Electric Motor Cable Size

[Kayode272]

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?

 

[sreid]

Assuming 6 mmSq = 6 mm^2 [six square mm]

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.

 

[electricpete]

The two considerations are voltage drop and steady state ampacity.

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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[7anoter4]

I agree with electricpete [but also with sride] as the 6 sqr.mm is approx.10 AWG [between 9AWG and 10 AWG] and the resistance is 3.08 ohm/km at 20 oC and 3.44 ohm/km at 50oC and reactance 0.08[at 50 Hz] ohm/km so could be neglected.
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.

 

[electricpete]

The starting current is only 1.7*Irated.

Good point.

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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[ScottyUK]

Any chance the starting and running currents might actually be the line currents with the windings connected in delta and star? The 1.7 factor looks just too close to be accidental, and no normal induction motor has such a low starting / running current ratio.


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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

Nice sleuthing Scotty.