Options to eliminate main breaker trips on hard motor starts

F

fastline

Senior Member
Location
midwest usa
Occupation
Engineer
I'm still collecting some info so bear with me but basically have a situation where industrial equipment is being started on single phase. It is configured with a 50kva transformer, and 200A main breaker at the meter, then another 200A inside the bld. The one at the meter trips very frequent. I did test work on this months ago and starting I think it was a 30-40hp motor, inrush was like 380A. I firmly believe there is nothing really wrong with any equipment here, just not matched and setup well.

they are checking the service conductor size but believe it is 350 Al. Because those conductors on the load side of the meter, and are also passing through another 200A main breaker only 100ft away, it sort of tosses this in an odd place. If the POCO moves this power before the meter, they rarely have an OCPD at all, but because it's after the meter, I'm not sure if the POCO would have enough authority to oversize the breaker?

Basically what I want is either that breaker raised to a 400A, or a motor rated breaker that has more tolerance for motor starts. Because this is after the meter, the meter head really needs pulled to do any swap, which complicates the red tape. We used to do this all the time before smart meters....lol
 
A few comments:

(1) 230.90(A) Exception 1 permits oversizing the service OCPD relative to the service conductor ampacity in accordance with 430.52, 430.62, and 430.63 as usual.

(2) A 50 kVA single phase transformer at 240V has a rated current of 208A. After 125% and the next size up, you'd be limited to at most 300A OCPD due to the transformer. That may be a lower limit than what Article 430 would otherwise allow.

(3) Table 430.250 gives an FLC of 104A for a 40HP 240V 3-phase motor. So if the service is 240V single phase, and the phase converter is 100% efficient, that would require conductors of 104 * sqrt(3) * 125% = 225A ampacity. I.e. a 240V single phase service with 200A conductors is too small for 40 HP.

Cheers, Wayne
 
wwhitney said:
A few comments:

(1) 230.90(A) Exception 1 permits oversizing the service OCPD relative to the service conductor ampacity in accordance with 430.52, 430.62, and 430.63 as usual.

(2) A 50 kVA single phase transformer at 240V has a rated current of 208A. After 125% and the next size up, you'd be limited to at most 300A OCPD due to the transformer. That may be a lower limit than what Article 430 would otherwise allow.

(3) Table 430.250 gives an FLC of 104A for a 40HP 240V 3-phase motor. So if the service is 240V single phase, and the phase converter is 100% efficient, that would require conductors of 104 * sqrt(3) * 125% = 225A ampacity. I.e. a 240V single phase service with 200A conductors is too small for 40 HP.

Cheers, Wayne
Click to expand...
Appreciate! Other than 50kva transformer is utility owned and they do what they want.
 
I'm sure if you shopped for VFDs you could just put a three phase motor on it and be done with it. You call any drive manufacturer and they will hook you up with something that will work. If this motor is really a single phase 40 you've got a rare bird. If you ever need to "replace" it nobody is going to have one sitting on the shelf

What frame is this big single phase thing?
 
fastline said:
Appreciate! Other than 50kva transformer is utility owned and they do what they want.
Click to expand...
OK, so it's a 120V/240V single phase service supplying a phase converter supplying a 240V 3-phase motor.

Article 455, which I've never had an occasion to look at before, covers phase converters. 455.6 and 455.7 cover conductor and OCPD sizing, and the effect is that you have to protect the conductors supplying the phase converter at their ampacity.

So it seems to me that if you want to upsize the 200A service OCPD, along with any other OCPD between the service and the phase converter, you'd need to correspondingly upsize all the conductors between the service OCPD and the phase converter. As well as possibly upsize the phase converter itself, as the OCPD is limited to 125% of the phase converter nameplate single phase input amps.

Cheers, Wayne
 
Yeah, it's going to get tricky because at least with prelim inrush data, the inrush should not be tripping that breaker. But I may have to get the scope involved to see the true 1-5 cycle situation. Not enthused to do that. regardless of NEC, I know that service set is never current protected if overhead, and dives right into the meter head. But this is underground so they kick this out with a breaker.

I need to find a compliant way to stop the trippies on that breaker. BUT, what I also learned is that is a milbank specialty busbar breaker. Super! I don't think you can even get a higher ampacity than 200A.

I might kick this to the POCO and make it their problem to solve because I suspect they may have to change some gear. Regardless of any start, this would come down to what inrush spec is acceptable. At the moment, I can't find either data for the breaker, or NEC code that explains this.
 
POCOs in this area quit providing meters with disconnects in this area years ago. They were expensive to maintain and failePOCOs in this area quit providing meters with disconnects in this area years ago. They were expensive to maintain and failed to turn back on when tripped. Often enough the breakers were removed by the area lineman. See if they will provide you with a 320. NEC doesn’t require a disconnect at the meter and as you said the POCO can do what it wants.
 
Like at least one other posted I would look into a VFD. We only had one soft starter for a 250 HP fire pump. I do not have any experience on soft stwarters other then PM 'ing the fire pump. If the motor runs all day a VFD set with at least a 10 second ramp up time. If you are not ul on drives would suggest contacting authorized drive companies to see how much they charge for a start up. I would install or rep!ace a few drives and install the basic parameters ( motor volts, ampere & speed, ramp up & down time & a few others ) then call our great drive compsny to perform start ups mostly to get the free parts & Labor warranty extended to three years.
 
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