Switches In Parallel

Smart $

Esteemed Member
Location
Ohio
That is not what I meant, I mean a load, a motor, a lamp, a heater etc. Now we can further confuse the issue with wall switches that have indicator lights that work by being placed in series with the load but please lets try not to continue drifting so far off for our own entertainment.

I have really lost all patience with trying to pick the pepper out of fly poop. I try to read the code for what it is and not rip apart every syllable. I have learned that once you start doing that the entire code book falls apart.
I know that's not what you intended to mean... but glad to see you are open minded enough to rescind with the lighted-indicator switch. :happyyes:

All in all, it's an age old debate which has no definitive resolution...
 

fifty60

Senior Member
Location
USA
Iwire let me see if I understand you correctly:

"The only conductors in parallel would be the short jumpers between relay contacts and they would not leave the control box in parallel only one conductor would leave the jumped contacts and continue to the light."

So two wires from each machine would go to the junction box. One for the coil, and one for the contacts of each relay. This would give me 6 wires going to the junction box. I can then have the load side of each contact go to the same terminal block, and then have a single wire leave this terminal block to activate the solenoid. Doing this would not create any code violations and also would not cause a problem if all 3 switches are on at the same time.

Smart $---Yes I would like switches on 3 separate machines to control a single solenoid.
 

fifty60

Senior Member
Location
USA
The machines are plugged into 3 different outlets. Is the problem in this the current imbalance? Can you please explain?

I could have the supply from one of the machines going through all 3 contacts at the junction box, but this would require that the machine whose source is providing power to the central coil be plugged in even if it is not the machine in use. Ideally I would want the solenoid to be activated by any 1 of the 3 machines regardless of the other 2 machines being plugged in or not. This is the reason for my original desire to have the central solenoid activated by 3 seperate switches.

Sounds like I cannot do this unless the 3 machines are combined to have a single power source. One plug to power them all.
 

Smart $

Esteemed Member
Location
Ohio
The machines are plugged into 3 different outlets. Is the problem in this the current imbalance? Can you please explain?

I could have the supply from one of the machines going through all 3 contacts at the junction box, but this would require that the machine whose source is providing power to the central coil be plugged in even if it is not the machine in use. Ideally I would want the solenoid to be activated by any 1 of the 3 machines regardless of the other 2 machines being plugged in or not. This is the reason for my original desire to have the central solenoid activated by 3 seperate switches.

Sounds like I cannot do this unless the 3 machines are combined to have a single power source. One plug to power them all.
Well for one, three cord and plug connected machines powering one solenoid is just outright bad design. For one, there's no guarantee the receptacles are powered by the same leg, and even if they are, they may not be the same circuit. Probably more, but that's enough in itself.

Exactly what are these switches? ...limit switch, for example... mostly need to know whether a dry contact (both sides not connected to machine power) or a wet contact (one or both sides connected to machine power).

As it stands now, I see only two options. Both require a separate power source for energizing the solenoid. One option is as iwire suggested using three relays with each coil energized by one machine's power. If wet contacts, this is the only option. The other option is dry contacts related to each machine. You'll either end up with four power sources in your three-relay enclosure or two power sources associated with each machine...
 

hurk27

Senior Member
I'm so confused, the OP started out by saying he wanted to control one light from three machines by using single pole switch's, which is all fine a dandy if the power source was kept to a single source, and as pointed out that you can't use different power sources even if they were on the same phase or leg but supplied by different breakers as then as each is turned on you now have conductors supplied by two or three breakers resulting in available current over the rating of the conductors and a very dangerous violation of the NEC, if they were on different phases or legs, then you would obversely have a dead short between two phase when the second switch is turned on.
But then the OP switch's to wanting to have the three switches to operate a solenoid? is the OP calling a contactor/relay a solenoid? strange wording if so.

My fix would be to supply an independent power source for the light, bringing two conductors to each machine for the single pole switch, or also with an independent power source use two 3-way switch's and one 4-way so the light can be turned on or off at each machine, if the machines are that close then why not just put a single wall switch close to them.

