How much overload can utility transformer do?

fastline said:
Here is where I'm at. After testing, I feel comfortable that the primary and transformer and performing admirably. Here is the first issue. Primary comes in and hits the dedicated 50kva xfmr and fed out on two line sets, one for house, one for shop, both with 200A mains right at the xfmr. That is an issue as the 200A breaker is getting trippy with big motor starts. The lineset is 4/0 Al and I need a bigger breaker on that! Can someone kick me a code compliant way to get this done? The panel in the shop also has a 200A breaker but has never tripped. Personally, I feel the breaker is not right but it also needs increased so that is not tripping. This is mostly an inrush issue, not FLA issue.
Click to expand...

If you have two 200A breakers in series, and only one is tripping during motor starting, then the first step before upsizing the breaker is to ensure that there isn't a problem with the breaker or its attachment.

It may be that the two breakers are in different parts of the allowed tolerance band and the more sensitive breaker is tripping.

But the other possibility is that something is causing excess heating at one breaker, making it prone to tripping.
 
winnie said:
If you have two 200A breakers in series, and only one is tripping during motor starting, then the first step before upsizing the breaker is to ensure that there isn't a problem with the breaker or its attachment.

It may be that the two breakers are in different parts of the allowed tolerance band and the more sensitive breaker is tripping.

But the other possibility is that something is causing excess heating at one breaker, making it prone to tripping.
Click to expand...
Different brands as well.
 
Indeed this is a new service, new transformer and breakers, etc. The panel at the xfmr is the one with meter above, then breakers on bottom right at the outfeed. Both breakers are Eaton, but when the one breaker trips, it is NOT into the time/current curve, it trips very instantly on a short circuit type trip, BUT that had just tested the load and I know it's fine! I think the reality is the inrush for that load is more than 2x the breaker size and it sees it as a short.

I'd rather just shoot the moon on that breaker so we can remove it as an issue. As well, these loads ran for years in other locations on a 200A breaker. It is possibly that one breaker has issues, but I also want it upsized. there is a main inside the building so yes, double breakered is what we got.
 
fastline said:
Primary comes in and hits the dedicated 50kva xfmr and fed out on two line sets, one for house, one for shop, both with 200A mains right at the xfmr. That is an issue as the 200A breaker is getting trippy with big motor starts. The lineset is 4/0 Al and I need a bigger breaker on that! Can someone kick me a code compliant way to get this done?
Click to expand...
Sure, 430.24 for conductor sizing, 430.63 for OCPD sizing. If the largest motor FLC is say 40A, and the rest of the load comes to 130A, then your MCA is 130+125% * 40 = 180A, which 4/0 Al at 75C is good for. And your OCPD size for an inverse time breaker is 130 + 250% * 40 = 230A. Which I guess you can round up to 250A.

[Although I still don't understand the "not less than" part of 430.63, seems like it should be "not more than when the conductors are sized per 430.24", just like 430.62 says.]

Cheers, Wayne
 
garbo said:
Like I posted in an earlier post it appears that utility companies think because a pole mounted transformer that is oil filled and has air circulation they can be continually overloaded by 250 to 300%.
Click to expand...
They also don't have same issues if it catches on fire when it is out on a pole vs when a transformer inside a building catches on fire because it is overloaded.

Just one reason they may allow intentionally allow overloading them.
 
I'd say if you go look at many pad mount xfmr installs by POCOs, the secondary side is nothing but polaris bricks with every home on the block on them and NO OCPD on that. Only OCPDs are in the homes as the main breaker, and of course the holy shi* candle in the xfmr on the primary side if thing get nutty.

My point is basically that the 200A breaker at the meter can is ridiculous IMO, considering how POCOs operate. I want to see it welded closed and let it rip. I was pretty say to hear the the little pipe that was used for the 4/0 so bigger party wires can't get done other than probably 4/0 copper and that would be costly with little return.
 
ptonsparky said:
At minimum the phase converter needs to be up and running before the compressor is allowed to start. It may need to run throughout the day.
Take enough meters to collect VD at all the pertinent locations at the same time. (Customers never have undersized conductors, right?:rolleyes:)
Click to expand...
Yeah, if they are starting the RPC at the same time as the air compressor, that’s a problem.
 
Might want to look into a soft start starter or a VFD if funds permit. Years ago we had an unloaded installed on a 100 HP air compressor that would trip breaker. Motor came up to speed then a few seconds later unloader would do its thing to have compressor producing air. Saw same thing in an old 8 cylinder refrigerant compressor maybe 150 HP. Would start up with the four cylinder unloaded then thru a timer one unloaded would start then maybe every ten seconds others would start.
fastline said:
I'd say if you go look at many pad mount xfmr installs by POCOs, the secondary side is nothing but polaris bricks with every home on the block on them and NO OCPD on that. Only OCPDs are in the homes as the main breaker, and of course the holy shi* candle in the xfmr on the primary side if thing get nutty.

