Overcurrent for transformer

[cowboyjwc]

OK I'm having a problem with a plan check sitting on my desk.

I have two 30kva 208-480 3 phase transformers. One is feeding a 17.8 kva A/C unit and the other is feeding a 14.4kva A/C unit.

One is being fed with a 70 amp breaker (3 #4) and the other is being fed with a 50 amp breaker (3 #6)

When I check my cheat sheet and run the calcs I come up with a minimum 83 amps of protection required.

Am I missing something or is the engineer mistaken? I'm guessing that because they sent out the wrong voltage A/C units and that the pipe has already been run (1") that they are trying to make the wire fit.

 

[charlie b]

There is no minimum. There is a maximum that depends on the rated current and whether or not you will be protecting the secondary separately. But you can protect a 30 KVA transformer with a 15 amp breaker, if you want. Probably a waste of resources, but not a violation.

 

[charlie b]

By the way, are they stepping up a 208 volt supply system, to feed a 480 volt load? Or is it the other way around?

 

[cowboyjwc]

They are stepping up the voltage.

Sorry you are correct I should have said maximum. I guess my concern is if everything is almost identical why the difference in overcurrent?

Thanks Charlie, right after I posted this I found some papers stuck in the back of a pile explaining the exact same thing you said. They are sizing it to the load that will be drawn.

 

[JohnJ0906]

Perhaps they are basing the OCP on the size of the load? Just a guess.:-?

 

[cowboyjwc]

That is what I found out they are doing. Thanks.

 

[dahualin]

I am just curious. Does the drawing show a fused disconnect or enclosed circuit breaker for each A/C unit? I believe the A/C unit will give the MOCP or MFA. Where are the 70A and 50 circuit breaker fed to the two transformers, panelboard in electrical room or enclosed circuit breaker next to the transformer?

 

[cowboyjwc]

The breakers for the transformers are in the MDP in the electric room. The Transformers are rainproof on the roof. There is a disconnect between the transformer and the A/C unit that is calling out for 30 amp fuses.

The first plan check did not call for the disconnects. Actually the original plans called for 208v units.

One of those things where they are spending a lot of money to fix something when I think they just should have told the supplier to send out the correct A/C units.

 

[LawnGuyLandSparky]

The breakers for the transformers are in the MDP in the electric room. The Transformers are rainproof on the roof. There is a disconnect between the transformer and the A/C unit that is calling out for 30 amp fuses.

The first plan check did not call for the disconnects. Actually the original plans called for 208v units.

One of those things where they are spending a lot of money to fix something when I think they just should have told the supplier to send out the correct A/C units.

I was going to comment on why the hay a design would call for 480v units when the supply isn't, unless the load was loacted far off in the distance.

 

[thinfool]

why the hay a design would call for 480v units when the supply isn't

Human error, likely. Some years ago we did a chiller replacement at a local university. Chiller submittals went from the mech. contractor to the engr. (we never saw them), were approved and the chiller (80 tons) was delivered.

After the piping was complete, we went to see what we would need to do our part...I was reading the nameplate...480/3....oops..in a building that was 40 years old with a 208/120 service.

I think the engr lost most of his fees in correcting that one. We had to provide a step-up xfmr to make it work.

 

[dahualin]

The breakers for the transformers are in the MDP in the electric room. The Transformers are rainproof on the roof. There is a disconnect between the transformer and the A/C unit that is calling out for 30 amp fuses.

The first plan check did not call for the disconnects. Actually the original plans called for 208v units.

One of those things where they are spending a lot of money to fix something when I think they just should have told the supplier to send out the correct A/C units.

Is the disconnect a fusible one with 30 am fuses? If it is not, it shall be corrected. I think the circuit breaker in MDP doesn't meet the name plate requirement of A/C unit.

 

[cowboyjwc]

Well to tell you the truth I'm not sure what's there yet as I haven't done an inspection.

That's the difference between plan check and job. I can only tell you what the engineer drew, I don't know what the contractor installed.