The other problem with the supplying the light circuit source from each machine is if the machine is turned off or unplugged like the OP mentioned then any of the other switch's would back feed the machine and create a dangerous shock hazard because the male hot prong on the plug would become hot when the other switches are turned on, not a good idea!!!

If supplying the light is a must from the machine power then three contactors or relays are a must to keep the power sources independent and the light circuit would have to have its own supply so that if any one machine is turned off or unplugged the other machines will still power their control relay to control the light, if you are wanting to make the light come on automatically when the machine is turned on, then the three relays are the only way to do this.

So what am I missing here?
 

gar

Senior Member
130504-1028 EDT

The first item to consider is the circuit logic. If a contact is assumed to be a logic 1 when closed, then contacts in parallel form a logical OR circuit. The truth table for a logical OR has a logic 1 as an output if at least one input is a logical 1.

Thus, in the proposed system if at least one machine is on, then the light is on, and all machines must be off for the light to be off.

If this is the desired logic, then a safe and code compliant method must be used. iwire's suggestion of three relays in a box with the normally open output contacts in parallel is probably the best solution. Make the relay coils 24 VDC or VAC supplied with power either at the input end (machine) or in the relay box.

A small transformer at the machine, with its input connected to 120 V, that connects from a power source when the machine is turned on and remains on while the machine is on can provide the signal to that machine's relay coil in the remote box.

A flaw in this approaches is, if work needs to be done on the machine, the light is needed for this service, and the machine is turned off for service, as it should be, then the machine power can not be used as the signal. If this is a problem there are solutions.

Also if DC is the power source, then a diode per machine, and one relay will perform the logical OR function.

.
 

fifty60

Senior Member
Location
USA
Thanks for all of the input. I understand the problem of the OCPD's being insufficient if they are paralleled from different sources. Thanks for pointing that out. The feeder supplying the paralleled OCPD's would have to be sized accordingly correct?

I would like to discuss the other situation a little bit more. If 2 of the lines are on different phases, and they both are switched on, I will have a short between phases. If the solenoid is between the phases, then it will not be dead short right?

If it is a split phase system, then I have the possibility of getting either half of the single phase 240VAC waveform that is creating the 120VAC. So the possible voltage available is 120-0(N)-120, with each 120V-N 180degrees out of phase with the other 120-N. If they outlets are connected to the same 120-N tap, then i would have 120VAC across the solenoid. If they are coming from different sides of the 120-N-120, then I would have the full 240VAC phase across the solenoid.

Is this correct? Please correct my wording if anything is incorrect. My main problem with the "dead short between phases" is that this would create an arc flash situation if it were a true dead short. Since the solenoid is between the 2 phases, then it would not create an arc flash unless the solenoid became obliterated and welded the two lines together.

But since they lines are 120-N-120 from a single phase, what happens if they are shorted together? I know it is extremely dangerous but is the incident energy much less than if they were from seperate phases. All very dangerous of course, but just trying to analyze some more.
 

Smart $

Esteemed Member
Location
Ohio
Thanks for all of the input. ....
As I said earlier, three cord and plug connected machines powering one solenoid is just outright bad design. Your electrical theory questions amount to creating a line-to-line short or paralleling ocpd's. The possibility is just too great to even consider it, so why discuss it? (BTW, that's a redundant question).

FWIW, have you ever heard the expression "getting information is like pulling teeth"....
 

broadgage

Senior Member
Location
London, England
I think that it is clear that the INTENT of the code was to prohibit the use of small conductors in paralel in order to share the load rather than use the correct size wire.
For example to prohibit the use of two #12 for a 40 amp range circuit.

An over pedantic interpretion of the code would seem to rule out many arrangements of switching lamps and fans etc. that are in practice perfectly safe.

IMHO the relevant code article needs re writing along the lines of "any arrangement of parralell conductors is permitted, provided that every conductor used has at least the minimum required ampacity for the circuit"

So you could parallell as many #14 as you want on a 15 amp breaker, but could not use 2 or more #14s on a 30 amp breaker.
 

gar

Senior Member
130505-1103 EDT

fifty60:

You need to draw circuits of what you are describing. Then do some circuit analysis.

Connect two 1.5 V batteries (really cells) negative terminals together. From one positive terminal connect to one SPST switch, from the other positive terminal connect a second SPST switch. Connect the output terminals of the switches together. Fundamentally the two switches are in series.

