Same circuit fed twice

[don_resqcapt19]

Re: Same circuit fed twice

Joe,

IMO this is still a 20amp circuit that will trip when 20amps is exceded

As soon as you tie the circuits together there are two paths for the current. The current will flow on both paths to the load(s). The division of current on each breaker will be a function of the impedance of the paths and will not result in a 50%/50% split, but the load currents anywhere on this circuit will have to far exceed 20 amps for the breakers to trip.
Don

 

[roger]

Re: Same circuit fed twice

Joe, this is not my theory, it is the laws of physics. Current will take all paths back to source and in the drawing there are two equal paths for the current to take.

So, the load is 40 amps which will divide equally and never load either conductor or breaker past 20 amps, now if one side of this circuit does exceed 20 amps it will trip it's breaker and the whole load will then shift to the other breaker tripping it. (these breakers tripping at 20 amps is only theory, and as Don points the breakers may not trip at 20 amps))

Now, this is just food for thought and as far as this thread is concerned, I agree that 2_BEARS problem needs to be located and corrected.

Roger

[ February 11, 2006, 03:06 PM: Message edited by: roger ]

 

[infinity]

Re: Same circuit fed twice

Is this your theory, because a 20amp Brkr. will trip after 20amps is exceded, where do you come up with 40amps, and how do know that the current divides at the center. IMO this is still a 20amp circuit that will trip when 20amps is exceded .

I guess that you never had an iron worker walk on the job at 2:30 on a Friday and the GC tells you that you have to hook up his 40 amp, 208 volt welder so he can work on Saturday. So while you search through every existing panel in the building for a 2 pole 40 you get the bright idea to parallel 2-20 amp breakers on the same phase. Do this twice on 2 different phases and presto, 40 amp, 208 volt circuit. And yes it's a violation.

 

[jimwalker]

Re: Same circuit fed twice

And i would tell the GC first thing monday morning we will deal with this.If he did not think to ask for it early then shame on him.No way would i create that many violations just to get the welder man happy.Now if he wants it done right now then sign here for the charges at OT rate

 

[hardworkingstiff]

Re: Same circuit fed twice

Just out of curiosity, if we put the NEC aside, where is the safety/fire danger in what Infinity's hypothetical example suggested.

 

[infinity]

Re: Same circuit fed twice

Originally posted by hardworkingstiff:
Just out of curiosity, if we put the NEC aside, where is the safety/fire danger in what Infinity's hypothetical example suggested.

That was kind of my point. For a temporary, few hour installation it isn't really a safety issue. I'm sure that some that post here would never do it and well, some others that post here, have done far worse.

 

[2_BEARS]

Re: Same circuit fed twice

Gentleman, thankyou for your input. GeorgeStolz, your right I thought this scenario to be impossible also in retrospect until I found the two seperate circuits connected to the uncut tab on the input of a stacked toggle switch that my helper had been toggling
What brand/system do ya'll recommend for a good wire tracer...for instance to help find buried boxes?

 

[al hildenbrand]

Re: Same circuit fed twice

Originally posted by 2_BEARS:
Did I understand correctly that you were also saying that I need to seperate the two circuits neutrals also?

When the two circuits are fed from the same phase, the neutrals of the two circuits need to be seperate.

Looking from that one point out in the circuits where the two different neutrals are tied together, back through the neutrals to the panel, the current will likely see two different resistances, as the two neutrals are not likely to be exactly the same length, etc.

Say one neutral is 0.5 ohm between the panel and the tie point, and the other neutral is 0.75 ohm. 2/3 of the "40 A" current will flow in the 0.5 ohm neutral.

 

[jes25]

Re: Same circuit fed twice

Originally posted by al hildenbrand:


Say one neutral is 0.5 ohm between the panel and the tie point, and the other neutral is 0.75 ohm. 2/3 of the "40 A" current will flow in the 0.5 ohm neutral.

True, but as the 0.5 ohm neutral heated due to overloading wouldn't the current start to even out again as the heat would raise the resistance?

 

[LarryFine]

Re: Same circuit fed twice

Originally posted by al hildenbrand:
Say one neutral is 0.5 ohm between the panel and the tie point, and the other neutral is 0.75 ohm. 2/3 of the "40 A" current will flow in the 0.5 ohm neutral.

Originally posted by jes25:
True, but . . .

Not true, actually. The currents' ratio would be 3/5ths and 2/5ths, respectively.

Yes, 0.5 is 2/3 of 0.75, but that's not the same as proportion of the total series resistance.

 

[al hildenbrand]

Re: Same circuit fed twice

Joe,

By using the formula in Note 2 Table 9 in Chapter 8, a 0.5 ohm wire will get to 0.75 ohm at a temperature of 230?C or 471?F.

