# 120/240 vs 240 2-pole breaker



## gunnut666 (Nov 24, 2011)

I've seen this discussed before, so I once knew, but I can't seem to find a good search term.
I can't seem to find out on the mfgrs site either.
What does this difference in voltage rating mean ?
I'm installing a welder receptacle, hot, hot, green. No neutral.
Thanks
Dave
Ampwise Electric


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## Jlarson (Jun 28, 2009)

What type of system is it? 120/240 single phase, 120/208 3 phase wye, 120 240 3 phase delta with a high leg?

A slash rated breaker can only be used where the voltage to ground doesn't exceed the lower rating, and the line to line voltage doesn't go over the higher voltage. I.E. ruling it out on the high leg of a three phase delta. 

The straight rated breaker can be used on a system where the voltage between line and ground or line to line doesn't exceed the rating.


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## gunnut666 (Nov 24, 2011)

Hi, 
The system is 120/208V 3 phase. No high leg.
All of the places I checked for the info had a generic pic of a breaker with no explanation of why one was twice the price of the other.
Also, this explains why the slash rated breaker is a stock item at the parts house, but not the straight rated.
I'm lucky, I get to use the cheaper breaker.

thanks
Dave


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## Big John (May 23, 2010)

Posted this in another thread regarding applying higher-voltage to breakers than they're labeled for. The second-to-last paragraph specifically addresses slash ratings:


> The higher the voltage on the system, the easier it is to sustain an arc and the more time it will last.
> 
> The higher the available fault current on the system, the more heat that arc produces.
> 
> ...


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## gunnut666 (Nov 24, 2011)

After being reminded what the slash rating referred to, it occured to me that multipole breakers are really just a convience because they have a factory installed common trip bar.
A 2-pole 120/240 is really: two 120V breakers with a common trip bar, 
and a 2-pole 240 is really: two 240V breakers with a common trip bar.
I hope I'm remembering correctly that my voltmeter reads 120V to ground on all poles when the circuit is operating correctly.

Am I just talking out of my hat, or do I understand the practical applications of slash rating.


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