Electric Motor Cable Size
Electric Motor Cable Size
(OP)
Please can somebody explain to me. I already pulled a 3 core by 6mmSq cable over a distance of 250 meters to power a 1.5KW electric motor. Unfortunately the motor size was changed to 4KW, starting current of this motor is 7.8A and running current is 4.5A. Can i still use my 6mmSq cable or do i need another cable size?





RE: Electric Motor Cable Size
Square Root (6 mm) = 2.5mm x 2.5mm [if the wire were square]
2.5 mm = 0.1 inch so this is bigger than a round 10 gage wire.
10 ga, wire has a resistance of 1 Ohm/1000 feet.
250 meters = 820 feet so the resistanc is no more than 0.8 Ohms.
V = IR so the voltage drop [running] is
V = 4.5 amps x 0.8 Ohms = 3.6 V RMS
That should be OK.
RE: Electric Motor Cable Size
I think the voltage drop calculation should consider the inductive voltage drop in addition to resistive discussed above . May also need to look at voltage drop during start.
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RE: Electric Motor Cable Size
First of all no chance the cable could be warmer than permissible [the minimum at 40oC ambient air on a very stuffed cable tray will be 35 A and in high thermal resistance ground at 30 oC will be 42 A].
If for 35 A the conductor temperature will be 90oC for 4.5 A will be less than 50 oC[40oC ambient]. If pf=0.925 and effic. =93% the voltage drop is indeed 3.6 V ph-N or 6.26 ph-ph. As at start the pf=0.4 then the voltage drop will be less 5V ph-ph.
The starting current is only 1.7*Irated.
RE: Electric Motor Cable Size
Kayode - are those currents measured values? If so it sounds like low voltage during start - you should do some measurements of voltage to see what you really have. In addition to voltage drop, low voltage supply can be a problem.
Can you provide the motor nameplate data?
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Electric Motor Cable Size
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RE: Electric Motor Cable Size