Question for electrical engineers?

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Ropata

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#2139407 4-Dec-2018 20:40

I have quite a few machines here running off a 20 amp breaker on 2.5mm cable over a distance of about 6 meters. Half of the machines are 3 phase running through a rotary phase converter and the biggest one which is 3 HP draws around 7 amps under load (I've tested them all at the board). The most high draw machine is a single phase 3 HP wood thicknesser which I recorded drawing 78amps on startup momentarily. My main house fuse is 60amps and it was a pain in the arse when it blew that. I'm running some 4mm cable in the next few weeks with a 32amp D curve fuse which my electrician will be coming to install certify at the board for me.

So my initial question seems to be answered in that a minimum would be a 20amp plug over 4mm cable to a 32amp fuse/breaker?

Thanks for all the help thus far.

Tracer

343 posts

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#2139413 4-Dec-2018 20:57

gregmcc:

To meet the "Electrically Safe" or "Electrically Unsafe" definition as per the Electricity Safety Regulation (Regulation 5)

Putting a 15A plug on a device which lists the maximum current of 27A would pose a risk of wiring catching fire, significant damage etc.. This would clearly fall in to the category of Electrically Unsafe.

As far as using duty cycle of 50%, unless there is some built in device to limit the duty cycle it would be assumed that the duty cycle is 100%

Ensuring that the circuit is protected by a suitably rated circuit breaker would not solve the problem either, as a 15A circuit breaker does not trip at 15A, there is a trip curve, the higher the current the quicker it will trip, you may find running 27A it may take somewhere in the vicinity of 500seconds, that is around 8min 20 seconds. For this 8mins and 20 seconds 27amps is coursing thru your standard 2.5mm power circuit cable, 2.5mm is simply not rated to pass 27A so it turns in to a big heater, plastic starts melting, the wood in your walls that the cable travels through starts to get hot, you have all the makings for a house fire.

Even if you don't reach the point of starting a fire the wiring in this circuit has now become damaged and unsafe.

It has been brought to the attention (in the past couple of years) of the testing and tagging companies that visit businesses to do the electrical testing and tagging that Welders are a problem, they should check the nameplate specifications and if the plug rating is less than the maximum current draw on the nameplate then it's a fail.

My advice:-

Don't listen to anyone here who is not an electrician.

(Yes I am an electrician)

Do not put a 15A plug on your 27A welder, it is electrically unsafe, regardless of you having a modern fuse board with circuit breakers fitted

Typically the only circuit in a house capable of supplying 27A safely would be your oven.

With my electrician's hat on, I agree with you, that is what we are taught, and that is what the standards say. With my EE hat on, the physics agree with the welder manufacturers, assuming the duty cycle is actually enforced. Perhaps it is not?

P.S. 2.5mm2 V-90 can carry 26 A touching a surface, 27 A in air for flex, and 28 A in air for solid, according to AS/NZS 3008.1.1:2017 Table 14. ;) Of course voltage drop will be substantial for any kind of length.

elpenguino

3577 posts

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#2139427 4-Dec-2018 21:25

I'd like the manufacturer to state whether the power consumption is during the peak or an average.

Did we ever find out what rating the internal breaker is?

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

SomeoneSomewhere

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#2139462 4-Dec-2018 22:44

Ensuring that the circuit is protected by a suitably rated circuit breaker would not solve the problem either, as a 15A circuit breaker does not trip at 15A, there is a trip curve, the higher the current the quicker it will trip, you may find running 27A it may take somewhere in the vicinity of 500seconds, that is around 8min 20 seconds. For this 8mins and 20 seconds 27amps is coursing thru your standard 2.5mm power circuit cable, 2.5mm is simply not rated to pass 27A so it turns in to a big heater, plastic starts melting, the wood in your walls that the cable travels through starts to get hot, you have all the makings for a house fire.

Even if you don't reach the point of starting a fire the wiring in this circuit has now become damaged and unsafe.

Provided the MCB is sized correctly, this *should* not be an issue. The cable has thermal mass too (how we can use motors with starting currents far higher than the circuit capacity), and there is supposed to be sufficient headroom in matching the MCB to the cable for this not to be an issue.

For example, plugging 27A worth of heaters/kettles/whatever into the circuit results in exactly the same effect for the cable. And limiting the maximum demand in the circuit by protection is perfectly valid, and used for practically every socket circuit.

The issue in this case is that the 15A plug might not be protected by a 15A breaker.

Aredwood

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#2139784 5-Dec-2018 13:05

AFAIK the duty cycle limit is only via an overheat cutout. And not via any sort of timer. Using the welder in an unheated workshop during winter, could allow the welder to operate just fine. Even when pushed much harder than designed. But the colder air temps might not help the rest of the circuit.

I have never managed to trip the overheat cutout on my welder. Even when using the plasma cutting function on some thick pieces of cast iron. I'm sure that I was using the welder far in excess of the 20% duty cycle rating. Managed to get the 32A breaker on the welder circuit to get slightly warm, along with the cable feeding the welder.

Before starting that job, I had to go around the house and make sure that no other major loads were turned on. As welder + air compressor (plasma cutting needs lots of compressed air) means approx 40A total load. Which is the limit of the krapi mains cable that feeds my house.

Agree with the sparky's. Upgrade your plugs and sockets. Supposedly there have been problems in Australia with using 3112 15A plugs and sockets for EV charging. Which shows that they are running at their limit even at 15A. And that some EV chargers even have temp sensors fitted to the plugs for that reason. Trying to put 27A through a plug that struggles with 15A - Madness.

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