Hi there,
A 380V 3phase Air Conditioner with a rated input 7700W and a rated Current 15.0A (48,000 btu)
Is the rated current PER PHASE or on all phases?
I am looking to have 10 of these units installed in a building and would like to know the appropriate cable size to supply all these units.. do they all add up to ~150A on all cable cores or ~150A on each cable core?

 

Hi,

Sounds like it's the total current, but for a project this big and expensive you think you should be asking for advice on a forum board? I think you should hire a responsible engineer. If you install the wrong wiring, you've got to replace it later, and that's the best possibly scenario if nothing else goes wrong.

 

That's the current in the wire of each phase.

 

Hi,

Sounds like it's the total current, but for a project this big and expensive you think you should be asking for advice on a forum board? I think you should hire a responsible engineer. If you install the wrong wiring, you've got to replace it later, and that's the best possibly scenario if nothing else goes wrong.

Hi, thanks for the reply..
It's a very long story, and I'm not going to install it myself anyway, I just need an advice because I need to buy the cable from a far area and there is a bit of logistical and financial problem where I live

 

Hi,

I would like to answer this but there is a lot more to it than for a non motor application for example.

If we had 8000 watts and 400vac single phase the current would be:
8000/400=20 amps.

Because it is three phase, then each phase will see about 20/3 which is roughly 7 amps per phase if it is set up as a Wye system.

That's with a resistive load of 8000 watts, with 1/3 of that on each phase.

With a motor however there is almost always a starting current, and that could easily be 5 times the normal run current.
Also, when the motor is running normally there will be a voltage drop along the wire, and that has to be accounted for as well as the drop when the motor is starting. This means the length of the wire run must be known in advance.

Do you know how long the wire run will be for ALL of the installations?
Also, is there some spec on the lowest voltage the line can go in this area?
Also, is the AC unit a wye load or delta load?

If you cant afford a good engineer to work this out for you then you should at least try ONE installation first to make sure it works properly. That's unless you dont mind oversizing the wire to start with or dont mind having to install a second wire run if the first does not work right.

We can go through some more detailed calculations, but we really have to know how long the wire run will be as that is important.

 

Hi,

I would like to answer this but there is a lot more to it than for a non motor application for example.

If we had 8000 watts and 400vac single phase the current would be:
8000/400=20 amps.

Because it is three phase, then each phase will see about 20/3 which is roughly 7 amps per phase if it is set up as a Wye system.

That's with a resistive load of 8000 watts, with 1/3 of that on each phase.

With a motor however there is almost always a starting current, and that could easily be 5 times the normal run current.
Also, when the motor is running normally there will be a voltage drop along the wire, and that has to be accounted for as well as the drop when the motor is starting. This means the length of the wire run must be known in advance.

Do you know how long the wire run will be for ALL of the installations?
Also, is there some spec on the lowest voltage the line can go in this area?
Also, is the AC unit a wye load or delta load?

If you cant afford a good engineer to work this out for you then you should at least try ONE installation first to make sure it works properly. That's unless you dont mind oversizing the wire to start with or dont mind having to install a second wire run if the first does not work right.

We can go through some more detailed calculations, but we really have to know how long the wire run will be as that is important.

I appreciate your kind reply, I tried contacting a trained engineer with no luck and I am running out of time and need to buy the cable, I understand there is a lot of factors to consider such as the ones you kindly listed, but I was trying to figure out if the rated current of the mentioned AC is calculate per phase or on all phases as this would make the biggest difference..
The Cable would be around 80 meters long, the line voltage should be fine without much of a voltage drop and the Unit is Delta Configuration I think..
After all I might have no choice but to wait for a trained engineer and cancel the cable purchase for now..
Thank you sir for your time.,..

 

\(\frac{380} sqrt(3) = 220 volts \)
So guessing the phase to phase volts = 380v
and the phase to neutral voltage= 220v
Could this be the case?

Yes

 

You shouldn't need an engineer. Any licensed or experience electrician should be able to size it. Your local electric jobber or supply house can size it....with the proper information.

 

f we had 8000 watts and 400vac single phase the current would be:
8000/400=20 amps.
Because it is three phase, then each phase will see about 20/3 which is roughly 7 amps per phase if it is set up as a Wye system.

That's not true if the 3 phases have a balanced load (no current through the Wye ground).
In that case the current in each phase is reduced by a factor of √3 (1.73) not a factor of 3, thus the 20A is reduced to 11.55A per phase.
You can look at it as each wire is carrying some of the current from the other two wires.

 

If the name plate power rating is in fact 7700 watts, then the per phase power is 2567 watts and the current is 11.67 amps per phase.

