Spilt system Circuit size and Circuit Breaker size?

[airflo]

Hello,

Can anyone tell me how to properly size up the cable & circuit breaker needed to supply split system air con's?

Is it like below?
RLA x 125%??? for the breaker size?

Cheers,
airflo

[paul_h]

In australia you aren't licenced to do this.
You need to determine other factors, like voltage drop for long runs, inrush current for starting conventional compressors, then FLA to determine cable size, and the c/b is sized for the cable and current curve, also bearing in mind any unfused cables in the actual unit.

You don't size a c/b on the air con FLA, the c/b is there to protect the cable, so it depends on the cable size in the run and inside the unit unfused.

[Brian_UK]

Can anyone tell me how to properly size up the cable & circuit breaker needed to supply split system air con's?Ask an electrician to size up the equipment requirements when he quotes you for the work involved.

[airflo]

I know i'm not allowed to do it but im interested to know how it is sized up...........thanks for you input Paul H.

[Giannis]

Ask an electrician becouse is not only the current , you need to know the type of cable, voltage drop , compressor characteristics and more.

[multisync]

Ask an electrician to size up the equipment requirements when he quotes you for the work involved.


Ask an electrician -you're having a giraffe!

What will he say "ERR what does the manual say?"

RTFM then work back from there..

Multisync
London

[nike123]

For correct sizing you need first to know starting characteristic of your equipment. For example, fix speed compressor has 5-6 times greater starting current than FLA .
Also you need to know allowed voltage drop for proper start of compressor. That is usually 5-10%.
Than you could size your power supply cable according to above data.
Some sizing of cable you could done online on some internet sites.

Circuit breaker should be sized by the rules of selectivity, starting characteristic of AC and by total power consumption of that exclusive circuit. Usually, in exclusive circuits like AC, cable size is greater than normal because of starting voltage drop and therefore circuit breaker is not dimensioned by cable size (it would be significantly higher value).

For proper sizing of circuit breaker and cable you could use this Schneider software (http://www.soft.schneider-electric.com/myecodialS.htm).

[paul_h]

I think electricians are more advanced in europe compared to domestic electricians down here :D
I've even noticed a few people in other threads recommending an electrcian to look at a fridge or split system to diagnose faults. They wouldn't touch a fridge or a/c down here as they don't have a clue.
For small stuff like split systems, I've never known a sparkie to look at any books or software when calculating anything to be honest, most of the stuff here is inverters anyway. Sure completely different for commercial and large conventional compressors, but that's the better electricians that work on them. Domestic installing electricians don't know anything about refrig and never use any calc software or books.

Normally they just look at fla and it's normally short cable runs here anyway, under 15M.
Here they normally chuck 1.5mm2 and a 10A c/b for any small inverter split that is rated at 5A or under FLA
2.5mm and a 20A c/b for any split that draws under 16A FLA, or for any 5A split with a long cable run.
4mm2 and a 25A or 32A breaker for any split that draws under 23A FLA, or for any split that draws under 16A fla with a long cable run.

They're not such rocket surgeons to worry about using isolators half the time, or when they do, they sometimes use crappy small undersized ones like the ones designed for outdoor light switches.

[Jeff Cox]

Hello,

Can anyone tell me how to properly size up the cable & circuit breaker needed to supply split system air con's?

Is it like below?
RLA x 125%??? for the breaker size?

Cheers,
airflo
hi
all you need to do is check the name plate of the FLC [full load current } in Amps or just A.
Some formulas for calculating the cable size required takes into consideration starting spikes or peaks.
In Australia in my state it is 6x the FLC
But now day with smarter starting technologies this is mostly not considered .

Hope this helps

[brunstar]

Well in regards to cable sizing there are various things that do need to be taken into account that has an advere effect on the current carrying capacity of a cable, as like any trades electrical is a quite difficult one and it is not as simple as you think.
The cable sizes in certain countries are smaller than others.
Points that must be considered to protect a cable are.
running current, the length of the run, the ambient temperature the cable is installed in, if it is ran in trefoil, on a cable tray, bunched, spaced on a cable tray, in a conduit, in a pipe duct, burried, catenary.
Another thing that must be looked at is the KVA rating as if this is undersized the potential for the circuit breaker to explode under fault current is there.
If it is 10KVA it can withstand a fault current of 10 000 amps before it will explode.

Another thing that must be carried out is a fault loop impedance, how long it takes for the protective device to shut down.
This is important because if the CSA cross sectional are of the cable is not big enough the circuit breaker will not trip in time.
The whole reason for earthing is when a system faults as there is low resistance down the earth it makes the fault current higher and faster, which is what you want to make sure that your protective device trips.

The electrical trade is not as easy as you think and i think it is harder than refrigeration in itself.

