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Hello,

I have a 7.5kw 3 phase motor 400/690v 50hz. Can anyone advise what size and type mcb is needed for this please? It’s currently being fed via a c16 which is tripping now and then.

many Thanks
 
D Type for motor starting.........Just noticed the DIY. Messing with 400v definitely not advised. Get someone qualified to sort it.
There are many factors to be taken into account.
 
Assuming this is running from 400v then the flc is approaching the rating of your protective device. Your start up current could be ×6 this so the 16A is bound to trip. As above seek some guidance.
 
Hello,

I have a 7.5kw 3 phase motor 400/690v 50hz. Can anyone advise what size and type mcb is needed for this please? It’s currently being fed via a c16 which is tripping now and then.

many Thanks

Welcome to the forum mate.
It's not just a simple question. Other factors are involved.
What's the motor for?
What's the starting system wiring?
 
Star delta will have a much reduced start current compared to direct, and given that it only trips intermittently you may be ok with a C20 mcb.

You will not need to provide overload protection as the starter will contain much better protection for overload than the mcb can offer anyhow.

BUT, you absolutely must provide fault level protection, which means you need to ensure the Zs is less than needed to operate the mcb in less than 0.4sec.

If you can measure this up front and determine the correct value for the C20 then you will know, if you don't know this, you need to get someone else to look at it.

Playing around with 3 phase stuff at this rating is way more of an issue than anything at home!
 
Hey,

thanks for the reply.

So the measured Zs at the commando socket is 0.30 so well below the 0.87 for the c20 and even the d20 at 0.43.

I’ll try the c20 and see how I get on. On further reading, a 10hp motor running current is around 18amps so think what you’ve pointed out is about right.

many Thanks
 
Star delta will have a much reduced start current compared to direct, and given that it only trips intermittently you may be ok with a C20 mcb.

You will not need to provide overload protection as the starter will contain much better protection for overload than the mcb can offer anyhow.

BUT, you absolutely must provide fault level protection, which means you need to ensure the Zs is less than needed to operate the mcb in less than 0.4sec.

If you can measure this up front and determine the correct value for the C20 then you will know, if you don't know this, you need to get someone else to look at it.

Playing around with 3 phase stuff at this rating is way more of an issue than anything at home!
You should buy an enclosure containing a 32A contactor with a 415v coil and an
18-25A motor starter.
You wii also need to replace your mcb with a 32A 3 phase model.
Upgrade the cables to 6mm tri-rated with 6mm cpc.
Run in flexible conduit to above-mentioned enclosure.
This is not a job for the DIYer!!
 
You should buy an enclosure containing a 32A contactor with a 415v coil and an
18-25A motor starter.
You wii also need to replace your mcb with a 32A 3 phase model.
Upgrade the cables to 6mm tri-rated with 6mm cpc.
Run in flexible conduit to above-mentioned enclosure.
This is not a job for the DIYer!!

You should not use an overload that big on this motor, the flc will be around 15A, whichever overload is used it should be able to be set at the full load current not circ 20 - 67% above it.

There should be no reason to add a second starter, the star-delta one if it's working fine can be retained.
 
We have no knowledge of the starter except that it is star delta. We assume this component is fit for purpose.
 
We have no knowledge of the starter except that it is star delta. We assume this component is fit for purpose.
Old forces adage, assumptions based on incorrect or no evidence are invariably wrong.
I would replace the lot, give a guarantee and leave a customer with the knowledge that everything was sorted.
 
You should not use an overload that big on this motor, the flc will be around 15A, whichever overload is used it should be able to be set at the full load current not circ 20 - 67% above it.

There should be no reason to add a second starter, the star-delta one if it's working fine can be retained.
You should not use an overload that big on this motor, the flc will be around 15A, whichever overload is used it should be able to be set at the full load current not circ 20 - 67% above it.

There should be no reason to add a second starter, the star-delta one if it's working fine can be retained.
Julie,

absolutely bang on. Swapped the C16 with the C20. Clamped on the ammeter and it was pulling roughly 15odd amps. I guess it was cutting it fine for the 16a breaker!

Many Thanks for your responses
 

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