Can't quite figure this out.....?????

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peter d

Senior Member
Location
New England
As an aside to this discussion, in the UK and I think Ireland as well, the purposely install this exact circuit, which is (shockingly ;) ) called a "ring circuit." I believe the UK ring is rated for 32 amps and they have to use fusible cord caps (plugs) because of the high current capacity of the ring.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
What would stop each breaker from protecting its conductor?
The other breaker connected to the load side of it in parallel.
But only downstream of the point where they join. If you joined pigtails from two breakers together, then yes, everything downstream is over-fused.

But, each pigtail won't be overfused, nor subject to overload. The two breakers each protect one pigtail.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Or any of the devices or equipment connected at any point in the circuit.
Absolutely. Any single load would be a tap, or a 'tangent' as I used the word, and would be "downstream" of the joining point.

Lets not forget supplying an appliance designed for 20 amp supply with 30 amps is a violation and could be dangerous if the appliance has a fault.
Absolutely agreed. The tap, and anything it supplies that is dependent on the circuit's OCP, is now unprotected.

But yes the conductor 'loop' from one breaker to the other can only have about 15 amps in each direction.:)
There you go! That applies whether we're talking about a pair of 6" pigtails or 100' of NM.

It also points out what's wrong with supplying a partial circuit outage from anything other than its original circuit.
 

e57

Senior Member
But only downstream of the point where they join. If you joined pigtails from two breakers together, then yes, everything downstream is over-fused.
True - in some cases but not in others - But still wrong on many levels...

e.g. two cb's of the same leg feeding a 3-wire cable - the noodle is way under protected....

conversely - two 2-wire circuits joined on the hot or noodle or both. Sure they are in parallel in parts or portions - but in most cases of a branch circuit under #4 it would be a code violation - and the reasons for that are that smaller conductors may not be capable of handling a short in that way - and since the OCP was not intentionally tied together - there is no way to determine an over-load on one vs. the other since they would only carry the same current in a perfect world in a vacuum - only on the 3rd day of a leap year under a blue moon....
But, each pigtail won't be overfused, nor subject to overload. The two breakers each protect one pigtail.
Under a short - who knows what would happen - they might not trip at all - or be capable of much much higher inrush currents - since each CB was not designed to do so.

(You're not advocating such an install are you - or all of this is hypothetical?)
 

220/221

Senior Member
Location
AZ
Are you (Larry) suggesting that, if the 30A load in yucan2's drawing was moved just ahead of the splice point, it would trip the 15?


Excellent input. I'm straight. Hope the OP is also

Not yet.

Most say yes, some say no, others seem to say only in part of the circuit.
 
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yucan2

Senior Member
But does that apply to the OPs question? :):confused:

Now that I'm focused again, I can say without a doubt that yes, it will take somewhere in the vicinity of 30 amps to trip this circuit, because all things being equal, it would be electrically impossible (improbable :roll:), for only (1) ungrounded conductor to carry an overcurrent and the other be significantly less.

In the OP's description of the connection and provided no loads are placed before the splice, the current in both legs will always be approximately equal (in proportion), and must rise relatively evenly. Thus 30 amps will be required to open that circuit, though once again acknowledging that upstream of the splice will never see more than 15 amps.
 

e57

Senior Member
As an aside to this discussion, in the UK and I think Ireland as well, the purposely install this exact circuit, which is (shockingly ;) ) called a "ring circuit." I believe the UK ring is rated for 32 amps and they have to use fusible cord caps (plugs) because of the high current capacity of the ring.
They actually fave fused or CB protected recepticals - and the ring might have OCP on both conductors at the panel.

I've worked with a number of Irish and a few Brit, and French electricians in the past and they of course swear by them... And the 240 does keep the wire size down...

http://en.wikipedia.org/wiki/Ring_circuit
 

e57

Senior Member
Now that I'm focused again, I can say without a doubt that yes, it will take somewhere in the vicinity of 30 amps to trip this circuit, because all things being equal, it would be electrically impossible (improbable :roll:), for only (1) ungrounded conductor to carry an overcurrent and the other be significantly less.
Just by conductor length they will differ - if by fault the will differ wildly. A resistive or open splice on one may go un-noticed and force the other to carry a higher current. So - it is very probable and yes very possible.... To what degree - well that depends - but we don't design circuits like this for a reason, or rightly - reasons....
 

yucan2

Senior Member
Just by conductor length they will differ - if by fault the will differ wildly. A resistive or open splice on one may go un-noticed and force the other to carry a higher current. So - it is very probable and yes very possible.... To what degree - well that depends - but we don't design circuits like this for a reason, or rightly - reasons....

