fuse down Air con unit

[RobFurn]

Hello all, I work mainly in the Air Con industry, the general wiring make up of an AC installation is, DB to External Compressor then Compressor to Internal Evaporator using a control cable. Since working in this industry I have always been concerned with the control cable. The current draw of the compressor can be high and therefore a large cable from DB to compressor is usually installed. But from Compressor to Evaporator the current draw is usually quite small so you could use a lower rated cable, however it will only be protected by the breaker in the DB, which is inevitably large to cope with the compressor. I would like to start fusing down so we can use a lower capacity cable from Compressor to evaporator. I was thinking a bit of din rail mounted in the compressor and an MCB mounted on that would be ok,. Any objections? The compressors are situated outside and although IP protected, there is little protection from cold and damp any ojections here?. Alternatively an inline cartridge fuse could be used to fuse down again any objections.
Thanks for any advice given .

 

[Spark123]

The control cables should be fused as such that the supply is automatically disconnected quickly enough to remove danger in the even to of a fault, basic principle. (Control cable immediately says to me that it is quite a bit smaller than a feeder, or am I getting the wrong idea?)
The last one I looked at consisted of a supply cable to a control box with a row of MCBs, one for the contactors, one for a heater, one for the fans and one for the compressor unit. In this setup each cable can then be sized to the MCB rating.

 

[RobFurn]

Yeah the control cable is quite a but smaller. As an example I am working on at the moment. Compressor unit is rated at 25A max. Internal evaporator is 0.15A so you can see my predicament, complete waste of money running the lot in 4mm. I think I will stick with a breaker and run the control cable in 1 or 1.5m from that.

 

[pte440]

although no help really, a lot of the IMI units i used to fit had a fuse in the condenser which was normally 3amp which protected the crankcase heater, fans and evaporator.

 

[Adam_151]

On a side note, can you answer me something...

On some jobs, I've seen aircon going in, and it seems to have a condensate pump on the internal unit... I though condensate normally came from external unit?

And is there reason this pump cannot be powered from internal unit?, rather tahn an FCU/single SO on a local power circuit?

 

[Steve]

The condensate pump is always indoors - it collects the condensation from the indoor unit - when hot air in the room meets the cold coils, drip drip . . .

On another note, I have seen indoor units fed from the outdoor unit, via a FCU mounted next to the indoor unit. I suppose the FCU could be mounted outdoors to minimise copper useage if you wanted.

 

[RobFurn]

Adam,

As Steve says the condensate pump is always indoors, some (ceiling cassette types) have them built in most dont so you have to wire one in Aspen are a popular manufacturer. Thats not to say condensation does not appear on the external unit, there is a drain their as well, no pump requied as their 99.9% outside but they are supplied with a drian adaptor to attach to the hose if necessary. The pumps are powered from the internal unit most pumps are also fitted with a trip to cut the supply to the evaporator if the pump fails so you dont get water all over the place.
Regarding Craftys sorry Steves note on FCUs next to the internal unit its somtimes the case that the supply goes to the internal unit first (off the ring) and the control cable is fed to the exteranl unit, but only on smaller units usually high wall type, I have one in my living room just like that, Aircon in your front room!!!! no im not bary its an extramly efficient way of heating your home, Im never turnign the gas fire on again! usefull in the summer as well

 

[ban-all-sheds]

Yup - call them air-source heat pumps and they sound very green....

 

[plugwash]

I was under the impression that when you considered the inefficiencies of generation and distribution heat pumps worked out at about the same overall efficiancy as gas heating.

 

[Stoday]

The current ratio of electricity to gas price is around 2:1, so the coefficient of performance of the heat pump only has to be greater than 2 for electricity to be cheaper.

These days you can get seasonal heating COP of 3.5 or even 4.

In making a comparison, don't forget that the gas price per kW is based on the gross calorific value. On the other hand gas use in apparatus is based on the net calorific value. The difference is 10%.