quick rule of thumb, 3 phase current

[telectrix]

now this is not exact, but comes close. say you have a 3 phase, 400v motor, given the kW, to get an approx. value for the current per phase, (can be done in your head), just multiply the kW x 1.5. e.g. a 20kW motor. I = 30A. ( not allowing for efficiency, pf etc. ) just to give a rough idea.

 

[Darkwood]

might be better if you double it as i do it more accurate but still a rough figure.
18.5 Kw motor = 35amps
22 Kw motor = 40amps ...... this trend to be a few amps off either side works up to 22kw then the values start to be several amps out after that.

30Kw = 54amps
37Kw = 65.5amps

Note im refering to motors with a plated voltage of 415v that are usually the majority of existing motors in use, you are using 400v plated value hence the difference but if you know both these rough guides its a good start.

 

[Knobhead]

Your 1.132A out Tel :tongue3:

 

[Knobhead]

I take it your using .85PF and 90% efficiency, Darkwood

Tel wasn't taking either in to account

 

[telectrix]

i get 53.4A for a 37kW.

37,000/(1.732 x 400)

 

[Darkwood]

Ive just done a 30kw and a 22kw motor existing on a machine and the plated ampage does match my examples give or take a few a few amps, i can only assume modern motors have a better efficiency than the older ones, its a rule of thumb that can catch you out if you are looking at older machinery and yes Tony i probably am as its a guide ive always used and found to be accurate, as it omits the need to allow for PF and efficiency issues.

 

[telectrix]

appreciate that pf and efficiency will increase the current required. my post was just to get a starting point. obviously when designing a circuit to power any machine, all factors must be taken into consideration, otherwise you either get an under rated circuit, or spend more on cable than necessary.

 

[Wazzajnr]

I normally work on 4.5A/kW when pricing up and it normally is reasonably close. However, i always go "next size up" so i don't come unstuck on the price.

20kw x 4.5 = 90A / 3phases = 30A/phase

Always worked for me

 

[telectrix]

which gives the same answer as my guesstimate. 20 x 1.5 = 30A.

 

[Knobhead]

It's good rule, stood me for 40 years

 

[Farmelectrics]

is that per phase tel

 

[telectrix]

yep, amps/phase.