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

[daiplayer]

ok here i go :

For deign current : 36.1 A

In : 40A
...................

Ok..... here goes...

Ca : 35 oc...... : factor of 0.94....table 4b1 ?
Cg : 0.82........table 4C1 ???

so ...It = 50.8 A ...????

Im lost as to where i go from here....

I can do the volt drop calc....but i cant suss how to find the correct table for the cable selection ?

Thanks

 

[i=p/u]

come on set the sum out clear as ice, peas

 

[telectrix]

how do you get 36.1A for a 3kw motor?

 

[telectrix]

come on set the sum out clear as ice, peas

frozen peas???

 

[i=p/u]

you set sum out for me and paste orig question in too. thank you if you could aint done 3phase before

 

[daiplayer]

how do you get 36.1A for a 3kw motor?

Its a three phase 400 v heater

 

[Sintra]

you set sum out for me and paste orig question in too. thank you if you could aint done 3phase before

The question is in post #49

 

[daiplayer]

edit...the load in q is i three phase (400v ) 25kw heater..

 

[Sintra]

The question is about a 25KW 400V heater.

 

[daiplayer]

you set sum out for me and paste orig question in too. thank you if you could aint done 3phase before

3 phase design current = power / (sqaure root of 3) x Uo (nominal voltage)

 

[daiplayer]

Ib -

Ib = Power / SQUARE ROOT of three x Uo = 36.1 A

In = Greater than Ib = 40A breaker

It : = In / Cg = 40 / 0.82 (table 4c1) ......It = 48.78 A

(these values have changed)

Cable : 4mm cable (Table 4G1A , column 9) ?????????????????

 

[i=p/u]

is that per phase

 

[daiplayer]

is that per phase

no...in three phase systems the ninal voltage between phases is 400v....

in single phase it is 230v

 

[i=p/u]

kk, so would you share that over 3 phases for balancing, i know of initial q. but but but

 

[Sparky Ninja]

Ib -

Ib = Power / SQUARE ROOT of three x Uo = 36.1 A

In = Greater than Ib = 40A breaker

Correct

It : = In / Cg = 40 / 0.82 (table 4c1) ......It = 48.78 A

You've identified Cg, but need to identify Ca. Table 4B1. Mineral, thermoplastic @ 35degrees = 0.93 factor.

 

[daiplayer]

If your talking about load balancing over phases.....then....Yeah...kind of...but that would be in the inititial design of the heater itself......not considered during the heater installation...

Seeing as it a three phase supply...You consider that the load in q has already been balanced by the Maker of it.

Installers may need to consider balancing when installing single phase items...into a premesis that uses/ are provided by three phases...( ie,, three single phase (identical heaters) would be installed on a phase each ?


Correct me on this if im wrong though guys..

 

[daiplayer]

Correct



You've identified Cg, but need to identify Ca. Table 4B1. Mineral, thermoplastic @ 35degrees = 0.93 factor.

Dammit...i had tha to start !!! ....then changed my answer cos i thought it would fall between the bracket ( 30 - 70 degrees ?)

 

[pushrod]

Ib = Power / SQUARE ROOT of three x Uo = 36.1 A

is that per phase?

The 36.1A figure is the design current for each line (L1, L2, L3) it is not a total value that is then shared between 3 cables if that is what you are asking

edit : you have to be a little careful with terminology because the phase current (Ip) only equals the line current (Il) when connected in star. If in delta, as it would most likely be in a motor, then Il = √3* Ip

When designing for cable size it is just the line current that you are interested in.

A complex subject (which i am rapidly forgetting) but just to be aware that phase current and line current are slightly different and are not always the same value.

 

[telectrix]

The question is about a 25KW 400V heater.

sorry, my fault for not reading all the posts. was going from original thread title 3kw motor. DOH!

 

[daiplayer]

The 36.1A figure is the design current for each line (L1, L2, L3) it is not a total value that is then shared between 3 cables if that is what you are asking

edit : you have to be a little careful with terminology because the phase current (Ip) only equals the line current (Il) when connected in star. If in delta, as it would most likely be in a motor, then Il = √3* Ip

When designing for cable size it is just the line current that you are interested in.

A complex subject (which i am rapidly forgetting) but just to be aware that phase current and line current are slightly different and are not always the same value.

Much better explanation than mine...