Thread: A 3HP (15A rated) DC in a shed

Originally Posted by BobL

The following is a link to a post from a sparky in the SHED forum regarding 15A GPOs.

Sorry but I'm not a sparky. I'm a structural engineer and deal with Australian Standards on a daily basis, including expert witness reports and statements. I have always been interested in electrical work (my dad was a sparky) and hence have absorbed AS3000 Wiring Rules, but I hold no electrical qualifications. I do have a lot more experience than the average sparky in the interpretation of Australian Standards though - it's my bread and butter.

The way to address this issue with a sparky who states that they're limited to 1x 15A outlet per 16A circuit is to ask the following questions:

1. Are they basing this off the calculation method of maximum demand per Appendix C and specifically Table C8?
2. If so, why can't they use the limitation method of determining maximum demand per Section 2.2.2 of the Code?
3. If the limitation method is used and the breaker rating determines maximum demand, what is stopping them putting multiple 15A outlets on a 16A circuit given that (a) you don't intend using more than one outlet at a time, so nuisance tripping won't be an issue and (b) if you do use more than one outlet, the breaker will trip and limit maximum demand as permitted by the Code?

In all likelihood you'll be met with blank stares and bluster, in which case move on to the next sparky. I would suggest asking them when you initially make contact (first phone call) whether they're comfortable using maximum demand by limitation rather than the calculation method guidelines in Appendix C. If they're not, keep phoning around.

Note that AS3000 Appendix C itself contains the following that supports my reasoning:



Of particular import is clause (b) which is directly applicable to a final subcircuit installation in a shed. The maximum demand of that subcircuit can be taken, by limitation, to be the current rating of the breaker protecting that circuit. Section C5 'Number of points connected to circuits' further states inter alia that "where more than one item of equipment is to be connected to a circuit, consideration needs to be given to- (b) the operating characteristics of the different items of equipment, including seasonal or daily variations; and (c) the circuit current under expected operating conditions..." Blind compliance with the informative Table C8 does not give due consideration to either of those aspects.

Ive got to commit that to memory.

DaveTTC

Turning Wood into Art

Originally Posted by DaveTTC

Ive got to commit that to memory.

You're building sheds for others though, correct? If so you'd really need to have an electrical layout agreed and signed off by the client before you could consider relying on the limitation method of maximum demand for subcircuits; otherwise the sparky would run a significant risk of the client being unhappy when they start up multiple machines on one circuit and trip the breaker. Different kettle of fish to your own personal shed.

On another note, I have a little giggle every time I read your name. On the baby forums that wifey frequents, the abbreviation 'TTC' means Trying To Conceive. Not sure what yours means?

This is interesting, for my sparky (a professional who is also a friend) knew of this and did exactly this in my studio. He enquired as to what machines I had, the draws (he looked at all the motors) and asked when and how they were used, plus likely upgrades (be enthusiastic!).

He then told me that I needed only 1/3rd the cabling and why. He was more than happy to do the whole "more than one 15A point per" thing.... He suggested it. It looks neat and I've far more options than initially hoped, plus gobs of room for upgrade.

I don't think there is a $$$ conspiracy. They just know that people can be complete tools when it comes to electricity.

Perhaps the admins will permanently pin this thread?

Originally Posted by woodPixel

This is interesting, for my sparky (a professional who is also a friend) knew of this and did exactly this in my studio. He enquired as to what machines I had, the draws (he looked at all the motors) and asked when and how they were used, plus likely upgrades (be enthusiastic!).

He then told me that I needed only 1/3rd the cabling and why. He was more than happy to do the whole "more than one 15A point per" thing.... He suggested it. It looks neat and I've far more options than initially hoped, plus gobs of room for upgrade.

Well that is really good to hear that there is a sparky prepared to do it.

The bizarre thing with the sparky that ended up wiring my shed was he was OK with using double point 15A GPOs but not with putting 2 single 15A GPOs on the one circuit.

TTC The Turning Cowboy
Im on Bubhub and found out what it meant there. Yes i had a chuckle too

DaveTTC

Turning Wood into Art

The sparky who wired my shed 35 years ago used exactly that thinking. He was also the one who advised me to put all lights on three pin outlets and that has proved to be the best thing I did and I have advised many others to do the same since.

With my recent shed subboard and wiring, I was having two 15 A GPOs wired up. On a whim, I asked the sparky to fit another 15A GPO on the same board as the sub board enclosure (400 mm away) so I had a 15A at each end of the shed and one in the middle. The sparky asked whether there would a chance that the front and middle would be used at the same time and I said no, so he put two single 15A GPOs on the one breaker.

In reality though, it only saved me ~$30 - the cost of a RCBO as both were wired into the one RCBO.

