3 Phase Circuit Breaker/Delta Leg

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kencoel

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Question: When there is an delta leg (stinger) does a standard 3P 120/240 breaker able to be used with this application?
 
Sure, as long as it's a 3 phase panel.

There ARE residential applications where the panel is 3 phase and the high leg only goes in and out of the 3rd pole of the special breaker.
 
See 240.85 for an explanation. The 120/240 means that where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker's voltage rating is where it is permitted to be used.
 
Here's my idea on why a straight 240V breaker is needed.

On a single phase system, there is 240V present between the two hots. Each hot is 120V to ground. A 120/240V breaker is capable of interrupting a 240V fault (up to its AIR rating) across both poles, (240V) and is also capable of interrupting a fault on either pole to ground (120V).

If a 2 pole breaker is installed on a 240/120 3PH 4W delta system, and one pole is on the high leg, it can interrupt a phase to phase fault, and a phase to ground fault on the 120V leg just like the single phase system. If a phase to ground fault occurs on the high leg pole, then there's 208V across one pole. It cannot safely interrupt a fault higher than 120V across one pole.

A straight 240V breaker can interrupt a fault across both poles at 240V, or either pole to ground at 240V.

This is also why a straight 240V breaker costs substantially more than a 120/240V one does.
 
220/221 said:
There ARE residential applications where the panel is 3 phase and the high leg only goes in and out of the 3rd pole of the special breaker.
Click to expand...

These special "Delta" breakers are no longer allowed in new construction.

120/240V breakers cannot be used on the high/stinger leg.
 
My bad...I assumed something.

But....how could/why would a 3 pole breaker be rated 120???


Bear with me. I am slow sometimes

See 240.85 for an explanation. The 120/240 means that where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker's voltage rating is where it is permitted to be used.
Click to expand...

Oh...yeah that :)
 
coulter said:
Wouldn't a 240/120 slash rating be okay in a 208/120, 3 ph panel?

carl
Click to expand...

Yes it would, on a 208Y/120 3PH 4W system.
208V L-L does not exceed the high voltage rating of the breaker
120V L-G does not exceed the the low voltage rating of the breaker.

But the OP was about a slash rated breaker on a 240/120 3P 4W system where
240V L-L does not exceed the high voltage rating of the breaker
208V L-G does exceed the the low voltage rating of the breaker

Most 240V three pole breakers are not slash rated so this discussion does not apply to them.
 
Clarrification

Clarrification

OK, somewhat confused with multiple answers. But...I believe then that a standard 3P breaker can be used when there is a delta phase connected?
 
kencoel said:
OK, somewhat confused with multiple answers. But...I believe then that a standard 3P breaker can be used when there is a delta phase connected?
Click to expand...

As long as the breaker is not slash rated for a lower voltage.
Not all 3 pole breakers are "fully" rated, although I don't know of any that are less than 208V L-G.
 
220/221 said:
My bad...I assumed something.

But....how could/why would a 3 pole breaker be rated 120???


Bear with me. I am slow sometimes



Oh...yeah that :)
Click to expand...


Slash rated components are intended for use ONLY on solidly grounded wye power systems.

What does a rating of 120/240 mean?
In plain english it means the device can interrupt a maximum of 240 volts across two or more pole and a maximum of 120 volts across one pole

On a 208 volt solidly grounded wye system the maximum line-line voltage is 208 (2 poles), and the maximum line-gnd voltage is 120 (one pole).
 
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