calculation of voltage drop

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Al Ameen

Member
cable of 3*240mm of 7sets are running from main distribution to load located at a distance of 265 foot length, 3 chillers are the load, each chiller have two compressors FLA 452A, if chillers run at 50% load what will be the voltage drop. supply voltage 230V 3 phase. please give me an example to calculate the voltage drop for parrell connected cable.
 

dkarst

Senior Member
Location
Minnesota
cable of 3*240mm of 7sets are running from main distribution to load located at a distance of 265 foot length, 3 chillers are the load, each chiller have two compressors FLA 452A, if chillers run at 50% load what will be the voltage drop. supply voltage 230V 3 phase. please give me an example to calculate the voltage drop for parrell connected cable.

The shortest answer is you can handle like parallel resistors using the same formula. I'm not sure you mean the two compressors have FLA of 452 so total load is 1356 A or total load is 2X or ~ 2700A. I assumed copper conductors and quickly looked and got a pretty small drop (few volts, although I did very quickly). Are you sure the compressors are rated for 208 and will run a bit below that (I saw your other post on same topic)? If they are rated at 230 and you are delivering ~ 206 or so you are already 10% low. You may also have an issue with starting but you didn't elaborate on that. Need some more information.
 

Al Ameen

Member
each compressor is 432Amps,one chiller have two compressor.... before starting i saw 228V, when we start one chiller voltage drop to 221V, again we start the second chiller there is further more drop in voltage it goes to 213V as the load increases, den when we start the third chiller voltage further drops and gets shutdown, and we capable of running two chillers at a time. so what to do?
 

Al Ameen

Member
at dkrst....before we start the chiller, voltage is 228V after starting first chiller voltage drops to 221V, den we start second chiller voltage further drops to 215V as the load increases, when we start the third chiller voltage further drops and gets shutdown, so we can run now only two chiller running in 50% of the full load... each chiller have two compressor, each compressor FLA is 452Amps..
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I see the voltage drop on the conductors as being less than 2.5%. Is the power source big enough to support that load?

Never mind, that is what you are getting.
 

Al Ameen

Member
yeah power source is enough, because when i check the voltage in supply side its been 220V after stating two chillers, but in the load end its 212V.. cable run for distance of 265foot.... so what to do now?
 

skeshesh

Senior Member
Location
Los Angeles, Ca
I believe you're confusing yourself by searching for a different formula. With parallel conductors you are still essentially calculating voltage drop for the feeders, but you're provided two or more parallel paths so that a smaller size conductor can be used by dividing the load between each parallel path.

So if each chiller has a load of "X" amps and you have 3 chillers you have a load of "3X", and if you have "N" sets of parallel conductors of equal length, you should have a load of 3X/N Amps which we can call "I". You already have the size of wire & length of conductor so you can find the Resistance of your run which we can call "R". Plug in your values and the voltage drop would be V(drop) = sqrt(3) x R x I .
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
240mm^2 3-core cable has voltage drop of approx. a 0.00021 V/A/m.

@ 265' (80m) and 7 x 240mm^2, with a load of 2712A, you will have about 6.48V drop from source to load.

That is assuming you have a source large enough to continually support the 2712A load.

The additional voltage drop you are experiencing is not due to cable, i.e. the VD is less than 3%, I would say you don't have a stiff enough source. The transformer stepping down to your 230V is possibly to low of KVA rating, or the system does not have enough reactance to support the motor load.

To fix the issue, I believe a power system study, including motor starting, is needed.
 
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