Switches In Parallel

[fifty60]

Hello everyone. I am trying to understand what happens to a load when it is started from three switches that are in parallel. My focus is on multiple switches that are in parallel and turned on at the same time. So imagine 3 switches with 2 switches in the on position.

The switches are in parallel, so the voltage remains the same. The resistance of the switch, which is miniscule compared to almost any load, does reduce to half if the switches are the same type. This is one minor change in the circuit. The resistance changes ever so slightly relative to the series circuit, the current perhaps increases ever so slightly.

In the end, I do not see anything wrong with having three parallel SPST switches located in different places, operating one circuit. Any one know what electrically could happen if all 3 switches are on at the same time. Perhaps I have had too much coffee here on a Saturday and am worrying about something that is seemingly pretty straightforward. Thanks in advance.

 

[iwire]

Any one know what electrically could happen if all 3 switches are on at the same time.

The load would operate fine,there would be no problems but very likely you would have a 310.10(H) violation as it puts conductors smaller than 1/0 in parallel.

 

[kwired]

Hello everyone. I am trying to understand what happens to a load when it is started from three switches that are in parallel. My focus is on multiple switches that are in parallel and turned on at the same time. So imagine 3 switches with 2 switches in the on position.

The switches are in parallel, so the voltage remains the same. The resistance of the switch, which is miniscule compared to almost any load, does reduce to half if the switches are the same type. This is one minor change in the circuit. The resistance changes ever so slightly relative to the series circuit, the current perhaps increases ever so slightly.

In the end, I do not see anything wrong with having three parallel SPST switches located in different places, operating one circuit. Any one know what electrically could happen if all 3 switches are on at the same time. Perhaps I have had too much coffee here on a Saturday and am worrying about something that is seemingly pretty straightforward. Thanks in advance.

Not only do you have a very small resistance in the switch contacts, you also have a small resistance in the conductors. Disregarding the switches for a moment, if the length of conductor to each switch location is not identical in length then you have unequal resistances in those conductors and still have imbalanced current in each path.

 

[jumper]

The load would operate fine,there would be no problems but very likely you would have a 310.10(H) violation as it puts conductors smaller than 1/0 in parallel.

Are you saying that I cannot have 3 SPST switches control a load?

 

[fifty60]

I have 3 seperate machines. Each machine can turn on the same light (hypothetical). Before any of the 3 machines can turn on the light, a control signal must first be sent to the "light".

So it is a 2 step operation. Control signal from any 1 of the 3 machines, and then another signal from any of the 3 machines to allow the light be illuminated to the individual machine. 1 light, 3 machines, 1 central control solenoid, 3 seperate supply solenoids (1 for each machine). All three machines can be illuminated from the single light source at the same time.

I would like to have the initial control switch located on each machine. I am getting tripped up a bit on how to get 3 seperate machines to access 1 solenoid. I worry about all three switches being on at the same time. It would probably be easiest to just have the control switch on 1 of the machines. The control voltage to the solenoid is 120V.

Any thoughts guys is appreciated. Thanks again.

 

[iwire]

Are you saying that I cannot have 3 SPST switches control a load?

I am not telling you what to do, and I may have done something like that a time or two.

But I am saying the code does not allow small conductors in parallel except for some specific things.

Exception No. 1: Conductors in sizes smaller than 1/0
AWG shall be permitted to be run in parallel to supply
control power to indicating instruments, contactors, relays,
solenoids, and similar control devices, or for frequencies of
360 Hz and higher, provided all of the following apply:

(a) They are contained within the same raceway or
cable.

(b) The ampacity of each individual conductor is sufficient
to carry the entire load current shared by the parallel
conductors.

(c) The overcurrent protection is such that the ampacity
of each individual conductor will not be exceeded if one
or more of the parallel conductors become inadvertently
disconnected.

Exception No. 2: Under engineering supervision, 2 AWG
and 1 AWG grounded neutral conductors shall be permitted
to be installed in parallel for existing installations.

So say you where trying to use two SPST wall switches one each in a mens and womens room to control a single roof top exhaust fan for both you would have code violation.

 

[Smart $]

The load would operate fine,there would be no problems but very likely you would have a 310.10(H) violation as it puts conductors smaller than 1/0 in parallel.

IMO, the conductors themselves are not in parallel. They do not terminate on the same equipment on both ends.

