Why no 125A 3 pole breakers?

[mbrooke]

Wild guess as stated above: heat dissipation. Middle pole might get toasty.

 

[Jraef]

It's about the potential energy in the circuit.

So if you look closely, you will see that the 2P 125A breakers are only listed as "slash" type, meaning 120/240V. That means that in the event of a asymmetric fault (line to ground), the fault energy seen by the circuit breaker is potentially never more than what will flow at 120V (line to ground). But in a 3 phase 240V delta system, even at the same amount of fault current the asymmetrical fault energy can be 240V to ground, so twice the potential fault energy . That's why you must use "straight" 240V rated 2 or 3 pole breakers in a 240V 3 phase panel and you can only use 1 pole slash rated breakers in a "High Leg" system on the 2 phases that are referenced to ground.

3 pole breakers would never be able to be used in a single phase panel, so they ALL have to be "straight" rated for 240V. When used in a 3 phase panels then, the worst case potential fault energy of an asymmetirical fault on a 3 pole breaker would be 2 phases to ground, so that would be twice the energy potential of a 2 pole 125A slash rated breaker.

 

[mbrooke]

So the question is, why can't they make a slash rated 3 pole breaker? Why can't say Eaton give their customers a $14 buck 3 pole?

 

[electrofelon]

It's about the potential energy in the circuit.

So if you look closely, you will see that the 2P 125A breakers are only listed as "slash" type, meaning 120/240V. That means that in the event of a asymmetric fault (line to ground), the fault energy seen by the circuit breaker is potentially never more than what will flow at 120V (line to ground). But in a 3 phase 240V delta system, even at the same amount of fault current the asymmetrical fault energy can be 240V to ground, so twice the potential fault energy . That's why you must use "straight" 240V rated 2 or 3 pole breakers in a 240V 3 phase panel and you can only use 1 pole slash rated breakers in a "High Leg" system on the 2 phases that are referenced to ground.

3 pole breakers would never be able to be used in a single phase panel, so they ALL have to be "straight" rated for 240V. When used in a 3 phase panels then, the worst case potential fault energy of an asymmetirical fault on a 3 pole breaker would be 2 phases to ground, so that would be twice the energy potential of a 2 pole 125A slash rated breaker.

I did propose this as a possibility in post #12

 

[LarryFine]

I did propose this as a possibility in post #12

Well, you mention the voltage ratings, but I claim bragging rights about the available energy in #19.

 

[oldsparky52]

In the past, I've had good luck talking to SQ D, Siemens, not so much.

Have you tried calling one of them?

 

[oldsparky52]

Reading through the square d documentation on 110/3, 125/3, & 150/3, it limits you to 20A cb’s on either side. So maybe this is the answer.


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You know, I bet that less than 2% of electricians are aware of that limit. Thank you.

 

[electrofelon]

You know, I bet that less than 2% of electricians are aware of that limit. Thank you.

Hey, if the breaker fits, even if you have to defeat the CTL rejection, it's all good

 

[oldsparky52]

Hey, if the breaker fits, even if you have to defeat the CTL rejection, it's all good

I worked for a man during the early 70's that would cut out that out of twin breakers (Bryant) so he could install them in panels not designed for them. I was too new to know that that's a no no.

 

[GoldDigger]

I like the idea of slash rating.

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[petersonra]

maybe the marketing people decided there was not enough demand for such a breaker so they didn't bother to spend the money on it.