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Discuss Float switch wire volt drop in the Electrical Forum area at ElectriciansForums.net

bigspark17

bigspark17

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Hi guys and gals.

a job that has got me second guessing my calculations. I have been asked to fot a float switch to a pump not a problem but the distance is 525m. Now there is no load on the float switch as i will be using a contactor to switch the pump Locally. So can i use a small 1.5mm 2c swa. I see no reason not as there is no load to take into account.
with the cable being so long it affects the price alot should it require a larger cable.
Thanks
 
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westward10

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I assume it controls the contactor so there must be load on it and wouldn't you double the distance to there and back.
 
Pete999

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Hi guys and gals.

a job that has got me second guessing my calculations. I have been asked to fot a float switch to a pump not a problem but the distance is 525m. Now there is no load on the float switch as i will be using a contactor to switch the pump Locally. So can i use a small 1.5mm 2c swa. I see no reason not as there is no load to take into account.
with the cable being so long it affects the price alot should it require a larger cable.
Thanks
What are your calculations
 
telectrix

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you need to determine the load of the contacor coil, then work out the VD for the cable selected. if within limits, OK, if not, select a bigger cable.
 
snowhead

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What's the purpose of the pump and float switch, why is the float so far away.?

The only thing I can think of is Spring water into a tank, then a pump in a house / building 500mtrs away controlled by the float switch, sounds like farmers idea, just run me a cable 500mtrs over there..
 
Last edited:
S

sinewove

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What's the purpose of the pump and float switch, why is the float so far away.?

The only thing I can think of is Spring water into a tank, then a pump in a house / building 500mtrs away controlled by the float switch, sounds like farmers idea, just run me a cable 500mtrs over there..
How about a solid state relay to switch the contactor,think they only draw milliamps?
S
 
James

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so just say a 32A contactor with a 230v ac coil uses about 7va,
7/230=0.03A
according to the data sheet for a shneider contactor, voltage limits for coil are 0.8 to 1.1 x nominal
therefore for stable operation the minimum return voltage to the coil should be 230*0.8=184v

this is not a distribution or lighting circuit so volt drop is calculated on component ratings as you are designing the control system.
assuming the nominal voltage is 230v you can deal with up to 46v dropped along the length of the cable.
mV/A/m for swa 1.5 2 core is 29
or for a 0.03A load that would be a drop of 0.87mV/m
max cable length for a 46v drop would be 52.8Km

i think you should be ok at 500m in my humble opinion.
 
westward10

westward10

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You would need to know the manufacturers voltage tolerance. To me the distance to calculate is 1050m not 525m.
 
James

James

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volt drop in the tables is already working on the assumption of a flow and return leg, at least that is what i have always assumed.
i.e. on a single phase supply cable voltage at origin is 230v
voltage at appliance may be 220v after volt drop
this is 5v dropped along the line conductor and 5v dropped along the N conductor.
i.e. in theory you should be able to measure 5v between N and E at the current using device end of the cable.
 
Lucien Nunes

Lucien Nunes

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Good practice would be to allow for coil volts to be in the 0.8-1.1 range with the mains at its limits, reducing the available drop in the cable to around 10%, less if there is significant drop in the circuit feeding the control unit. Obviously still OK by miles using 230V, but if you wanted a lower voltage control circuit, 48V works well for a typical 7VA coil (4.6% VD) but 24V does not (19% VD). A small relay with 1-2VA coil power would be OK at 24V though.

BTW I suspect the 29mV/A/m VD figure is at 70°, so it's very slightly pessimistic as the cable will never dissipate measurable heat.
 
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