

mepengineer

Member
Location
New York, NY
Depends on assumptions...

(150A?2)?(120V?3?)=108kVA

112.5kVA?(480V?sqrt3)=135.3... x250%=338.3A.... round down to 300A primary OCPD.
400AF/300AT disconnect, 300kcmil primary conductors.

112.5kVA?(120V?3?)=312.5A.... ?125%=390.6A.... round up to 400A secondary OCPD.
400AF/400AT disconnect, 600kcmil secondary conductors (also post-fuse because already using it, but could be downsized to 500kcmil, economics factor into decision).

(2) 200A MCB panels, 3/0 feeder taps

All ratings, sizes based on 75?C terminations, 75?C or better Cu conductors , no adjustment and correction required.

I leave grounding conductors as an exercise for the OP'er.... :D


why are you going with 300A primary OCPD why not the next size up 400A?
also i have seen on a 112.4kva xfrm with a 175A breaker
 

Smart $

Esteemed Member
Location
Ohio
why are you going with 300A primary OCPD why not the next size up 400A?
also i have seen on a 112.4kva xfrm with a 175A breaker
Refer to Table 450.3(B).

My assessment is using the primary and secondary protection scheme. Note 1 only applies to the secondary protection. You could go higher where the Note 3 condition applies.

The 175A breaker scenario is using primary only protection. Note 1 is applicable.
 
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