Calculating volt drop, cable selection and mcb selection on an 3kw motor

[daiplayer]

Any explanations on how this is calculated ?

Diversity :

By referin to table 1b in the osg, it states : 100% largest motor ?? ...3kw ....???

3000/230 = 13.04 A....is this it !!?? max demand ??


How there on would you make cable selection , mcb selection ?

 

[ezzzekiel]

once design current is established select a protective device (in) rated equal or greater to ib then select a cable rated equal or greater than in

 

[daiplayer]

ok, thankyou. ...How then is volt drop calciulated...eg///length of run = 6mm...13A = ib....cable=2.5mm

 

[smartspark]

Is it going to be run in armoured cable or twin and earth?, is the circuit a radial? A standard ring with 2.5mm t+e will take anything up-to 3kw on a 32A breaker and a radial on a 20A breaker.
Check your table in BS7671 for voltage drop per milliamp per metre, is the cable being clipped direct or going through thermal insulation, blah blah blah.

 

[ezzzekiel]

vd = mva taken from regs x length of circuit x design current / 1000

 

[Sav]

Is it going to be run in armoured cable or twin and earth?, is the circuit a radial? A standard ring with 2.5mm t+e will take anything up-to 3kw on a 32A breaker and a radial on a 20A breaker.
Check your table in BS7671 for voltage drop per milliamp per metre, is the cable being clipped direct or going through thermal insulation, blah blah blah.

I think that an A3 radial 2.5mm cable within a max floor area of 50m2 can take upto 5kW and be protected by a 20A breaker.

I think thats right.......
Sav

 

[smartspark]

I wouldn't, The max I would put on a radial is 3KW, ie: immersion heater 16A or 20A breaker with a 20A double pole switch. Any higher I would go 4mm or 6mm cable 32A breaker. Just to air on the side of safety.

 

[telectrix]

I think that an A3 radial 2.5mm cable within a max floor area of 50m2 can take upto 5kW and be protected by a 20A breaker.

I think thats right.......
Sav

5kw >20A. thats a bit high for a 20A MCB. then there's volt drop to consider. 4mm and 32A minimum in my mind.

 

[daiplayer]

this is the drawing ive been given to design for my final practical exam. A 3 kw motor will be wired in. It will run from the cu to a dp switch via twin and earth in conduit ( 2.5m run) from the dp switch, then run in swa cable to motor , 500mm, then back through it all. Ive have to show how i calculated all the cable design, considering volt drop and diversity. ??? For some reason i come up with : volt drop ( approx 2v) ....for 1.5mm cable, And since the osg says a 1.5mm cable can carry 17.5 A (ref method b) table 6D1. ?????????????? surely im wrong !!!!!!

 

[ezzzekiel]

3kw load at 230v is just over 13a so nearest mcb would be a 16 so although a 1.5mm cable can handle it (going off basic info given) then your design would be correct although not ideal, but do they want exact or for your judgement, now providing all your calcs are correct and you can show how you have designed the circuit then all should be good.

 

[SimpleSimon]

this is the drawing ive been given to design for my final practical exam. A 3 kw motor will be wired in. It will run from the cu to a dp switch via twin and earth in conduit ( 2.5m run) from the dp switch, then run in swa cable to motor , 500mm, then back through it all. Ive have to show how i calculated all the cable design, considering volt drop and diversity. ??? For some reason i come up with : volt drop ( approx 2v) ....for 1.5mm cable, And since the osg says a 1.5mm cable can carry 17.5 A (ref method b) table 6D1. ?????????????? surely im wrong !!!!!!

No your not wrong.
3kW motor.
Diversity= 100%. So 13A
Volt drop= mV/a/m (in osg) x 13 x 2.5m/ 1000.
So 1.5 is ok as a minimum but not the best. You would be better off on 2.5mm but I'm saying that out of instinct so I'm sure some would say you can get away with 1.5mm

 

[smartspark]

I.5mm would be ok if you had a 2inch run of cable from the CU, if the run of cable is two floors away in a 5 bed house, I think you would be in trouble.

 

[pushrod]

Not asked to consider power factor for the 3kW motor? It will increase Ib

 

[malcolmsanford]

this is the drawing ive been given to design for my final practical exam. A 3 kw motor will be wired in. It will run from the cu to a dp switch via twin and earth in conduit ( 2.5m run) from the dp switch, then run in swa cable to motor , 500mm, then back through it all. Ive have to show how i calculated all the cable design, considering volt drop and diversity. ??? For some reason i come up with : volt drop ( approx 2v) ....for 1.5mm cable, And since the osg says a 1.5mm cable can carry 17.5 A (ref method b) table 6D1. ?????????????? surely im wrong !!!!!!

Are you sure that is right. I would have expected the question if it's a design one to want you to calculate the CPC sizing as well by using the adiabatic equation in reg 543.1.3.

So would it not be singles in that conduit?

 

[daiplayer]

yes, it is all run in singles. And in 20mm pvc conduit.