[paul_h]

I wasn't having a go at all electricians, I know a few personally that work in the commercial/industrial sector.
I was having a go at the domestic/installer sparkies, like the ones that ask me what size cable they should use, (it's happened every time I do an install and sub out the electrical work) or the ones who don't fit isolators (or the wrong type), the ones that run 32m of 2.5mm to run a 3hp motor (kept buring out the soft starter due to excessive voltage drop, or the ones that don't even fit a c/b (connected it to GPO power), or put two units on one 10a c/b. The list goes on...
No offence intended to commercial sparkies, just domestic a/c installer sparkies don't seem to know too much or have ever read the AS book or did any calculations..

[TRASH101]

The electrical trade is not as easy as you think and i think it is harder than refrigeration in itself.

I don't think this is a fair comparison. There are various tiers of understanding/ aptitude for all things and we shouldn't cast judgment.

e.g.
split monkeys - wire chuckers

I'm sure we all know somebody who performs at a level that they shouldn't ( both up and down ).

Furthermore, refrigeration in itself is unlike any other mainstream engineering science as it is firmly rooted in thermophysical manipulation as well as the more conventional flow/current, pressure/emf and resistance/impedence.

[glenn1340]

Ask an electrician -you're having a giraffe!

What will he say "ERR what does the manual say?"

RTFM then work back from there..

Multisync
London

Couldn`t agree more. Been there plenty of times before. Or they`ll say "Dunno mate I fought you were the expert"

[CMC]

if your in the uk, I would suggest that you buy 17th edition wiring regs BS7671 from the IEE. This will describe in great detail how to size cables and select circuit protection. But you will still need to know the starting or inrush current as well as running current weather it is inverter driven or not. Also you will to know the incomming supply details, such as External Earth Loop Impeadance and Prospective Short Circuit Current, as this will determine the PSC of the circuit protective device. ie breaking capacity under fault conditions selection of the incorrect type could lead to the protective device failing to disconnect the supply in the event of a short circuit fault. I could go on but it should give you an idea of whats involved.

[frank]

But you will still need to know the starting or inrush current as well as running current weather it is inverter driven or not.
Starting current is not used when sizing cables. Running current is.

Also you will to know the incomming supply details, such as External Earth Loop Impeadance and Prospective Short Circuit Current,
The cable protective device (mcb or RCD) has a maximum fault current tolerance before it fails, usually 6000A or 10000A, instantaneous. The installed circuit should be measured to ensure that it doesn't exceed this rating. (Potential Short Circuit 'PSC'). The measurement is the resistance of the circuit conductor and the CPC.

as this will determine the PSC of the circuit protective device. ie breaking capacity under fault conditions
As stated, this is the other way around

selection of the incorrect type could lead to the protective device failing to disconnect the supply in the event of a short circuit fault.
The disconnection time of the protective device is determined by Zs , not the PSC

[nike123]

Starting current is not used when sizing cables. Running current is.



I think you are wrong here Frank.
Starting current have main role in dimensioning cable size because it affects voltage drop during start of compressor.

[frank]

I think you are wrong here Frank.
Starting current have main role in dimensioning cable size because it affects voltage drop during start of compressor.
Hi Nike

The formula for calculating volt drop does not refer to starting current, which is ignored.

Volt Drop =((L x A x Mv/m) / 1000)

Where

L = length of cable
A = Running current of device
Mv/m = milli-volt drop per metre of cable to be used (taken from tables)

The maximum volt drop allowed for any given cable is 4% (9v for single phase and 16 volts for 3 phase) here in the UK

[nike123]

Hi Nike

The formula for calculating volt drop does not refer to starting current, which is ignored.

Volt Drop =((L x A x Mv/m) / 1000)

Where

L = length of cable
A = Running current of device
Mv/m = milli-volt drop per metre of cable to be used (taken from tables)

The maximum volt drop allowed for any given cable is 4% (9v for single phase and 16 volts for 3 phase) here in the UK

Ok, this subject is one that we cannot agree for now.

Please read this PDF (http://www.electrical-installation.schneider-electric.com/ei-guide/pdf_files/EIG-G-sizing-protection-conductors.pdf), especially page 20!

[coolhandlew]

Hi all,
something else which is often overlooked is the size of the interconnecting cable. Assuming the unit requires a 32A supply to the outdoor unit how many a/c guys automatically install a 1.5mm 3 core cable to the indoor unit? This is only acceptable if the outdoor unit has a fuse protecting the interconnecting cable, eg 6.3A. Quite often this is not the case and the interconnecting terminals are connected direct onto the incoming supply terminals. (Daikin are making units like this now). In this situation you must install an interconnecting cable with a current carrying capacity (ccc) of at least 32A. Also bear in mind that if you cable tie this cable to the insulated pipes you are dramatically reducing it's ccc as it is in contact with thermal insulation on one side. This may mean it would be necessary to increase the cable size even further.

