Using float switch to control pump via relay

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I am trying to utilize a float switch to turn a pump on and off. The float switch is rated for 50VA/115V (0.435 amps). The motor is 1.4A at 115V. Obviously that is higher than what the float switch is spec'd for. Would there be any foreseeable NEC issue with just having the float switch power on a relay or contactor within its specs, and then have the relay itself toggle the motor?
 

jap

Senior Member
Occupation
Electrician
I am trying to utilize a float switch to turn a pump on and off. The float switch is rated for 50VA/115V (0.435 amps). The motor is 1.4A at 115V. Obviously that is higher than what the float switch is spec'd for. Would there be any foreseeable NEC issue with just having the float switch power on a relay or contactor within its specs, and then have the relay itself toggle the motor?


Your on the right track.
A very common practice.


JAP>
 

templdl

Senior Member
Location
Wisconsin
I am trying to utilize a float switch to turn a pump on and off. The float switch is rated for 50VA/115V (0.435 amps). The motor is 1.4A at 115V. Obviously that is higher than what the float switch is spec'd for. Would there be any foreseeable NEC issue with just having the float switch power on a relay or contactor within its specs, and then have the relay itself toggle the motor?
Will the float switch that you choose prevent short cycling? That is do you have a given liquid lever range that you want to stay within. When the liquid recedes to a specific level the motor turns on and when it reaches the max height turns off the motor.
 

Aleman

Senior Member
Location
Southern Ca, USA
Will the float switch that you choose prevent short cycling? That is do you have a given liquid lever range that you want to stay within. When the liquid recedes to a specific level the motor turns on and when it reaches the max height turns off the motor.

That's correct. Need at least 2 switches or the relay will chatter and bounce and the sump will only run long enough to break the switch. Look at level guard float switches,
solid state and it is a stand alone pump controller or you can use the output to switch your relay. Or buy a smart relay or time delay relay set to off delay to let the pump run
awhile before it turns off. Or find a switch described above.
 

iwire

Moderator
Staff member
Location
Massachusetts
That's correct. Need at least 2 switches or the relay will chatter and bounce and the sump will only run long enough to break the switch

It depends on the float switch and how it is installed.

For sure I have seen sewer ejection pumps burned up in a short time due to a bouncing float so I know it can be an issue.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Not sure what he's pumping but check valves will prevent cycling.
Check valve valves may prevent bleeding down of the fluid level with no actual flow, but they will not prevent cycling caused by a steady low flow rate and a high pumped flow.
Rapid cycling under load is potentially just as harmful as cycling with no load.
A wide dead band between on and off levels, whether it requires two floats or uses mechanical hysteresis in one float, will help with both problems.
 

mgookin

Senior Member
Location
Fort Myers, FL
Check valve valves may prevent bleeding down of the fluid level with no actual flow, but they will not prevent cycling caused by a steady low flow rate and a high pumped flow.
Rapid cycling under load is potentially just as harmful as cycling with no load.
A wide dead band between on and off levels, whether it requires two floats or uses mechanical hysteresis in one float, will help with both problems.

+1
 

chris1971

Senior Member
Location
Usa
How about a high level float switch along with a low level float switch to prevent the pump motor from cycling to much.
 

iwire

Moderator
Staff member
Location
Massachusetts
How about a high level float switch along with a low level float switch to prevent the pump motor from cycling to much.

For sure that is one way to do it. Why not three floats, run, stop, alarm? Or why not two pumps and four floats, run 1, run 2, stop, alarm?

The OP just wants to know if he can use a relay with a single float and single pump and the answer is often yes. :)
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
For sure that is one way to do it. Why not three floats, run, stop, alarm? Or why not two pumps and four floats, run 1, run 2, stop, alarm?

The OP just wants to know if he can use a relay with a single float and single pump and the answer is often yes. :)

Yes, it was starting to sound like a software shop discussing the features for the next release...:D
 

kwired

Electron manager
Location
NE Nebraska
Conversation has kind of trended toward waste water pumps, though the OP said he had 1.4A at 115V pump motor which doesn't sound all that typical for waste water pumps. Many suggestions related to having a dead band between off/on points are still valid for most any pumping application though.
 

mgookin

Senior Member
Location
Fort Myers, FL
Conversation has kind of trended toward waste water pumps, though the OP said he had 1.4A at 115V pump motor which doesn't sound all that typical for waste water pumps. Many suggestions related to having a dead band between off/on points are still valid for most any pumping application though.

From the OP I thought it was just a basement sump pump. They already have the dead band for off/on but if you have too high of a rise for your discharge you will still be cycling the pump without a check valve.

To dig a basement up north you do a 5' deep cut. Then when you grade you come up about 1' and slope away from the foundation for a minimum of 3' horizontally. That leaves 2' rise to the sill and another 12" or so to walk up to the finished floor level upon entry. That's why many stoops are about 4-5 risers usually.

So if you have 6' rise in elevation from basement floor to grade and the pump is 1' or more below the basement floor, that's 7-8'+ of pipe. While that's still not a lot of water, it does make the pump cycle faster. I used to see them fail prematurely for that reason. You could stand there with a hose and watch the pump come on, go off, then watch the pipe water gush back into the sump.

Put the check valve in there and problem solved; the new pump lasts it's rated service life. Nothing sucks more than a failed sump pump in a nice basement. (well maybe a sewer backup)
 
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