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# Choosing cables to load - 3-phase engine.

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#### zitrex2

##### Newbie level 5
Hi everyone,
I need to wire up 3-phase motor.
5,5kW | 400V | 11,3A | direct delta connection | cos(phi)=0,86 |
Cable length must be 25m. | Tmax=60*C | Voltage drop max. 3%
From calculation, I see that 2,5mm^2 would be ok, but I have doubts. Can someone give me advice? Thank You.

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Hi everyone,
I need to wire up 3-phase motor.
5,5kW | 400V | 11,3A | direct delta connection | cos(phi)=0,86 |
Cable length must be 25m. | Tmax=60*C | Voltage drop max. 3%
From calculation, I see that 2,5mm^2 would be ok, but I have doubts. Can someone give me advice? Thank You.

Show us your calculations and we'll review them. But it is essenially quite simple:
1) For the absolute voltage drop and the plate's current rating, calculate the wire's allowable ohmic value.
2) Divide that value by your length, and you'll have an ohms/meter requriement.
3) Go to a Copper Wire Table, and select the gage that meets that requirement. Give yourself a little margin.

BTW, I like your avatar of the jumping transmission tower!

My H07RN-F cable have 60*C rating. R(20*C)=7,98Ω/km -> 25m cable have R=0,2Ω (20*C) and R=0,23Ω (60*C) Ur=I*R=2,6V -> Voltage drop = 1,13% < 3%. This is in theory, if I'm right ofcourse. I'm concerned about inrush current which should be like 5-8 times bigger than normal current. I don't this electronics way of calculating wire across section would work in my problem.

Inrush I*5=56.5A -> Ur=13V -> Voltage drop = 5.65%

PS: Thanks, this is how voltage spikes and drops are made.

Calc. update

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Looks like you didn't actually calculate. 2.5 sqmm is suffcient but not necessary.

- - - Updated - - -

Voltage drop is calculated for rated (steady state) current, not inrush. The relative voltage drop has to related to phase rather than phase-to-phase voltage.

An additional requiremet is that apart from acceptable voltage drop, the short circuit current must be high enough to trip the overcurrent protection means. But that's usually no point with short cables up to 100 m length.

zitrex2

Points: 2