3 phase supply to motor cable sizing

[Maestro]

I need to run in a supply for a 3.52kw 3 phase motor 70m from DB i worked out that 1.5 swa in free air would just about be sufficient so thought i would use 2.5mm 4 core

length of Run 70m
P= 3.52kw
v=400
I = P/V = 8.8A

Table 4D4A USING INSTALLATION METHOD E
1.5MM can take 19A

Table 4D4B
voltage drop is 25mV/A/m
length of run is 70m
current is 8.8
total volatage drop would be - 25*70*8.8/1000=15.4
allowed voltage drop
400/100*4=16v

is there some other calculation you do when doing 3 phase - i am not running a nuetral to the motor

 

[Marvo]

I'm not doubting your calcs but there's no way I'd install a 70 meter 1.5mm cable to run a 3.5kW inductive (AC3) load.

Is the motor DOL? Is there a VFD or soft starter? Does the motor need to start under high torque conditions?

 

[Maestro]

thats why i am using 2.5 - i thought before i worked it out it would be 4 or 6 i even put it into the calculator on Doncastercables and that confirmed 1.5mm, but it is a small motor really it runs a belt driver for fan - its also a Delta wired

 

[Simonslimline]

just out of interest why have you not worked out the current using route 3 to start with ?

3520/(1.732 x 400) 5.08A

I am still training so just wondering if I am missing something here

Thanks

 

[Darkwood]

My 1 question is what is the motor doing is this a high inertia load ... although its a very small motor .... and as already asked what is the starting arrangement of the motor.

Thats 2 questions but just expressing Marvos question as it a good point!

 

[Spoon]

Its nice when people get back to you eh....
No mention of fuse/breaker size.

 

[Knobhead]

I'd like to know where he got his figures from (I do know). Such a simple mistake.

Not that it matters as he seems to have vanished.

 

[ian.settle1]

I'd like to know where he got his figures from (I do know). Such a simple mistake.

Not that it matters as he seems to have vanished.

Give the poor guy a chance Tony it was around 9-10 last night and he might not get to a PC until tonight

 

[Lucien Nunes]

OP: Are you sure about this bit:

P= 3.52kw
v=400
I = P/V = 8.8A

Consider: How many phases? What power factor?

Have you checked actual line current given on the motor plate? And as others have mentioned, what is the starting current and permissible voltage drop at that starting current?

is there some other calculation you do when doing 3 phase

Yes!

 

[Maestro]

The starting arrangement is Delta - i know i have not added power factor or efficiently rating but i thought if i done it at 100% what would be the big deal the amps on the side of the motor was 7.58A i see know i forget to root 3 the current as its 3 phase but that means i would have got an even lower current so the 1.5 cable size which still seems really small

I just 4mm as there was plenty of that lying around the yard

 

[Darkwood]

Your missing the point here many factors need to be taken into account - yes you probably have a plated FLC of 7.58amps but this is an highly inductive load also you need to consider your front end device and its ability to handle the starting inrush.

What is your front end device?
What is your method of starting this motor?
What is the starting duty likely to be? (Is this an high inertia load with a long run up time!)
You can't just apply domestic calcs to sizing the cable, you have been asked several questions and not yet answered them so how can we give guidance ?

This is for a belt driven fan and may have an extended starting duty this will create tripping issues if mcb's are used ... is the motor on its own with a starter or is it part of a bigger system and controls.. did you get the ampage directly off the motor plate or the side of a control panel.

 

[Maestro]

i will answer best i can i only ever done motors a college and trade test

What is your front end device?
it a belt driven fan
What is your method of starting this motor?
the installer of the fan left DOL Starter with D range Contactor with a thermalover to be conected in it
What is the starting duty likely to be? (Is this an high inertia load with a long run up time!)
i was taking it to be about 10x so was gonna use type C or D MCB

 

[Darkwood]

i will answer best i can i only ever done motors a college and trade test

What is your front end device?
it a belt driven fan
What is your method of starting this motor?
the installer of the fan left DOL Starter with D range Contactor with a thermalover to be conected in it
What is the starting duty likely to be? (Is this an high inertia load with a long run up time!)
i was taking it to be about 10x so was gonna use type C or D MCB

You need to select you mcb size and type ensuring its large enough to handle the starting curve of the motor usually by referencing the tech' support chart of the manufacturer 'check online' this will give the mcb size required...(its not as simple as type C for motors etc)... once you have a mcb size then select your cable to comply with VD and ELI permitted values .... you may find due to starting duty of the motor you might need a larger mcb to account for this.

The recommended (merlin) mcb for a 3.75kw motor (nearest listed to yours) is a 25amp(b) 25amp (c) or 16amp (d) ... its now up to you to ensure you VD and ELI are met by choosing the correct size cable. Its now showing that your 1.5mm original thought will probably be inadequate .... for the sake of this thread we will omit the OL omit clause for fixed loads as you need to learn to design for motor circuits.

If you are out of your comfort zone here you could do with going on circuit design course or if you feel confident do some web searching and brain cramming ....its an area often messed up and can be costly when it happens.

 

[Knobhead]

You new current value?
Is that from the motor plate?
If it is I’d go and have another look because it’s the most inefficient motor in the world if it’s right.