However, the 80 meter distance from source to load is enough to produce significant voltage droop at full load, due to distributed inductive reactance in the transmission wire.

The inductive reactance will cause a low voltage problem unless you can adequately compensate for it ... a larger than necessary wire gage, or a capacitive reactance at the load.

 

A 380V 3phase Air Conditioner with a rated input 7700W and a rated Current 15.0A (48,000 btu)

It's also a horribly inefficient air conditioner with an EER of 6.23 (not counting the power used by the fan motors).

 

However, the 80 meter distance from source to load is enough to produce significant voltage droop at full load, due to distributed inductive reactance in the transmission wire.

I don't think so.
The inductance for that length of Romex should be less than a 100μH, giving a voltage drop of less than a half volt for 11A of current at 60Hz.

 

That's not true if the 3 phases have a balanced load (no current through the Wye ground).
In that case the current in each phase is reduced by a factor of √3 (1.73) not a factor of 3, thus the 20A is reduced to 11.55A per phase.
You can look at it as each wire is carrying some of the current from the other two wires.

Hi,

I am not sure what you are talking about here. A three phase wye is like a system of three independent systems. Just because the currents cancel in the neutral wire does not mean the currents are somehow shared in the loads. The neutral current is a different aspect.

Example:
A set of three 10 ohm resistors as load driven at 100vrms for each of the three phases A,B,C, with a neutral wire.
The rms current per phase is 100/10=10 amps.
The power per phase is 1000 watts.
The total power is 3000 watts.

If we had a single phase system, that 3000 watts would be drawing 30 amps, as 30 amps times 100vrms is 3000 watts (resistive).
If we divide 30 by 3 we get 10, the current in each phase of the three phase system.

Back to the main topic:
80 meters is a very long run. The voltage drop must be taken into account, and also the voltage drop during motor start up. If the voltage drop is too high during start up the AC units may keep trying to start, then shut down, then try to start, then shut down, repeatedly over and over again. This is a very important aspect of this kind of system. There will probably be about 2 to 3 minutes between tries as the thermal protecting device keeps opening the circuit, then needs time to cool down, then tries again, etc., etc.

 

I am not sure what you are talking about here. A three phase wye is like a system of three independent systems. Just because the currents cancel in the neutral wire does not mean the currents are somehow shared in the loads.

Your are correct.
I was confusing wye and delta connections.

 

It's also a horribly inefficient air conditioner with an EER of 6.23 (not counting the power used by the fan motors).

7700W is the rated input, I'm sure the running wattage is much lower, I've seen another system 60,000 btu with a rated input of 12,000W they're both V5 ACs

 

Hi,

I am not sure what you are talking about here. A three phase wye is like a system of three independent systems. Just because the currents cancel in the neutral wire does not mean the currents are somehow shared in the loads. The neutral current is a different aspect.

Example:
A set of three 10 ohm resistors as load driven at 100vrms for each of the three phases A,B,C, with a neutral wire.
The rms current per phase is 100/10=10 amps.
The power per phase is 1000 watts.
The total power is 3000 watts.

If we had a single phase system, that 3000 watts would be drawing 30 amps, as 30 amps times 100vrms is 3000 watts (resistive).
If we divide 30 by 3 we get 10, the current in each phase of the three phase system.

Back to the main topic:
80 meters is a very long run. The voltage drop must be taken into account, and also the voltage drop during motor start up. If the voltage drop is too high during start up the AC units may keep trying to start, then shut down, then try to start, then shut down, repeatedly over and over again. This is a very important aspect of this kind of system. There will probably be about 2 to 3 minutes between tries as the thermal protecting device keeps opening the circuit, then needs time to cool down, then tries again, etc., etc.

Thank you very much, I would take all that into consideration ..

 

Why R22?

http://www.energyvanguard.com/blog/...2-for-Air-Conditioners-May-Close-the-Loophole

I know your not from the US, but http://www.brighthubengineering.com/hvac/61968-timeline-for-phase-out-of-r22-refrigerant/

in the US it will not be manufactured or imported soon.

http://en.wikipedia.org/wiki/Chlorodifluoromethane

 

Hi,
I got a good understanding on 3 phase wye wiring(connection), Is there any big difference using a delta (3phase wiring).

Can any body explain...

 

Look here: https://en.wikipedia.org/wiki/High-leg_delta

 

Why R22?

http://www.energyvanguard.com/blog/...2-for-Air-Conditioners-May-Close-the-Loophole

I know your not from the US, but http://www.brighthubengineering.com/hvac/61968-timeline-for-phase-out-of-r22-refrigerant/

in the US it will not be manufactured or imported soon.

http://en.wikipedia.org/wiki/Chlorodifluoromethane

I understand your point, but these are the ACs available here, I can't help it