I think you miss the gist of the speculation of this now theoretical scenario. The key statement of this purely mathematical exercise is what if? As we know this was an accidental occurrence. I merely infer on the merits of operation. Not installation. Purely speculation as no one would knowingly install such an abortion, at least not in America.

We all understand the limitations. But the assumption(s) have been made that length of wire and resistance and other parameters are equal and will remain so for the purposes of simply analyzing the circuit.
 

iwire

Moderator
Staff member
Location
Massachusetts
Now that I'm focused again, I can say without a doubt that yes,

I just see it differently.

The OP never asked how much current would flow in any part of the circuit.

The question was simple and direct

Will it take 30 amps to trip this circuit?

IMO the simple and direct answer is is yes, approximately 30 amps.

But I am forgetting where I am, we never stay on topic. :cool:
 

yucan2

Senior Member
I just see it differently.

The OP never asked how much current would flow in any part of the circuit.

The question was simple and direct



IMO the simple and direct answer is is yes, approximately 30 amps.

But I am forgetting where I am, we never stay on topic. :cool:

In that context, I now agree as well.
 

e57

Senior Member
I think you miss the gist of the speculation of this now theoretical scenario. The key statement of this purely mathematical exercise is what if? As we know this was an accidental occurrence. I merely infer on the merits of operation. Not installation. Purely speculation as no one would knowingly install such an abortion, at least not in America.
[brit major]Sir - I do not intend to impugn your veracity SIR! But do you not realize that you contradict yourself in the statements above????[/brit major]
We all understand the limitations. But the assumption(s) have been made that length of wire and resistance and other parameters are equal and will remain so for the purposes of simply analyzing the circuit.
How could we assume equality of characteristics - when the hypothetical scenario is clearly made by error or ignorance.

(FWIW - I'm not looking to boost my post count - I'm shooting for a record high syllable count.... :D)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Are you (Larry) suggesting that, if the 30A load in yucan2's drawing was moved just ahead of the splice point, it would trip the 15?
No, only that (with equal path impedance, and thus, current division) each side of the parallel paths would still carry only half of the total current.

The relocation of such a 30a load would merely relocate the 'tap' position, and, in reality, shift the current division somewhat.

With enough imbalance, less than 30a total could overload one breaker, which will eventually trip and shift all of the load to the other breaker, which will trip quickly.
 

yucan2

Senior Member
[brit major]Sir - I do not intend to impugn your veracity SIR! But do you not realize that you contradict yourself in the statements above????[/brit major]
How could we assume equality of characteristics - when the hypothetical scenario is clearly made by error or ignorance.

(FWIW - I'm not looking to boost my post count - I'm shooting for a record high syllable count.... :D)

The OP made no mention of how long each conductor was. In order to provide any meaningful dialog, some assumptions must be made. Mine are too assume equal length and resistance. Obviously you choose to differ.

Please feel free to have at it.
 

Volta

Senior Member
Location
Columbus, Ohio
... though once again acknowledging that upstream of the splice will never see more than 15 amps.
That is, on the ungrounded conductor. The grounded will carry the full load.

Just by conductor length they will differ - if by fault the will differ wildly. A resistive or open splice on one may go un-noticed and force the other to carry a higher current. So - it is very probable and yes very possible.... To what degree - well that depends - but we don't design circuits like this for a reason, or rightly - reasons....
Of course, if the splice is open, we are back to a 15 amp circuit, overloaded as it may be.

I think you miss the gist of the speculation of this now theoretical scenario. The key statement of this purely mathematical exercise is what if? As we know this was an accidental occurrence. I merely infer on the merits of operation. Not installation. Purely speculation as no one would knowingly install such an abortion, at least not in America...
Never underestimate the power of the hacks! :D
 
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