Air Con. Volt Drop

[RobFurn]

I have a question regarding the calculation of V.D for Air con units.

The supply to an AC unit is generally connected from DB to external compressor unit. The internal (fan) evaporator is then connected from the external compressor unit via a 3 core and earth cable. Its easy enough to calculate the V.D from DB to external compressor but what about the calculation from External compressor to internal evaporator. I’m concerned with the cable length. Do I simply take the length of the cable (3 core +Earth) from external unit to internal unit or do I have to use the entire length from DB---- to----- Compressor---- to ----Evaporator ?

 

[ban-all-sheds]

Surely

1) The distance between the two units is usually pretty short? (Or are you talking about commercial installations?)

2) The load of the internal unit is pretty low?

 

[tim west]

Unless seperatly fused I'd treat it as part of the same circuit and include the entire length in the calc.

 

[Space cat]

The volts drop at the compressor can be calculated using the COMBINED currents of the compressor and fan along with the cable size/length to the COMPRESSOR ONLY. For the ADDITIONAL volts drop at the fan, use the fan current along with the compressor-to-fan cable.

When you do this you should find that the extra drop between compressor and fan doesn't amount to much. (BAS is correct; the compressor is the heavy load.)

Note that the current drain of an inductive load - which you have - is bigger than the power divided by the voltage. If current isn't actually given, look for a VA (volt-amp) rating and divide that by the voltage.

 

[RobFurn]

Thanks one and all . I will do the following:

1) Calculate V.D at the compressor using the combined Compressor and Fan current. Use cable to suit from D.B to compresssor as per the outcome of that calculation.

2) Calculate the V.D from Compressor to Fan but using the entire cable length from D.B to Fan this time using the Fan current only. likewise use a suitable cable as per the outcome of that calculation.

Pretty sure thats right, shoot me down if not

 

[Space cat]

Your first calculation is correct. For the second one, use ONLY the length from compressor to fan BUT the voltage drop along this length must be added to the drop you already have at the compressor.

Example: You've chosen a cable from DB to compressor that drops a volt (that's just a number I picked out of thin air) and you don't want to lose more than two volts at the fan. You're one volt down already so your maximum drop on the fan cable is one volt.

Did that make sense?

 

[RobFurn]

Space Cat.

Ok so from Compressor to fan calculate the V.D in that length only and add whatever is dropped to the V.D from D.B to compressor. If the total does not go over the 4% allowed i.e. 9.2V then were ok. Right

 

[tim west]

I would hope that the 3 Core & Earth is seperately fused and therefore can be calculated seperately otherwise there should not be any derating of cable from source to the fan and any VD calc should be done over the whole circuit ie when seperately fused the fan supply becomes a sub circuit with its own set of calcs, the main supply to the compressor of course takes this fan load into account(whether minimal or not it still forms part of the calculation).

 

[Space cat]

I would hope that the 3 Core & Earth is seperately fused and therefore can be calculated seperately otherwise there should not be any derating of cable from source to the fan --

Good point Tim. In the absence of a separate fuse, the cable to the fan must be big enough for whatever fuse the compressor is on but --

I don't see how the presence or absence of a fuse makes any difference to voltage drop calculations. That's just Ohm's law.

 

[tim west]

It can influence the selection of the cable and the fuse protection of the sub circuit, in other words by derating i mean choosing a smaller csa.

If there is not seperate fusing then the csa must stay the same(dependant on choice of cable and circumstances of install) throughout the entire circuit( not always practical or cost effective) and the vd calc is done on the entire length of the circuit, if seperately fused then the cable to the fan can be derated accordingly if the volt drop calc allows (a seperate vd calculation because of the difference in csa,cable type etc etc).

When i say if the calc allows, it has to be as you rightly said an overall volt drop including both cables together but as you know you cant use just one calculation when there is a difference in csa,cable type etc (due to the different values in the tables), you have to break up the circuit and then add the two volt drops together so that the overall volt drop is still permissable.

 

[RobFurn]

Tim

you have lost me a tad on your last post.

Your saying that if not separately fused then the same cable must be used in the entire length of the cable run. I cant see how you would need a separate fuse if the fuse back at the D.B is sufficient to protect the derated cable from Compressor to Fan. Surely this is ok?

 

[Space cat]

I cant see how you would need a separate fuse if the fuse back at the D.B is sufficient to protect the derated cable from Compressor to Fan. Surely this is ok?

If the fuse at the DB is small enough (NOT big enough) to protect the derated (ie smaller) fan cable, why would you use a bigger cable for the compressor?

It works like this:

1) Your compressor and fan together need a certain amount of current.

2) The DB fuse must be big enough to carry this.

3) All the cable must be big enough to carry the current rating of the fuse.

You can't 'derate' the fan cable. It must be big enough to carry the fuse current or it won't be protected.

 

[tim west]

Tim

you have lost me a tad on your last post.

Your saying that if not separately fused then the same cable must be used in the entire length of the cable run. I cant see how you would need a separate fuse if the fuse back at the D.B is sufficient to protect the derated cable from Compressor to Fan. Surely this is ok?

No in that case the derated cable would be protected but you would then probably suffer from nuisanse tripping or fuses blowing as the main fuse probably wouldnt cope with the demand from the compressor.

The main fuse must not only protect the cable throughout the circuit but must also be of sufficient rating to be used with the appliance.

If the Fan cable is derated from the main supply cable then seperate fusing must be used

 

[RobFurn]

Tim thanks for your response

Every AC job I have seen always has a derated cable from Compressor to Fan (without fusing). this is obviously a No No but I was under the impression that so long as the main fuse is suitable for both cable types than that is fine.

I think the best way to fuse down if your going to use a deratred cable is to use an in line fuse inside the compressor where the 3 core and earth leaves the teminal block. Either that or use an FCU where the cable enters the building and derate from there. What do you reckon, any other suggestions.

 

[tim west]

Tim thanks for your response

Every AC job I have seen always has a derated cable from Compressor to Fan (without fusing). this is obviously a No No but I was under the impression that so long as the main fuse is suitable for both cable types than that is fine.

I think the best way to fuse down if your going to use a deratred cable is to use an in line fuse inside the compressor where the 3 core and earth leaves the teminal block. Either that or use an FCU where the cable enters the building and derate from there. What do you reckon, any other suggestions.

I was careful to state "type of cable" previously, you would have to look up the tables to see if this smaller sized cable is suitable for use without seperate fusing first. It may or may not have a similar current rating? Or be of short enough length to comply with the regs regarding not having to fuse down ie adequate protection against mechanical damage. A similar dilemma came with smaller sized cables off of busbars within a chamber to Isolators/dist boards where as long as they were not longer than a certain length you could get away with downrating up to if i remember right no less than half the largest conductor