120v photo cell/240 volt contactor

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Talley1013

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Location
Houston, TX
Is it code compliant to use a 120v photocell to switch one leg of a control circuit for a 240v lighting contactor coil(one leg directly to coil, other leg wired through photocell w/ neutral)? I am installing a new contactor that unfortunately is 240v in a situation that already has a 120v photocell installed w/ wire pulled to the LP/photocell. If it is OK I would like to use the existing wire/photocell. Thanks.

ET
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Is it code compliant to use a 120v photocell to switch one leg of a control circuit for a 240v lighting contactor coil(one leg directly to coil, other leg wired through photocell w/ neutral)? I am installing a new contactor that unfortunately is 240v in a situation that already has a 120v photocell installed w/ wire pulled to the LP/photocell. If it is OK I would like to use the existing wire/photocell. Thanks.

ET

I'll take a shot at it. I have not found anything in 410 that states both legs of a controller need to be disconnected. The breaker IMO is the disconnect.
 

hurk27

Senior Member
240 volt running through a 120 volt photo cell will make for a short life of it, most photocells are an active device meaning electrical the components in them are part of the circuit (IE the heater, photo senser), and when it shuts off the light it will have 240 volts running through it from the load, now if your saying that you have a 240 volt load switch by a contactor with a 120 volt coil then there is no problem.
also most 240 volt photo cells are a 3-wire device also and will only break one leg of the load, but they are rated for 240 volts, and they get tied in white to L2 and load L2, black to L1 and red to load L1
 
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kwired

Electron manager
Location
NE Nebraska
240 volt running through a 120 volt photo cell will make for a short life of it, most photocells are an active device meaning electrical the components in them are part of the circuit (IE the heater, photo senser), and when it shuts off the light it will have 240 volts running through it from the load, now if your saying that you have a 240 volt load switch by a contactor with a 120 volt coil then there is no problem.
also most 240 volt photo cells are a 3-wire device also and will only break one leg of the load, but they are rated for 240 volts, and they get tied in white to L2 and load L2, black to L1 and red to load L1

But the photocell driving components are connected from black lead to white lead and the switched contact is from black lead to red lead, so when the cell is "off" the controlled 240 volt load is isolated from the driving components by the open contact. May not be a great idea but will work.

Another question would be if the contact is rated for switching 240 volts?
 

Talley1013

Member
Location
Houston, TX
240 volt running through a 120 volt photo cell will make for a short life of it, most photocells are an active device meaning electrical the components in them are part of the circuit (IE the heater, photo senser), and when it shuts off the light it will have 240 volts running through it from the load, now if your saying that you have a 240 volt load switch by a contactor with a 120 volt coil then there is no problem.
also most 240 volt photo cells are a 3-wire device also and will only break one leg of the load, but they are rated for 240 volts, and they get tied in white to L2 and load L2, black to L1 and red to load L1

The pc would only see 120v. I would have a multi wire branch circuit on a 2 pole breaker with one pole going directly to one side of the 240v contactor coil and the other pole running through a 120v pc(with neutral of course for the pc)and then to the other side of the 240v contactor coil. I know when a 240v light is operated directly through a pc only one leg is broken but I just want to make sure it is ok (per nec)to do the same thing when using a hoa/pc/contactor.
 

hurk27

Senior Member
But the photocell driving components are connected from black lead to white lead and the switched contact is from black lead to red lead, so when the cell is "off" the controlled 240 volt load is isolated from the driving components by the open contact. May not be a great idea but will work.

Another question would be if the contact is rated for switching 240 volts?

So then explain why is there 120 volt PC's and 240 volt PC's?

I think if you looked at a diagram you will see that some of the components are in fact in series with the load wire (red) this is why if you remove the load the PC will turn on after the heater cools off, this is the reason that when changing a failed incandescent lamp the new lamp will light then turn off, this doesn't happen with a ballasted fixture since the ballast provides a load to keep the PC off.

So if you connect a 120 volt PC to a 240 volt load it will see 240 volts when the PC turns off from back feeding through the load.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Take this with a grain of salt as it comes from wayyyyyy back in my memory bank, but I believe I connected one that way and it did not work. When the PC turned off there was still a current path that held the coil in at a reduced voltage until I disconnected the neutral to the PC. It would not pull in until dark, but after the PC turned off it held.
It may have been unique to that one type photocell
 

kwired

Electron manager
Location
NE Nebraska
So then explain why is there 120 volt PC's and 240 volt PC's?

I think if you looked at a diagram you will see that some of the components are in fact in series with the load wire (red) this is why if you remove the load the PC will turn on after the heater cools off, this is the reason that when changing a failed incandescent lamp the new lamp will light then turn off, this doesn't happen with a ballasted fixture since the ballast provides a load to keep the PC off.

So if you connect a 120 volt PC to a 240 volt load it will see 240 volts when the PC turns off from back feeding through the load.

What is in series with the load and why? The resistance of the component(s) would reduce voltage to the load. Maybe parallel across the contact I could see happening. Then the component(s) would be subject to open circuit voltage when the load is off.

Why is there 120 and 240 volt PC's, even without the possible components we just mentioned you have a L1, L2 and a controlled lead that has a contact between it and L1 on most PC's. The components connected between L1 and L2 must be able to withstand L1 to L2 voltage.

Whether or not the PC will work for the OP depends on the design of the PC.
 
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