Breaker application

randy

Supporting Member
R
Join Date
Apr 2002
Posts
158
Using a three pole breaker on a single phase transformer is there any special wiring technique that should be used? This is for the primary side of the transformer, which is supplied with 480V.

Thank you

rmonroe
 
Depends on the type I suppose. Most of the ones we use are just three single pole MCB's joined together, and the operating levers combined with a sleeve. So I would say yes. An over current in any of the poles will cause the device to trip. Take a close look at the one you have you may be able to remove one of the poles with some careful work. Or just leave it it wont do any harm.

cheers

myles
 
As usual I am not sure I understand what is being asked.

If 3ph in and 1ph out then a 3 pole breaker should be used on the primary.

If single phase in with single phase out, example 2 wire (1ph) 480 in and 1 ph out then a 2 pole breaker should be used AND care not to cause an imbalance on the supplying 3ph source.

There may be other variables involved depending on the devices fed by the 1ph transformer.
 
Genrally speaking, I normally try to protect transformers on both sides: primary and secondary , and using a three pole breaker for a single phase depends on the voltage I'm using:
  • If a single phase lets say has a Neutral (120V, 277V), I don't need to protect the neutral, I only protect the phase.
  • If a single phase is reffered to as (dual phase) (208, 240, or in this case 480V), Then you should use a " dual pole breaker" to protect both phases. But since a three pole breaker is handy, I would use it and i would double protect one of the phases( in and out to the third pole) as long you keep it Standard: your line/top and load/bottom,
  • I will make sure that my breakers are sized accordingly to my demand.


fav quote: The reason is Abstract, but the reaction is clear!!!!
 
There is nothing special that needs to be done. All UL Listed 3-pole breakers can be used to feed 1-pole or 2-pole loads with no loss of circuit protection or sensitivity.
 
The only "special technique" is to wire the incoming line to the TOP of the breaker, and the outgoing load to the BOTTOM. I know of cases where electricians have been electrocuted when the breaker was wired backwards. One fellow was troubleshooting a 13.6KV motor. He did not know that the main breaker (in OFf position) was wired backwards and stuck a meter probe on the bottom and was killed by the arcing current to ground.

The electricans should have checked, but we are all only human and make mistakes. A good trial lawyer will put you in the witness box and make you wish you had wired it standard.
 
Last edited:
Lancie1 said:
The only "special technique" is to wire the incoming line to the TOP of the breaker, and the outgoing LOAD to the bottom. I know of cases where electricians have been electrocuted when the breaker was wired backwards.
Click to expand...

That is why you should ALWAYS test it before you touch it. This is a fundamental part of a lockout/tagout program.

Never assume anything when it comes to electricity.
 
That is why you should ALWAYS test it before you touch it.
Click to expand...
He thought he WAS testing it. In this case the test is what killed him. This was the incident at the XXXXXXX XXXXXXXXXX Plant in Cherokee, Alabama. The floor was covered with damp (conductive material), the high voltage probe was dirty and untested, the breaker enclosure was poorly grounded, and the electrican was not wearing any protective equipment because he thought he was checking the dead end of the breaker to see if power was indeed off. He was an outside contractor called in to get the motor going. Later it was found that the plant electricians had wired the high-voltage line to the bottom of the breaker because the cables were cut too short to reach the top.
 
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Electric, mechanical, fluidpower etc., just plain NEVER ASSUME, check/test.

The best thing to do when approaching something broken or the first time is "KEEP HANDS OFF" and do a visual check then follow appropriate steps to verify all is de-enrgized before accessing the equipment.

Sometimes breakers are also mounted "sideways" ie horizontal instead of vertical. The "top" or input side of the breaker should be the side away from the switch when in the OFF position AND incoming power should be put there.

It gets "hairy" because some people will put breakers in a position where it is not obvious where the power supply is coming from and may wire it from the "wrong" side.

This can also happen with other energy devices. So please follow proper procedures.
 
abdou said:
Genrally speaking, I normally try to protect transformers on both sides: primary and secondary , and using a three pole breaker for a single phase depends on the voltage I'm using:
  • If a single phase lets say has a Neutral (120V, 277V), I don't need to protect the neutral, I only protect the phase.
  • If a single phase is reffered to as (dual phase) (208, 240, or in this case 480V), Then you should use a " dual pole breaker" to protect both phases. But since a three pole breaker is handy, I would use it and i would double protect one of the phases( in and out to the third pole) as long you keep it Standard: your line/top and load/bottom,
  • I will make sure that my breakers are sized accordingly to my demand.


fav quote: The reason is Abstract, but the reaction is clear!!!!
Click to expand...

Are you talking about a breaker that's hard wired on both ends (vs snapping/screwing into a buss bar)?

I would just put the breaker in, and wire to 2 of 3 the poles. We are running a few services like this. But it's on a big system, also with 277 loads, so it wasn't too hard to balance.

Had a crappy electrician wire in a breaker once, left the middle bus bar screw a bit loose, burned the lug. We now put a breaker over it, straddling that burnt lug on the buss bar, and use the other 2 legs to feed a 480 to 240 single phase transformer.
 
Bolt said:
Are you talking about a breaker that's hard wired on both ends (vs snapping/screwing into a buss bar)?

Yes, "that's what I m talking about" If it was a breaker used by itself not from a "panel of breakers".
Click to expand...
 
The NEC 110.3(B) allows breakers (actually it is the UL standards that do) to be wired "backwards" unless the brealer is specifically marked with LINE and LOAD designations. The only NEC requirement is that up is ON and down is OFF NEC 240.81.

OSHA (via NFPA 70E) requires PPE for all circuits (>49V) UNTIL they have been verified (i.e tested) as de-energized.
 
The NEC 110.3(B) allows breakers (actually it is the UL standards that do) to be wired "backwards" unless the brealer is specifically marked with LINE and LOAD designations.
Click to expand...
Yes, the NEC provides the minimum guidelines. If you do go against the "conventional method" or the "normal way", or the "what is usually expected", you had better have an excuse that is good enough to sacrifice some person's life.

Before you run your test to check for safe conditions, make sure you know the actual situation and double check that your test can not endanger your life.
 
Lancie1 said:
Yes, the NEC provides the minimum guidelines. If you do go against the "conventional method" or the "normal way", or the "what is usually expected", you had better have an excuse that is good enough to sacrifice some person's life.

Before you run your test to check for safe conditions, make sure you know the actual situation and double check that your test can not endanger your life.
Click to expand...

I am not judging the parties involved in the situation you detailed.

Bottom feed equipment is very common. Every manufacture of panelboards offers bottom fed main breakers. Roof top air handlers are normally bottom fed. Back in the '80s some motor control center buckets were bottom fed to the breaker and then out the top to the starter.

My point, which is the same as yours, is: Electricity Kills. Always assume the circuit is energized until you have tested it. Always wear the appropiate PPE for the task you are performing.

Never assume there is a normal manner, a normal color code, or a normal description. What you are used to is probably different somewhere else. If it is not prohibited by a code, you can be sure someone someplace is doing it that way.
 
I have no idea if this was something I was taught or just got into my head as the proper way to do it.

3pole.jpg


This is a basic 3 pole panel or din mount breaker, when in the OFF position I expect the terminals the switch is pointing towards to be de-energized.

I have no problems with vertical mounting but hate to see a breaker mounted upside down. If ON/OFF looks like this:
onoffupdown.jpg

then be very carful.

Just be careful, wear PE, and follow good sense even if the rules do not require it.
 

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