Sizing a Contactor.

[YoungBart]

Morning all,
Possibly a simple question But a mix of doctor prescribed medication and post Wembley joy have left my brain elsewhere.

I have a sub DB that needs the supply isolating if a fire is detected (commercial Kitchen) So The 3 phase supply for this DB needs a 4P contactor. Do I size the contactor to the current load of the sub board(10A) so Say a 20A contactor.
Or do I need to size the contactor to 63A same as the Sub DB main switch?

Thanks Chaps...

 

[Leesparkykent]

what size is the OCPD for the distribution circuit feeding the kitchen DB?

 

[YoungBart]

Sorry, I've given the wrong info already. 63A breaker supplying new sub board with 80A main switch.

 

[Leesparkykent]

Sorry, I've given the wrong info already. 63A breaker supplying new sub board with 80A main switch.

Put a 63A contactor in then. probably around the same price anyway for a 20A or 63A 4pole contactor.

Edit: I didn't insert the skim link...you can get a 63A 4pole contactor for around £25-£30.

 

[Brightspark2]

As said size contactor for 63A, protective device rating. Remember correct AC rating depending on load being switched.

Sent from my E5823 using Tapatalk

 

[Darkwood]

As above you need to know your load types IE resistive/inductive AC1/AC3 respective, the price different can be 2-3 times but get the wrong contactor and it may be damaged when operating under load. If in doubt or you have a high Inductive nature to the loads then fit a 30kw AC3 contactor with a coil rating of your choice dependent on the control voltage.

 

[YoungBart]

Thanks for the help guys.
The load is pretty straight forward. A couple of vent units. I'll check the data for them and find out more on the load type.

 

[HandySparks]

Clearly, the 'safe' thing to do is to size the contactor to the rating of the OCPD of the distribution circuit. This is probably the pragmatic thing to do if the additional cost for a higher rated contactor is small.

However, if the sole purpose of the board is to supply the kitchen ventilation system, which comprises several known fixed loads, I would not think it wrong to size according to the actual load current. If there is the possibility of overload, one could consider rating to the sum of the OCPDs in the board, especially if there is no possibility of adding more ways.

 

[YoungBart]

Clearly, the 'safe' thing to do is to size the contactor to the rating of the OCPD of the distribution circuit. This is probably the pragmatic thing to do if the additional cost for a higher rated contactor is small.

However, if the sole purpose of the board is to supply the kitchen ventilation system, which comprises several known fixed loads, I would not think it wrong to size according to the actual load current. If there is the possibility of overload, one could consider rating to the sum of the OCPDs in the board, especially if there is no possibility of adding more ways.

That's Where my confusion was coming in, I know the load, etc So was initially planning to size the contactor to that, But with it being a commercial kitchen the chance of someone adding a fairly high load item of equipment is always a threat.
So I've sized it to the OCPD. As you say, To be safe.

 

[Spoon]

Can you not just add a shunt trip to the breaker supplying the DB?