what size cable for a 20amp motor

[mpooley]

Hi
I am installing a dust extraction system in my woodworking shop which uses a 20amp motor - i dont know what it will draw at start up and as yet have not decided if i will need to just use capacitor start or capacitor start and capacitor run,

an "electrical friend" (no not a robot) has told me a 2,5 twin and earth radial supply will be ok as its only using 20amps but i am not so sure
can any one give me some advice please?

BTW this is single phase and the workshop has a 10mm supply from the house if that makes any difference.

Mike pooley

 

[Chivers7]

Mike I'd tend to use a 4mm radial here, which will give you 26 amps or even 6mm giving 32 especially as it's probably a short run? As you say the motor on start up may draw some initial exra current. If you use 2.5 / 20amp then you've l'd think the cable won't be up to the job as it's working on it's capacity fella and I'd hazzard a guess that it will be tripping out from time to time.

Not sure what size protection you intend to use has the workshop got it's own board?

 

[JohnD]

When you've worked out what protective device (MCB or fuse) to use, the cable must be capable of carrying that current or greater, subject to any correction factors like thermal insulation or bunching.

 

[bernardgreen]

When sizing the cable consider the voltage drop along the cable when the start up current is flowing. This is the very time the motor needs full voltage if it is a direct on line start.

 

[GaryMo]

As you say the motor on start up may draw some initial exra current. If you use 2.5 / 20amp then you've l'd think the cable won't be up to the job as it's working on it's capacity fella and I'd hazzard a guess that it will be tripping out from time to time.

That's where type C MCB's come in.

What's the length of run? Installation method?

 

[RF Lighting]

The type 'c' only means the MCB will not trip when the initial starting current is drawn. The cabling still needs to be able to deliver this initially high current.

 

[mpooley]

Thanks for all replies

To answer some of your questions
I reckon the run from the workshop consumer unit will be about 15 meters
I will be using a contactor and to be honest i dont know what mcb i should use. a 30 amp i suppose?
the cable will be run accros the loft over the insulation.

thanks again

mike

 

[GaryMo]

The type 'c' only means the MCB will not trip when the initial starting current is drawn. The cabling still needs to be able to deliver this initially high current.

Yes I know (5-10 x In as opposed to type B being 3-5 x In), I was replying to the poster who said that tripping may occur due to the extra current on start-up.
Earth loop impedance values for a circuit protected by a type C MCB will have a maximum value lower than that of a type B though (approximately half).

I then asked about the length of run and installation method, this will be required to choose the correct cable.

 

[GaryMo]

Thanks for all replies

To answer some of your questions
I reckon the run from the workshop consumer unit will be about 15 meters
I will be using a contactor and to be honest i dont know what mcb i should use. a 30 amp i suppose?
the cable will be run accros the loft over the insulation.

thanks again

mike

Probably a 25A type B MCB, if you get nuisance tripping on start-up then uprate to a type C assuming Zs at the DB and R1+R2 for the circuit is under 0.77ohms (1.54ohms for a type B).
4.0mm2 T+E run over the insulation in the loft area (better clipped above the insulation if you can) and in mini trunking or conduit as it comes down the wall (ref method 3) to your isolation point and DOL starter, assumed 40 degree's temperature for the loft and not bunched with other cables.
Volt-drop is well within for the length of run.
If you think temperature will rise above that then it may be a better bet using 6.0mm2 T+E.
Make sure you select a suitable range of overload unit for your DOL starter.