How to choose the right 3 pole isolator

[Richiespark100]

Hi guys,

i need abit of help here I have a 3phase, 440v, 50hz, 1.7kw squirrel cage motor. I need to know how I can calculate and choose the right boxed rotary isolator switch. From my calculations I would do PV which equals 3.86. Now this is where I come stuck because it's 3phase do I now times the 3.86 by 3 and say that is my full load current? At the moment I'm thinking of getting a 25 amp isolator switch would this be ok? Please help us out.

Ok guys il try reply to a few of your queries. First of all the type of isolator I want is this: http://www.meteorelectrical.com/25-amp-3-pole-polycarbonate-boxed-isolator-switch.html

also yes it is a 1.7 kW motor 3ph motor it's only a small motor serving as a toilet extract fan working with another 1.7 kW motor as duty/standby. I'm looking to see what is the right isolator to choose. Please give us hand guys


Regards

Richie

 

[Spoon]

Hi OP. Just a few questions:
Are you an electrician?
What does is say on the motor plate for the current rating?
Is this a new cricuit or are you just adding an isolator?

 

[Spoon]

Also please dont edit your original post with the answers as it will not notify my you have replied.

 

[Richiespark100]

Ok guys il try reply to a few of your queries. First of all the type of isolator I want is this: Meteor Electrical


also yes it is a 1.7 kW motor 3ph motor it's only a small motor serving as a toilet extract fan working with another 1.7 kW motor as duty/standby. I'm looking to see what is the right isolator to choose. Please give us hand guys

 

[uksparks]

Oh i see, so its not exactly as I thought, let me get a book off the shelf, I've got a few, I am sure all will be revealed in there!

 

[LeeH]

Is there such a thing as a standard size?

Motors come in nominal sizes. 1.7 isn't a nominal size.

1.1, 1.5, 2.2KW are industry standard sizes.

Or so I thought....

 

[Richiespark100]

Here's the proof of the motor plate guys

 

[davesparks]

So the motor plate says 1.27KW and 2.4A at 440V
Not the 1.7 KW in your OP?

 

[Richiespark100]

Yeah sorry that was a typo by me!

 

[davesparks]

Ok, but in the OP you say you need help calculating the current, but the current is written on the data plate?

Just take that current and select an isolator based on that current.
If you want it to also be a switch then you will need to check its rating for switching motor loads (not necessarily the same as it's rating as an isolator)

 

[Richiespark100]

Thanks Davesparks. Yeah I wanted to know to know the calculation because it's been a while since I've don 3phase calculation. This has spurred me on the get the study books out. I forgot about the square root and P.F serves me right tbh! Ok well I good on the calculations and I know that it's pulling 2.4. Here's the part which gets me I was gonna use a 25A 3P switch disconnector. Here's the link Usablenet also here's the data sheet of the isolator I'm looking at it's the KG20. It's this suitable? The most it's gonna pull is 2.4 and I want a 25A isolator is this ok or shall I go smaller?

 

[Knobhead]

I've got a slightly older book on the subject, I believe it was printed in 1939 and makes reference to the 10th edition regulations as being the latest edition. I can highly recommend it, it's called 'Stubbs electrical encyclopedia'

I've got three volumes of Stubbs 2, 3 & 4. Still usefull for theory.

 

[davesparks]

Ive got all four volumes of what I believe is the last edition to be printed, minus the last two pages of volume 3 (pages 1055 and 1066)

Mine all have blue covers with a picture if a pylon on the spine, I think the earlier editions had different pictures on the spine of each volume.

I think many people could benefit from reading the entry for 'electrician' amongst other things.

 

[Knobhead]

The bottom of page 1056 says "End of volume three"

You've only two pages missing.

 

[davesparks]

I know, I meant 1056 not 1066 in that post!

 

[Knobhead]

I guessed you did, just after I clicked post.

So you're missing the kinky bits..........