My point is basically that the 200A breaker at the meter can is ridiculous IMO, considering how POCOs operate. I want to see it welded closed and let it rip. I was pretty say to hear the the little pipe that was used for the 4/0 so bigger party wires can't get done other than probably 4/0 copper and that would be costly with little return.
Click to expand...
Years cargo After a house had what was termed an electrical fire was due to old cloth covered service cable shorted out and luckily most of the damage was to the vinyl siding that melted. Line man from PECO told me they rather see you house on fire then have to spend a couple hundred dollars to send a guy out with his bucket truck to replace a property sized high voltage fuse feeding the transformer primary so they rather install a much higher ampere fuse.
 
I don't know if I missed it, but did you measure voltage at the service yet?

You could be end of line for the utility if it is rural and they might need to tap it up to help with the voltage drop issue. When they changed out the transformer they might not have tapped it up and it might have been previously tapped up. It happens.

The motor could be compensating for the low voltage and it could be tripping the breaker. If it has tripped quite a bit, it could also be a winding fault or intermittent issue with the condition of the equipment. You should ask them to replicate what they are doing when it trips. It is wierd to have a large motor trip the main and not their breaker or fused disconnect. As well as trip the main after replacing equipment but not before. It leads me to their being a safety issue that they might have been wiring around before hand and now that it is wired correctly, it is tripping.

That or it is all just normal inrush. Do we know the HP?
 
Elect117 said:
I don't know if I missed it, but did you measure voltage at the service yet?

You could be end of line for the utility if it is rural and they might need to tap it up to help with the voltage drop issue. When they changed out the transformer they might not have tapped it up and it might have been previously tapped up. It happens.

The motor could be compensating for the low voltage and it could be tripping the breaker. If it has tripped quite a bit, it could also be a winding fault or intermittent issue with the condition of the equipment. You should ask them to replicate what they are doing when it trips. It is wierd to have a large motor trip the main and not their breaker or fused disconnect. As well as trip the main after replacing equipment but not before. It leads me to their being a safety issue that they might have been wiring around before hand and now that it is wired correctly, it is tripping.

That or it is all just normal inrush. Do we know the HP?
Click to expand...
yes, I was there when it got trippy. Was started several times with normal values, which was near 500A inrush, and standing voltage of 242, dropped to 210. I find those values to be well within range considering the lineset alone should see an expected 15V drop, and if I consider the primary and xfmr, I'd say all of that is within norm. Actually damn good all things considered.

When it got trippy, it was obvious there was no delay or attempt for load to start. That breaker tripped instantly.
 
Back in the day I used to come across a lot of larger reciprocating air compressors that the motor ran continuously. They had an air governor on them like an engine driven compressor. Little air cylinder in the head kept the intake valve open once it got up to high limit.

I would think with the right size motor and some start and run capacitors it could work like that without having the rotary converter.
 
A continuous run, combination, start/stop operation head unloader. Control Switch selectable. I Put one together for a paint shop 20HP unit that was running on a weak open delta service, Shared with residential services.

Running a slightly smaller motor sheave will help too, as long as you don't take pump RPM lower than minimum, for proper crankcase lubrication.

But we have no clue of what type of pump it is. As mentioned previously, welding the breaker contacts might work too.
 
MTW said:
slightly smaller motor sheave will help too, as long as you don't take pump RPM lower than minimum, for proper crankcase lubrication.
Click to expand...

Yeah I've had to do that on a few that were in old buildings with insufficient pneumatic plumbing. Set the high limit up at about 200 to make the machinery function right, and a smaller sheave to keep the FLA in spec. Sometimes an additional receiver at the other end of the building, or at whatever machine uses high volume intermittently works even better and you can run a smaller compressor. But yeah, we know nothing about this system.

The problem with doing that stuff is the customer doesn't want to pay you for what you know, or for doing the math to make the POS cobbled together system in his 70 year old plant work
 
Joethemechanic said:
Yeah I've had to do that on a few that were in old buildings with insufficient pneumatic plumbing. Set the high limit up at about 200 to make the machinery function right, and a smaller sheave to keep the FLA in spec. Sometimes an additional receiver at the other end of the building, or at whatever machine uses high volume intermittently works even better and you can run a smaller compressor. But yeah, we know nothing about this system.

The problem with doing that stuff is the customer doesn't want to pay you for what you know, or for doing the math to make the POS cobbled together system in his 70 year old plant work
Click to expand...
Back in the 1970's the company that I worked at was having problem with not enough air pressure due to additional equipment added over the years. They had their own railroad siding and purchased a used we were told a liquid tank that was over 60' long..They would use it to store 125#'s of plant air life the receiver mentioned. Eventually the air pressure would drop causing equipment to shut down. They finally purchased a larger screw compressor and had a contractor replace 4" air line with a 6" pipe looped around production area. With a loop air was able to flow in both directions of 6" pipe.
 
Interesting, never thought about doing that. The main building at the plant has an air compressor at both ends of the line, one is the original, used as a backup if the other one goes down. I’ve got two 15 hp compressors alternating at one end now, so the old five horse is disconnected now.
 
You must log in or register to reply here.
Top