Originally Posted by BobL

The bizarre thing with the sparky that ended up wiring my shed was he was OK with using double point 15A GPOs but not with putting 2 single 15A GPOs on the one circuit.

That is indeed bizarre as the notes to Table C8 state "For the purposes of determining the number of points, a multiple combination of socket-outlets is regarded as the same number of points as the number of integral socket-outlets in the combination." In simple terms, a double outlet GPO is two points for the purposes of Table C8.

Just another example of misunderstanding of the Code, unfortunately.

Originally Posted by Chris Parks

The sparky who wired my shed 35 years ago used exactly that thinking. He was also the one who advised me to put all lights on three pin outlets and that has proved to be the best thing I did and I have advised many others to do the same since.

That's what I wanted as well. One of the sparkies who quoted for the wiring refused to do this and said lights had to be hardwired. The sparky that eventually did mine did put in 3 pin plugs but I have them on a separate circuit. Losing lighting at night in a cluttered shed is a bit disconcerting.

The 3 pin idea turned out to be useful when I changed the lighting to LED tubes. As well as changing all the existing tubes to LED I was able to add 4 extra doubles just by piggybacking onto the 3 pin outlets.

Just as a point of interest, in South Africa, the standard domestic socket is 16A, and they are wired with 2.5mm wire (in the past 4mm was more common), usually on a 20A (sometimes a 30A) breaker per circuit. AFAIK there is no legal limit to the number of sockets on a circuit, but typically an average house will have two, maybe three, circuits for plug sockets, so often half of the sockets in the house will share a single breaker. Obviously it is possible to get nuisance trips, especially in winter when there are a couple of heaters running, but it does mean that you are free to hook up enough sockets to connect all your toys, and if you turn too many on, you might have to reset the breaker, which is less inconvenient than burning the house down.

Additionally we're big users of multi-plug adapters, and each adapter should have an over-current breaker built in, which allows you to draw 16A in total. I have a double 16A socket in my garage, and I run my lathe, bandsaw and dust extractor off one of those sockets, using a multi-plug adapter. The lathe and bandsaw don't usually run at the same time, but my 3hp dust extractor and either lathe or bandsaw run at the same time with no issues.

This thread is perfect timing, I've been wondering about this stuff lately. My old man has a 15a outlet in his shed but he wasn't sure if we could run more than one machine off it. So if I understand things correctly I should be able to check the rating on the breaker it's wired to and that will tell me the total capacity?

Edit: never mind, just thought about it and it looks like I'd need another point wired up anyway. No double adaptors for 15a from what I can tell.

Originally Posted by bueller

This thread is perfect timing, I've been wondering about this stuff lately. My old man has a 15a outlet in his shed but he wasn't sure if we could run more than one machine off it. So if I understand things correctly I should be able to check the rating on the breaker it's wired to and that will tell me the total capacity?

Edit: never mind, just thought about it and it looks like I'd need another point wired up anyway. No double adaptors for 15a from what I can tell.

You can replace the single 15A GPO by a double like this https://www.bunnings.com.au/deta-15a...point_p4430422 but you may not be able to run 2 x 15A machines at the same time from this outlet. Well you can try but it will probably trip the breaker.

If the cable is 2.5mm (preferably 4mm^2) and there is a 20 A breaker on it you could safely run a single 15A rated machine and a 10A rated machine on that circuit.

This is no different than plugging in multiple 10A rated machines into a single circuit protected by a 16A circuit breaker. the assumption is that none of the machine are drawing their full power under load.

It's even more subtle than this because even 15A machines don't use anywhere near 15A when free running so its not just about them being on but the load they are under.
My BS, belt sander and DC are all 3HP 15A rated machines
The BS uses 1.8A when free running while the Belt sander uses 3A free running and the 3HP DC uses ~7-9.5 A constantly.

I don't know why I would do this but it means I could connect all 3 machines to my 20A breaker circuit and leave all three machines on, the DC, and the BS and Belt sander free running (13.8A total).
Then I could move between using the BS and Belt sander.
Under load he BS rarely goes above 5A in 150 mm wide cuts - it just doesn't seem to need the juice to cut.
The total current would then go up to 17A.

The Belt sander seems to max out at about 6A so when using that the total current would be about the same as using the BS under load.

What this highlights is knowing what the actual current drawn by each machine is, is instructive and useful.

The only 3HP (15A rated) WW machine in my shed that uses 15A is the TS and I have to be really hard on it to do that.

Cheers for the advice. I've also been pointed to an Arlec 15A power centre which could do the trick. I'm happy to muck around with electronics and motors etc but I draw the line at touching mains power. Parents place is out bush too so sparkies charge like a wounded bull for callout.

https://www.bunnings.com.au/arlec-15...block_p7050064

Realistically I'll probably only need it for a dust collector and table saw so hopefully that shouldn't be too much.