[brunstar]

hi guys i know it is getting heated on this topic but i agree with Frank, the cable size will be calculated to all of the above that frank states and then the last thing that is done is the voltage drop and if this is over the percentage drop the next size cable must be used and this way, if the fault loop impedance is generally correct, you should only have to go up one size in your CSA of cable.
all is looked at in cable sizing.
when you are doing MSB'S maximum demand, you do not calculate the starting current of everything on that board, it is a percentage depending if it is domestic or a commercial application.

[frank]

One other point to consider is, if you use the calculation provided by Nike for volt drop during start up, and it is excessive, then a star delta method could be adopted to minimise volt drop and prevent motor stall.

Something I always do when looking at motor start current (as well as following manufacturers recommendations)

[nike123]

hi guys i know it is getting heated on this topic but i agree with Frank, the cable size will be calculated to all of the above that frank states and then the last thing that is done is the voltage drop and if this is over the percentage drop the next size cable must be used and this way, if the fault loop impedance is generally correct, you should only have to go up one size in your CSA of cable.


And what is that than sizing of cable according to permissible voltage drop of motor (or permissible voltage drop by regulations, whichever is lower) which depend on maximum predictable current which in case of squirrel cage motors of compressors is starting current?
If we speak about direct on-line starting of squirrel cage motors, that current could be 5-7 times of normal running current. If we speak about starting current of inverter compressors then we could say that sizing should be done according to maximum operating current if starting current is not higher than maximum running curent.

http://farm4.static.flickr.com/3379/3253405899_7cca32e654_o_d.png

[nike123]

One other point to consider is, if you use the calculation provided by Nike for volt drop during start up, and it is excessive, then a star delta method could be adopted to minimise volt drop and prevent motor stall.

Something I always do when looking at motor start current (as well as following manufacturers recommendations)

If you use star delta starting method on device which is not able to overcome minimal reqired starting torque in star configuration, you will end up with stalled motor.

[paul_h]

Hi all,
something else which is often overlooked is the size of the interconnecting cable. Assuming the unit requires a 32A supply to the outdoor unit how many a/c guys automatically install a 1.5mm 3 core cable to the indoor unit? This is only acceptable if the outdoor unit has a fuse protecting the interconnecting cable, eg 6.3A. Quite often this is not the case and the interconnecting terminals are connected direct onto the incoming supply terminals. (Daikin are making units like this now). In this situation you must install an interconnecting cable with a current carrying capacity (ccc) of at least 32A. Also bear in mind that if you cable tie this cable to the insulated pipes you are dramatically reducing it's ccc as it is in contact with thermal insulation on one side. This may mean it would be necessary to increase the cable size even further.
You could just add your own fuse or CB in the outdoor unit like many installers do, instead of running 32A interconnect :D
Cheaper option.

[Cody82]

I wasn't having a go at all electricians, I know a few personally that work in the commercial/industrial sector.
I was having a go at the domestic/installer sparkies, like the ones that ask me what size cable they should use, (it's happened every time I do an install and sub out the electrical work) or the ones who don't fit isolators (or the wrong type), the ones that run 32m of 2.5mm to run a 3hp motor (kept buring out the soft starter due to excessive voltage drop, or the ones that don't even fit a c/b (connected it to GPO power), or put two units on one 10a c/b. The list goes on...
No offence intended to commercial sparkies, just domestic a/c installer sparkies don't seem to know too much or have ever read the AS book or did any
calculations..


LOL as opposed to the commercial sparkies who have worked on big sites and maintained fluro's/GPO's and data points.. You find ****heads everywhere, I work in the industrial sector and they are even here too.

[manender]

Hello,

Can anyone tell me how to properly size up the cable & circuit breaker needed to supply split system air con's?

Is it like below?
RLA x 125%??? for the breaker size?

Cheers,
airflo

hi,

it ll depend on the machine capacity,,,,,,,,,,,,
can u plz tell me the m/c capacity.

[coolhandlew]

I think you are wrong here Frank.
Starting current have main role in dimensioning cable size because it affects voltage drop during start of compressor.

Frank wins, regulation 525-01-02 of BS7671 states;

A greater voltage drop may be accepted for a motor during starting periods and for other equipment with high inrush currents provided that it is verified that the voltage variations are within the limits specified in the relevant British Standards for the equipment or, in the absence of a British Standard, in accordance with the manufacturer's reccomendations.

Yes, I know this is 16th edition but I suspect the reg won't have changed.

[nike123]

Frank wins, regulation 525-01-02 of BS7671 states;

A greater voltage drop may be accepted for a motor during starting periods and for other equipment with high inrush currents provided that it is verified that the voltage variations are within the limits specified in the relevant British Standards for the equipment or, in the absence of a British Standard, in accordance with the manufacturer's reccomendations.

Yes, I know this is 16th edition but I suspect the reg won't have changed.


Where I oppose to that?


And what is that than sizing of cable according to permissible voltage drop of motor (or permissible voltage drop by regulations, whichever is lower) which depend on maximum predictable current which in case of squirrel cage motors of compressors is starting current?

From what i read here, British standards are not giving recommendations lower than manufacturers.