If both cells are exactly 1.5 V and both switches are closed, then a low impedance short exists between the two cells, but no current flows. Change one of the cells to 1.4 V. Now a huge current flows when both switches are closed. You can apply this test to any AC circuit as well.

In general when you have different machines you want them electrically isolated from one another. As Smart $ said your initial proposal is just bad design.

There are many ways to get isolation. Use an isolated contact at each machine, and do not use machine power in the circuit controlled by the switch. Use transformer isolation as I suggested in a earlier post. Use a fiber optic cable. Use an RF or infrared link. Use an acoustic signal. Use air pressure.

No matter what you do for isolation you must consider the desired system logic.

.
 

kwired

Electron manager
Location
NE Nebraska
I think that it is clear that the INTENT of the code was to prohibit the use of small conductors in paralel in order to share the load rather than use the correct size wire.
For example to prohibit the use of two #12 for a 40 amp range circuit.

An over pedantic interpretion of the code would seem to rule out many arrangements of switching lamps and fans etc. that are in practice perfectly safe.

IMHO the relevant code article needs re writing along the lines of "any arrangement of parralell conductors is permitted, provided that every conductor used has at least the minimum required ampacity for the circuit"

So you could parallell as many #14 as you want on a 15 amp breaker, but could not use 2 or more #14s on a 30 amp breaker.
I am mostly with you on this, one flaw to it though is you could still parallel two #14's on a 15 amp breaker for the purpose of voltage drop compensation. I don't think that is intended to be allowed either. I do think it is ok to have conductors that end up parallel to one another as part of a control scheme, and we do this all the time with control wiring. But that control wiring is typically limited load on the controls. Though it doesn't happen as often, use similar control method for controlling a heavier load and we have everyone question it.

I may open another can of worms by mentioning "ring circuits" that have been heavily debated on here before. This is on a similar order when it comes to whether or not this is a "parallel" conductor or not.
 
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iwire

Moderator
Staff member
Location
Massachusetts
I think that it is clear that the INTENT of the code was to prohibit the use of small conductors in paralel in order to share the load rather than use the correct size wire.
For example to prohibit the use of two #12 for a 40 amp range circuit.
Again I point out if that is the only intent there would be no need for the exceptions and thier very specific limitations.

They were just updated in 2011 so I doubt the intent is limited to what you suggest. :)
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
2017 proposal. "Conductors of any size shall be permitted to be connected in parallel where the ampacity of the smallest conductor is equal to or less than the rating of the OCPD."
 

Smart $

Esteemed Member
Location
Ohio
2017 proposal. "Conductors of any size shall be permitted to be connected in parallel where the ampacity of the smallest conductor is equal to or less than the rating of the OCPD."
Ampacity less than OCPD? So I would be able to install two a #14 in parallel for a 1000A feeder circuit????

I'm thinking that should be equal or greater than.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
I know I'm aiding and abetting in taking this thread off topic, but what would be the advantage of paralleling two 14AWG on a 15A circuit. Or any conductors if any one of them is capable of carring the current that matches the OCPD?

Also, @ the OP...Can you describe better what you're trying to accomplish besides just saying you want to turn on a light from 3 different locations?
Such as, if it is an alarm light, "in use" light, Christmas light:)D) etc.

If just a simple light, then the two 3-ways and a 4-way would be the way to go.
 

kwired

Electron manager
Location
NE Nebraska
I know I'm aiding and abetting in taking this thread off topic, but what would be the advantage of paralleling two 14AWG on a 15A circuit. Or any conductors if any one of them is capable of carring the current that matches the OCPD?
I think one advantage has already been mentioned - voltage drop. Of course running a single larger conductor also works.

I don't think this is too far off topic, we are still talking about parallel conductors instead of what you had for lunch today:happyyes:
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
I think one advantage has already been mentioned - voltage drop. Of course running a single larger conductor also works.

I don't think this is too far off topic, we are still talking about parallel conductors instead of what you had for lunch today:happyyes:
Well since you asked, I had ...............:p

Still would like more info on what the OP is trying to accomplish!
 
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