A 20 amp 120volt breaker feeds a transformer that steps it up to 277volts to run 4 lights to light of a breezway. The current draw on the primary side of this circuit is about 13 amps. It keeps tripping. It will go for about 1-3 month and trip again. We have changed the breaker, switch. checked the wiring and light fixtures. Could it be that the 20 amp breaker just can't handel the inrush of the transformer.
Breaker Tripping
- Thread starter JdoubleU
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jwjrw
Senior Member
Sorry but what do you mean switching the secondary side?
Install a switch on the 277v side.
jwjrw
Senior Member
You meant that kinda switch! Duh
Hameedulla-Ekhlas
Senior Member
- Location
- AFG
Dear friend,
yes, ofcourse it will trip becuase on the load side you draw 13 ampere with 277 volts. Since you have steped up the transformer ratio becomes 277/120 wich equals to 2.3. Because you have steped up voltage and with the same ratio current will fall from 20 ampere to 20/2.3 = 8.69 ampere and load needs 13 ampere that is the reason it trips.
best regards,
Hameedulalh Ekhlas
Electrical Engineer
Kabul Afghanistan
yes, ofcourse it will trip becuase on the load side you draw 13 ampere with 277 volts. Since you have steped up the transformer ratio becomes 277/120 wich equals to 2.3. Because you have steped up voltage and with the same ratio current will fall from 20 ampere to 20/2.3 = 8.69 ampere and load needs 13 ampere that is the reason it trips.
best regards,
Hameedulalh Ekhlas
Electrical Engineer
Kabul Afghanistan
I didn't see the transformer size, but with a 13 amp primary current it's not unusual to trip a 20 amp breaker due to inrush.
My suggestion would be to see if the gear manufacture has a high-inrush breaker (such as Square D QO120-HM)
My suggestion would be to see if the gear manufacture has a high-inrush breaker (such as Square D QO120-HM)
Reread the OP... Primary amps is 13.
I am not sure what you mean by this. I understand ratio and the current difference with the voltage difference. But I don't understand the rest. I have 13 amps on the primary side of the circuit at 120 volts. I am interested in what you are saying could you elaborate.
Hameedulla-Ekhlas
Senior Member
- Location
- AFG
Could you please tell me the wattage and number of lumps please
Does it make any difference whether it's five 60watts or or three 100watts?
The only thing I can tell you at this point is that I measured the current on the primary side and with everything on it read 13 amps. Why would you need to know the wattage and lumps. Not sure what lumps is. Sorry for not knowing the lingo.
mivey
Senior Member
Don't worry about it. H-E mis-read your post.
H-E meant lamps. Does not matter anyway.
RETRAINDAILY
Senior Member
- Location
- PHX, arizona
what is the name plate say ?
dont you size the OCD by the name plate
dont you use table 450.3(b) .
dont you size the OCD by the name plate
dont you use table 450.3(b) .
brian john
Senior Member
- Location
- Leesburg, VA
If you have 13 amps one would assume maybe a 2 kva transformer? With a rated primary current of 16 amps inrush might be 64 amps -160 amps, maybe higher if it is a actually a step down wired as a step up. It is reasonable to expect the 160 amps to trip a 20 amp CB every so often.
kwired
Electron manager
- Location
- NE Nebraska
How many lumps of sugar do you want in your coffee or tea? 
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Is this transformer switched on and off? If not, the inrush current would only appear the first time you turned it on. If it stays on the 13 amp load should not trip a 20 amp CB.
hillbilly1
Senior Member
- Location
- North Georgia mountains
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- Owner/electrical contractor
Yes,this is a common problem, from what I've been told by engineers, if you control this with an SCR, you will not have this problem. It seems to have something to do with at what point of the wave form it is shut off/on at. Sort of a capacitive effect, increases the inrush current. Changing to a larger primary breaker, or if it is a step down transformer with an adjustable magnetic trip breaker feeding the primary (which is not the case here) the trip would have to be increased. Run into this problem all of the time when a backup generator with an auto transfer switch is added to the equation. Usually trips on transfer back to utility.
RETRAINDAILY
Senior Member
- Location
- PHX, arizona
so if it has a primary current of 16 @1.25 is 20 and its tripping can you go 25? with out secondary protection. ? or install SOCD can you then go 250%?witch a 30 would work.
Either way it should be at lest 10 AWG IMO
looking at 450.3(b)
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