are the suppliers right about this contactor?

[lewislee]

Hi, a few weeks back i started a job on six new apartments, they were to have combi boilers for heating and hot water but the developer changed his mind and deciced to go all electric! great! The heating system he opted for were panel heaters controlled by a central room stat. I wired the system as per the diagram supplied by the heating company.

32 amp supply from c.u to 32 amp relay module(supplied by them) cable along with cable from stat, then on to a another c.u supplying the individual heaters.

I picked up the controls today to start second fix and they have supplied 25 amp 4 pole contactors, not a 32 amp relay module as per supplied paperwork, one per apartment.

I rang them to query this and they told me to bridge across 3 of the poles???

 

[Steve]

25 amps per pole, 4 poles. If each heater is 2kw, you can have two heaters through each pole.

Not sure how many heaters are in each apartment, you dont say. But 32A for heating a flat seems a bit "on the edge".

That contactor will do the job.

 

[lewislee]

Thanks Steve. Each apartment has 6 heaters total load 6kw. The thing is it has been wired to the diagram given to me at the start of the job which was set up for one 32 amp relay doing all the switching.

All the cables have been drilled through ply in the relevant positions for c.u, relay and c.u for the heaters so cant change the layout.

If id of known they were suppling a contactor id of wired it all into one large c.u as normal.

Because of this i dont have all the heater cables at the contactor position, only a 6mm from the main c.u a 6mm to the c.u controlling the heaters along with the 1.5 3 core from the stat.

They are saying i dont have to change the wiring layout ( obviously i dont want to do this!) i can just use the contactor in place of the 32 amp relay and bridge across 3 of the poles!!!

 

[ericmark]

To supply 32A you need 6mm cable but you would not want 6mm cable to each heater so a contactor in an enclosure with three 20A or less MCB's feeding the contactor with 6mm to all three contacts and feeding out through individual contacts to three MCB's would then allow 2.5mm cable to be used from the enclosure to each of the FCU's supplying each heater.

However 32A seems a little on small size. I know there may be some diversity but using the contactor will mean they will all switch on together so with 2Kw for water heating that leaves 5Kw to heat the apartment. So three heaters only I would guess?

 

[ericmark]

Sorry you post was as writing mine. I seem to remember you can get bridging blocks to join the 4 inputs and outputs together but without the proper bits I would have thought sticking bits of 6mm to bridge each way would look unprofessional and you may have problems getting 2 x 6mm cables in the holes?

You may need some of these to be able to get cables connected? If you want it to look good anyway. Assuming a small relay like

The screw is in centre of each terminal and only way to get 6mm in is to use forks you may be able to position one up and one down but not a very satisfactory way of doing the work. Wonder if you can back charge them for extra work?

 

[lewislee]

Hi ericmark. The water heating is being done by 2x immersions which are not conected to this system!!!!

They are panel heaters 1kw in bedrooms, hallway and kitchen, 1.5kw in lounge and 0.5kw towel rail in bathroom.
Because this is a new build its really well insulated for heat and sound loss.The heating company did a site visit to calculate the heater sizes taking into account the type and amount of insulation.

Basically they are telling me i can use a 25amp 4 pole contactor to switch a 32 amp supply if i bridge 3 of the poles together is right??

I just cant see it

 

[electronicsuk]

Why the confusion? You could link all 4 input contacts on one side and all 4 outputs on the other to give a total 100A single pole switching capacity, or you could wire as two pairs to switch live & neutral giving you 50A capacity.

Linking three poles will also work and give 75A switching capacity, but it makes more sense to have two pairs to allow you to switch live and neutral, and provides a more convenient solution than linking the neutrals in a terminal block inside the enclosure.

 

[lewislee]

This is the problem. There saying its feasable to bridge the poles and it wil switch the required load, i was just trying to confirm this before i waste my time connecting it all up only to find once the new supplies go in it doesnt!!!

Even if it is possible, as you say you wont get 2x 6mms in the terminals!! Iv tried a few places today to find some sort of shunt but had no luck!

I just cant understand why they have sent these contactors and not the relays that were shown in the original paperwork!!

 

[lewislee]

Thanks electronics uk. Do you know of anywhere that i could get something to use as a link??

 

[lewislee]

Thanks to you to ericmark. I looked at fork and finger crimps earlier but still wont work.

Maybe ill just back these and get 32 amp contactors and save myself the hassle!

 

[ericmark]

Sorry again writing at same time. I agree a botch but can't really see any way out. Think you would have to use forks to terminate cables or pins. Even then using 12A relay it will be tight. I think a phone call to relay manufacture and ask if it is recommended to use relay in that way. And if so do they do bridge kit. I think you will find they say no. As putting such big cables into the contactor is likely to warp it slightly and as a result all contacts may not make and break at exactly the same time so one contact could be overloaded?
If they say no then easy phone heater people and say what was told to you and if yes then you know you are doing right thing.

 

[lewislee]

Surely there must be something out there to get over this?! Being a domestic sparks i dont come across the use of contactors much but usually dont have to get over this problem, contactor(S) inside main c.u each pole fed from individual mcb.

Well iv got tonight to find something, just dont like the thought of jamming the cables in!!

 

[ColJack]

photo's of the contactor concerned, or a maks and model will help in the search..

as for a "tonight" solution you'll be hard pressed, it might take a few days to get the parts..

 

[lewislee]

Hi coljack its the PRE9030DRF.

My original wiring diagram showed the use of PRE9030 http://www.prefectcontrols.com/

 

[ColJack]

ok, now peel the sticker off and give us the real manufacturers name..