 

[jumper]

Why would 3 SPST switches on the same phase turning on a light/load matter?

The load/light would just stay on until all are off. No?

 

[iwire]

IMO, the conductors themselves are not in parallel. They do not terminate on the same equipment on both ends.

That is certainly one view, of course if that were the case would there be any need for the exceptions?

Also the code specifies electrical joined at each end, not physically connected at each end.

 

[iwire]

Why would 3 SPST switches on the same phase turning on a light/load matter?

The load/light would just stay on until all are off. No?

Yes it would and I really don't see the safety issue, just a code issue.

 

[Smart $]

That is certainly one view, of course if that were the case would there be any need for the exceptions?

I have never run into any need to use either exception... but then again, I am going by how I interpret that section... so touch?.

 

[Smart $]

...
Also the code specifies electrical joined at each end, not physically connected at each end.

Yeah... but if you go by that interpretation, all conductors we typically install are paralleled. You can't have a circuit without a loop. Typical wiring is a bunch of parallel loops... making all the conductors electrically joined at each end...!!!

 

[iwire]

Yeah... but if you go by that interpretation, all conductors we typically install are paralleled. You can't have a circuit without a loop. Typical wiring is a bunch of parallel loops... making all the conductors electrically joined at each end...!!!

I see your point and it certainly has truth in it.

But in my personal opinion the CMP was considering the supplied device the end point of the loop. Just like the load itself more or less is the division point between phases or neutral.

 

[fifty60]

So exception No. 1 is certainly in my favor:

Exception No. 1: Conductors in sizes smaller than 1/0
AWG shall be permitted to be run in parallel to supply
control power to indicating instruments, contactors, relays,
solenoids, and similar control devices, or for frequencies of
360 Hz and higher, provided all of the following apply:

(a) They are contained within the same raceway or
cable.

This first part of the exceptions seems a little trickey. The 3 seperate machines are located right next to each other. They can eventually run through the same conduit once they get to a joining point, but they of course will not be contained within the same raceway for the first couple of feet as they leave each respective machine. They will join into the same raceway, and then be ran to the solenoid.

I do not see any way that 3 seperate wires in this type of situation could instantly be in the same raceway. They are coming from seperate sources over a couple feet distance and then will be joined. Does this seem valid?

 

[jumper]

Yes it would and I really don't see the safety issue, just a code issue.

I gotta think about this one.:?

 

[Smart $]

I see your point and it certainly has truth in it.

But in my personal opinion the CMP was considering the supplied device the end point of the loop. Just like the load itself more or less is the division point between phases or neutral.

I agree... with your words, that is. A typical wall switch under Code is a device... is it not?

 

[iwire]

So exception No. 1 is certainly in my favor:

I agree.

This first part of the exceptions seems a little trickey. The 3 separate machines are located right next to each other. They can eventually run through the same conduit once they get to a joining point, but they of course will not be contained within the same raceway for the first couple of feet as they leave each respective machine. They will join into the same raceway, and then be ran to the solenoid.

I do not see any way that 3 separate wires in this type of situation could instantly be in the same raceway. They are coming from separate sources over a couple feet distance and then will be joined. Does this seem valid?

I think what you would have to do is have circuits from each machine run to a common control box that contains 3 relays. Each machine switch would operate the coil on individual relays so that the conductors between machines and this common control box would never be in parallel. 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.

 

[Smart $]

...

This first part of the exceptions seems a little trickey. The 3 seperate machines are located right next to each other. They can eventually run through the same conduit once they get to a joining point, but they of course will not be contained within the same raceway for the first couple of feet as they leave each respective machine. They will join into the same raceway, and then be ran to the solenoid.

I do not see any way that 3 seperate wires in this type of situation could instantly be in the same raceway. They are coming from seperate sources over a couple feet distance and then will be joined. Does this seem valid?

Need clarification... are you saying there is a control circuit originating in 3 separate machines and powering a single solenoid?

 

[iwire]

I agree... with your words, that is. A typical wall switch under Code is a device... is it not?

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.

 

[Smart $]

...
I think what you would have to do is have circuits from each machine run to a common control box that contains 3 relays. Each machine switch would operate the coil on individual relays so that the conductors between machines and this common control box would never be in parallel. 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.

Essentially concur... However I see proposed more as a violation of 240.8 if each machine has separate OCPDs (240.8). Defintely not wise if separate control power supplies.