allowable wire current

[svizoman]

Air condition installer told me that I need either:
two parallel cables 5x2.5mm2 with 6x16A brakers or
one cable 5x4.0mm2 with 3x20A brakers.

We are talking about inverter heat pump Daikin Altherma HT, with termal power output of 11kW and with aditional 6kW inline electric heaters.

I have an older house where single core cables are build into the walls rather than multicore cables beeing laid into the plastic cable hose. I am using 2.5 cables and 3x16A brakers. My main house brakers are 3x20A (3 phase 50Hz, 400V) so using 3x20A brakers just for one consumer is not a good idea.

Considering EU regulation for house electric lines we can affor up to 3% of voltage drop for regular 70C termoplastic cables. I found two equations to calculate needed size of vire (in square milimeters) and power it is transported using 3phase, 400V supply.


Since inverter (variabile freq drive) acts as resistive load and electric heater is a resistive load we can neglect sin fi in our equation. So at 400V, 3 phase, 50Hz AC suply with 16A brakers we can transport round 11kW of electric power which should be enough for both heat pump as for additional el heaters.

Using second equation we calculate that 0.5mm2 wire (20 AWG) is more than enough to carry 11kW load at 400V, 3 phase AC at the distance of 10meters, considering EU regulation (up to 3% voltage drop and cable thermal insulation).

So why does offical Daikin installer want two paraller cables of 2.5mm2 with 6x16A brakers or one parallel cable of 4.0mm2 with 3x20A brakers?

Did I miss something?


My question to you is - am I right at my thinking or not.

 

[KlausST]

Hi,

Considering EU regulation for house electric lines we can affor up to 3% of voltage drop for regular 70C termoplastic cables.

Never heared about this.

It makes no sense, because temperature rise then depends on cable length.

An example: 17kW max. total power. 3% voltage drop on the cable is about the same as 3% power dissipation at the cable.
This means about 500W heating on the cable.
If the cable is 1m in length, then call the fire brigade.

If the cable is 500m in length then it is OK.

***********************
i´d say 2W...5W per meter is acceptable.
************

Best is to keep on your local regualtions. Your installer may be right.

Klaus

 

[BradtheRad]

From what I know about heat pumps, they contain a compressor similar to an A/C unit. The motors are known to draw 3-4 X normal running amperes. (Example, on startup, or when stalled due to restarting too quickly after cutting power.)

Could this high current draw be what the installer is thinking about?

 

[svizoman]

KlausST I agree with you. Regulation in fact is way more complex and there are several online calculator made acrording to the regulation and for the input data you must also chose cable type (single/multi core), cable insulation (termoplastic, shilded), cable length, fuse type (quick, slow...), where cables are installed (wall, ground, plastic hose) all to get the cable size. I agree that max few watts per meter is acceptable as heat looses, otherwise vire will act as a heater not a conductor and if I am not mistake only .5 watts per meter length is allowed.

But still 2.5mm2 vire should be well enough to carry constant 20A load. Also 16A brakers on 3 phase 400V AC supply should provied enough juice to run both units (split system) + heater.

BrattheRad I know of spikes. This unit has a "smart" inverter and it should not draw more current that specified, not even at startup or shutdown.

More of a question are other inhouse consumers like washingmachine or dishwaser that don't use inverter and don't have smart logics and runs only on one phase and can draw much amps at startup. Today ovens and induction stove can also eat much juice up to 5kW

 

[chuckey]

Wire rating is found out first from size of cable and its mountings. Then the length and voltage drop is calculated , if the voltage drop is greater then 2 1/2% (UK), then a larger cable must be used. I have seen 25 mm^2 cable used for a 10 KW shower heater, because the length of the cable was so long( 50m?).
Frank