15 amp power lines

I recently installed 15 amp power to my tool cupboard so that I could hide my compressor in there. It used to work fine when it was plunked in the middle of my workshop but it has been blowing its condensers lately, and at $30 a pop I don't want to replace them again.

The details are:
The line used to be 5 metres long and worked OK.
I extended the line by 7 metres to put it in the tool cupboard.
The wire I used is stranded copper wire with the following dimensions
Positive wire: 5 strands of .65mm dia
Negative wire: 7 strands of .65 mm dia
Earth wire: 7 strands of .65 mm dia
The compressor also has a 2 metre 15amp lead on it
The motor is 240v 3 hp (or so I am told, the plate is illegible) and (supposedly) draws about 40 amps on startup. :(

The condensers are 108-140 MFD 220v.

The question is, are the wires heavy enough to support this load and enable the condensers to do their work without melting?
When they blow, they make a reasonable sized bang and the room fills with smoke. :eek:

If they aren't heavy enough, what size should I be using for a run of that length?

Bob

A couple of questions.

Is the 15 Amp socket permantly wired with flat Twin and Earth cable or are you using round flexible cable.

The twin flat cable should be what is called 2.5mm² and is rated at 23amps.

The cable you have stated is uneven between the positive and the negative. You say one has 5 strands and the other has 7 strands. Both should be the same which should be 7 x 0.67

It should be protected with either a 20 amp circuit breaker or a 15 amp fuse.

Hi Barry
The cable is permanently wired flat cable.
I just stripped another bit of the red wire and it is also 7 strands. Sorry, must have broken a couple of wires when I did it the first time.

So this cable is not good enough then? .325 * .325 *(Pi) 3.14 = .33 mm
.33 * 7 = 2.3 sq mm

Hi Bob

That is 2.5mm² cable and it should be capable of carrying the current. 2.5mm² comes as 7 x 0.67 or 1 x 1.78. I think stranded cable will actually carry a slightly higher rating.

What size is your fuse or circuit breaker.

Not sure what could be blowing the capacitors, maybe need to get the motor looked at.

Bob,

What cables feed the board. Maybe there are lighter cables or too long lengths for the area etc which causes a voltage drop. Take a voltage reading at the motor with it running. I am sure that you'll find a huge voltage drop and that is what is causing the problem. It is quite common with contractors using long undersized extention leads on compressors.

:)

Maybe having the compressor hidden inside a cupboard is causing heat to build up (the cooling fins are there for a reason).

The circuit breaker is 20 amp.

Damn! I was hoping you weren't going to say that about the motor.

How is the motor actually checked?
Is this something I can do myself with a meggameter?
I assume that you are thinking that part of the motor windings is blown thereby putting extra load on the rest of it and thereby increasing the amperage draw drastically?

I wrote that last posting about two hours ago before I went to sleep to get ready for work tonight but then got distracted and forgot to press the send button.

Dan, it's a big cupboard. about 3m * 2 m. It is the coolest place in the house. :)

Gatiep: Hmmmmm. I will try that. That is what I was asking before, whether or not the cable was thick enough for a run of that length; I just didn't know how to test it.

Bob

Not sure about testing motors, may be worth taking it to someone who fixes motors.

I think Joe was talking about what cable size to your sub board if you have one or does the 15 Amp circuit go back to the main switch board.

Is the run 7 metres or 12 metres. Even a 12 metre run is not excessive for a 15 amp circuit if it comes off the main board.

If it is coming of a sub board it depends on how far that run is and what size cable that runs to it.

My main board is out in the paddock on the transformer pole. My run to the sub board is in 6mm² cable and the run to the sub board is 43 metres. Admittedly my 15 Amp circuit is only 2 metres from the sub board, but I am going to put another 15 amp circuit in that will be 8 metres from the sub board but I think I will run it in 4mm².

Test the voltage at the switch closest to the motor with a multimeter before switching on, then again with the motor running. I suspect that somewhere in the circuit there may be some non standard wiring, which with the extra few meters of 2.5 mm 2 that u have added has made the drop too much for the motorcapacitor to be comfortable with. Ofcourse you could just run it with the previous setup that you had when it did run good. If it keeps working there, it proves the point. How about phoning a sparky mate?


:)

Quote:

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Originally Posted by gatiep

How about phoning a sparky mate?

---

Used to have one next door but he moved. Is a plumber OK? I still got one of those. :)

I can't really run it in the old place as it was the middle of our garage and I had to move it too often.
I will try a voltage measurement before and after as you suggest.
Gotta skeddadle off to work now. :( 3 years before I retire. :D)

Bazza, you are 100% in with what I am thinking. Like in your case for instance, if you checked to the sub board from the plug all would seem ok, but if you only had 2.5 istead of 6 mm2 running from the paddock, it could've caused a problem.Most these things work out well be retracing one's steps.....a process of elimination.

I bought a new saw and a mig welder a while ago.Everytime I switched one of them on they would trip the rcd. I eventually connected the circuit to the input side of the rcd, so that it didn't go thru rcd at all. However, every time I switched one of these it tripped the rcd further down the circuit. Aaah faulty rcd....altho its further down and shouldn't trip. Replace rcd....same result. Suddenly I realised that it didn't trip if the fluoros in the shed weren't on. I had a sparky spend some time here, after he told me on the phone that I was talking nonsense. He tried things, swapped things, gave up and left, telling me that it is not possible. A second sparky had a go, no success, meggared the saw and the welder. Gave up and left. Both put it in the too hard basket. When I found out that did not trip when the lights were in circuit as well ( ie not just switched off but both live and neutral removed out of circuit ), I started disconnecting one fitting at a time. Lo n behold, one fitting had a leak which caused all the nonsense. Removed that one from the circuits, connected up as all should be and no problems. Often people buy a new machine, have rcd's tripping, blame the new machine, which in fact it just accsentuated a problem that existed before .

Anyway, back to Bob's problem. Eliminate.

:)

Bob, I think that maybe you should re-think what dan said.. the COOLING fins are on the motor for a reason.. to get rid of heat build up in the motor circuit.

They cool the motor by passing air over them. locking the compressor away will only lead to heat build up.

IF you really really have to have it in a cupboard, at least drill a few hole top and bottom to assist with some kind of ventilation flow to remove the heat.

Just humour us :rolleyes: , place the compressor out in the workshop again and see if the problem still exists

Hi Bob,

To check the voltage drop, measure the voltage at your 15 Amp point with a voltmeter when the compressor is off, then measure it again after the compressor has started and is running normally. The difference should be not much more than 5%. If your cables are big enough, but you still have a big voltage drop, check all connections between you motor and where the street wires connect to your house for signs of burning. A prime candidate on a compressor is the pressure switch.

Capacitors are susceptable to harmonics on the power supply. When you lengthened the circuit, you may have set up a tuned path to your capacitor for these harmonic currents. These harmonic currents heat up the capacitor. The source of harmonics can be quite difficult to find, but they can be from inside your shed ( fluro light ballasts), your house, or your street. One cure is bigger cables to supply the load.

Doing the above will mean you will be working with live exposed electricity. It will kill you. If you are not experienced doing this, get someone who is to check it with you. I'm not being unnecessarily alarmist, I've seen the dead bodies, and the burnt ones.

Hope this helps,

regards,

John

Very sound advice John.

:)