Seems to me the question would be whether the surrounding material is burning by then, or not.

 

[al hildenbrand]

Re: Same circuit fed twice

Originally posted by LarryFine:
The currents' ratio would be 3/5ths and 2/5ths, respectively.

How did you come to this?

 

[Jhr]

Re: Same circuit fed twice

So, the load is 40 amps which will divide equally and never load either conductor or breaker past 20 amps, now if one side of this circuit does exceed 20 amps it will trip it's breaker and the whole load will then shift to the other breaker tripping it. (these breakers tripping at 20 amps is only theory, and as Don points the breakers may not trip at 20 amps))

As I stated this is still a 20 amp circuit, if one breaker trips after the 20 amps is exceded (which you yourself mention in the quote above) you will not have a 40amp load, it will be what ever load is put on it and what makes it trip would be a load in excess of 20 amps, the circuit in question does not create a load, much less a 40 amp load, a load would be created by what ever current drawing equipment is put on it. The comment

these breakers tripping at 20 amps is only theory

is incorrect, a 20 amp breaker will trip not at 20amps but as Don and I stated at the ampreage that excedes 20amps. But to say that this circuit will create its own load just by the way its wired and the breakers holding till the wiring burns, is IMO incorrect. And as I stated in my first response JUST FIX THE DARN PROBLEM and 2 Bears will sleep well at night.

[ February 12, 2006, 06:10 PM: Message edited by: Jhr ]

 

[iwire]

Re: Same circuit fed twice

Joe I think your still missing it.

If you feed one receptacle from two 20 amp breakers on the same phase you will have a receptacle that can supply approximately 40 amps before either 20 amp breaker will trip.

It is not theory it is fact.

 

[roger]

Re: Same circuit fed twice

Joe, look at my graphic and think about it. Where do you interpret anything to mean a set of wires and breakers create a load, the zigzag gold line represents a resistor, i.e. a LOAD.

As far as 2_BEARS fixing his problem, we all agree, but it doesn't change the fact that these two 20 amp breakers can supply a 40 amp load without overloading a 20 amp conductor.

Roger

 

[growler]

Re: Same circuit fed twice

I did a fire repair job on an old house that had antique armored cable. There was no real fire just melted insulation all over the house. They had replaced 15 & 20 Amp. fuses with 30's but at first I couldn't figure out why even the 30's hadn't blown but after mapping out the circuits I found that they were fed from both ends. The insulation on the wire was ruined through out the house because everything was on those two circuits. Two circuits with four fuses. Really bad smell, nothing but burned insulation.

 

[Jhr]

Re: Same circuit fed twice

I guess that you never had an iron worker walk on the job at 2:30 on a Friday and the GC tells you that you have to hook up his 40 amp, 208 volt welder so he can work on Saturday. So while you search through every existing panel in the building for a 2 pole 40 you get the bright idea to parallel 2-20 amp breakers on the same phase. Do this twice on 2 different phases and presto, 40 amp, 208 volt circuit

. Worse case: Why go to all the trouble find a circuit that you know is not going to be used during the weekend turn the breaker off and disconnect the wiring and connect the welder to it and see you Monday with the extra breaker

 

[templdl]

Re: Same circuit fed twice

Part of the original post:

Initially it appears that a circuit is being fed twice from two separate breakers. The first clue to this is that two 20A breakers in the sub-panel are popping. Upon further investigation I find that if only one breaker is energized at a time then it holds HMMM...Yet when I flip the second they both trip. AHAH! So I think well must be the two feeds are coming from seperate bus legs...so when they both are energized the circuit sees 220VAC ?

This is a common characteristic of the electrician simply forgetting to clip that jumper between the two line screws of a "common yoke" convenience outlet fed via a home run circuit. You get a L-L bolted fault"The first clue to this is that two 20A breakers in the sub-panel are popping." and I find that if only one breaker is energized at a time then it holds HMMM...Yet when I flip the second they both trip.
or, what am I missing?
Dave T

 

[Jhr]

Re: Same circuit fed twice

If you feed one receptacle from two 20 amp breakers on the same phase you will have a receptacle that can supply approximately 40 amps before either 20 amp breaker will trip.

, besides why would you put 40amps of load on a circuit (the one in question) that you know is only rated at 20 amps . I will say it one more time JUST FIX THE DARN THING!

[ February 12, 2006, 06:50 PM: Message edited by: Jhr ]

 

[iwire]

Re: Same circuit fed twice

Sorry Joe, my mistake, most come here to learn new things others just to yell.

[ February 12, 2006, 06:56 PM: Message edited by: iwire ]