I have a breaker tripping and can't seem to figure out whats causing it. When ever we have a power outage, one breaker powering an out building trips, I will start @ the utility and walk you down stream to give the whole picture. Utility is 300 KVA MV-480/277 and is feed by 10 sets of 600MCM to a 4000A switchboard, we are then feeding a distribution panel with a 800 amp breaker. Out of this we are coming off a 125 amp breaker feeding a 75 KVA trans 480/208/120 feeding the load center in the building.The 125 amp breaker is the one tripping. Under full load it has 63.5 amps on A phase and 53.5 on B phase and about 8 amps on the neutral. What am I missing?
Breaker tripping
- Thread starter dconrad
- Start date
- Status
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Your missing inrush, it is possable your 125 amp breaker might be tripping on inrush, code allows a larger primary protection, but it depends upon if there is secondary protection also as to how much.
Well, at least the 800 isn't tripping
Joking, but when the selective coordination was addressed they may have just looked at the 125 in reference to the breakers ahead of it.
I agree with Hurk that inrush is most likely your problem. You need to look at 450,3 and 240.21 and see if you can increase the breaker to accommodate the inrush.
zog
Senior Member
- Location
- Charlotte, NC
Does the breaker have a trip unit or is it thermal mag? knowing the type of trip you are getting is the key to starting your troubleshooting.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Selective coordination does not look at the loads, impedance of the feeding conductors, and the transformer impedance is what will determine the inrush current, this is over looked all the time by some engineers.
brian john
Senior Member
- Location
- Leesburg, VA
Inrush current,
ptonsparky
Tom
- Occupation
- EC - retired
You're lucky it start with just the inrush, let alone any additional load.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The latest generation of transformers must meet federal guidelines (TP-1) for energy efficiency. These transformers experience a greatly different inrush current than most people are used to considering. In my experience, 75kVA units have an 'unbelievably' high inrush.inrush
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The loading of a transformer has very little effect on the inrush current of a transformer.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I'm sitting here trying to remember how to calculate the inrush and proper breaker size, but I'm not sure I have enough info from the OP, he PM me with the transformer info that has an impedance of 3.9 ohm's and the feeder from the MDP is 2/0 copper at approximately 300' run from the MDP, I know inrush can be up to 50 times the FLA of 90.2 amps, I'm thinking that I need the primary winding resistance to make this calculation?
The OP did say in post 1 that the MDP is fed via a 4ka service.
Been a while since I have done this one
The OP did say in post 1 that the MDP is fed via a 4ka service.
Been a while since I have done this one
Last edited:
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
He also stated that they ran a #6 EGC, which if the 2/0 was installed for voltage drop then the EGC would have to be up sized the same 4 times to a 2 awg. 250.122(B)
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I am not surprised that a 125A breaker cannot handle the inrush of that transformer, especially if it is a type HJ or EJB.
It is a Type -SO. Sorry about the bad picture
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Any chance your breaker has an adjustable mag trip setting, and it's set low?
If my memory serves me correctly, Square D KA Frames do have adjustable trip.
If so, as Jraef, mentions, you can possibly adjust the trip to withstand the inrush.
I do not know what effect that might have on your system coordination.
If so, as Jraef, mentions, you can possibly adjust the trip to withstand the inrush.
I do not know what effect that might have on your system coordination.
If you can duplicate the fault get a scope trace on the current going through the breaker that trips.
The amps squared x time of this current pulse should be less than the breaker's trip curve.
If it isn't you can go downstream with your current probe until you find what is putting the RMS value of this pulse over the limit.
The amps squared x time of this current pulse should be less than the breaker's trip curve.
If it isn't you can go downstream with your current probe until you find what is putting the RMS value of this pulse over the limit.
- Status