Thread: Is this enough electrical supply to shed?

The most useful answer to this is , "it depends" on the power requirements of your machinery, the number of machines running simultaneously, and the number of persons likely to use the shed at the same time.

Each 15A machine should really be on its own breaker/circuit.

Solo Wood workers are most likely to have t most two machines running at the same time - ie a DC and a machine. Then it depends on machines power ratings.

2 x 2HP machines will be fine on a 16A breaker but not so for 2 x 3HP machines.

If you run a heater, or an AC as well as even two x 2HP machines then 16A is going to be marginal.

If you supply the required information we can provide you with better advice.

Originally Posted by BobL

The most useful answer to this is , "it depends" on the power requirements of your machinery, the number of machines running simultaneously, and the number of persons likely to use the shed at the same time.

Each 15A machine should really be on its own breaker/circuit.

Solo Wood workers are most likely to have t most two machines running at the same time - ie a DC and a machine. Then it depends on machines power ratings.

2 x 2HP machines will be fine on a 16A breaker but not so for 2 x 3HP machines.

If you run a heater, or an AC as well as even two x 2HP machines then 16A is going to be marginal.

If you supply the required information we can provide you with better advice.

Fair point, thanks BobL.

It's just me at home using the shed. As you say, I'll only be using two big machines (dusty + tables/bandsaw/etc). So max use at any one time:
- Biggest machine will be a 2.5hp table saw
- Dusty 2hp (but considering a 3hp)
- Two or three extraction fans (the 6" leather jacket grow fans)
- Small speaker, battery chargers, etc
- Shed lights (all LED)

A few random thoughts in no particular order for consideration.

With the kit you have, and considering scope-creep (you may add more things to the shed without thinking about overall power limit), I'd say 16A is already marginal, and it's to the point where I personally probably wouldn't be comfortable (everyone is different though).

Is the cable running from Granny Flat to shed rated only to 16A (ignore the circuit breaker for a minute, concentrate on the cable)? If it can only do 16A (and your sparkie can calculate this/voltage drop etc), then you're already behind the 8-ball. If it can handle a bit more, can the circuit breaker be replaced with a 25A breaker? That will buy you a bit of scope-creep/time.

If the cable can't handle more than 16A, what is the possibility of running a new circuit from the main switchboard, avoiding the Granny Flat entirely?

Also think - will someone ever be in the Granny Flat at the same time as you are in your workshop? If so, power loads must be combined between both locations, and fit inside 32A

Originally Posted by Midnight Man

A few random thoughts in no particular order for consideration.

With the kit you have, and considering scope-creep (you may add more things to the shed without thinking about overall power limit), I'd say 16A is already marginal, and it's to the point where I personally probably wouldn't be comfortable (everyone is different though).

Is the cable running from Granny Flat to shed rated only to 16A (ignore the circuit breaker for a minute, concentrate on the cable)? If it can only do 16A (and your sparkie can calculate this/voltage drop etc), then you're already behind the 8-ball. If it can handle a bit more, can the circuit breaker be replaced with a 25A breaker? That will buy you a bit of scope-creep/time.

If the cable can't handle more than 16A, what is the possibility of running a new circuit from the main switchboard, avoiding the Granny Flat entirely?

Also think - will someone ever be in the Granny Flat at the same time as you are in your workshop? If so, power loads must be combined between both locations, and fit inside 32A

Running a new cable from the main switchboard is probably not an option just based on the layout of our property and the fact that there is a big slab of open driveway between the main board and the shed, so the cable can't be buried. I think it would probably have to be strung from the main house to the shed? I'm not sure, but either way, too much faff.

However, upgrading the cable from the granny flat to the shed is possible. The existing cable is buried under concrete in conduit. So maybe a sparky can thread a new cable? Otherwise there is a solid structure to run conduit along. But I'm just considering the cost of doing that.

If I can get away with rewiring the shed with a new sub-board and gpos, then that is ideal. But it does sound like it's borderline, so probably best to just bite the bullet and upgrade the line from the granny to the shed.

Running a mid sized saw and some dust extraction from a 16A circuit in a location a moderate distance from the house is definitely doable, I had a barn in my yard in Melb that was fed from a 15external GPO via a 40m long 4mm² 3 core extension cord, and that worked fine for 10 years. But it is a juggle to do so and also cope with other moderate loads like lighting, recharging batteries for portable tools, drill presses, mains portable tools, jointers and thicknessers, drill presses etc simultaneously.

New barn is 1m wider, closer to the house but further from the street. I have 63A 3 phase to the house with 40A 3 phase cable capacity from house to shed, and are putting in 20KW of 3 phase solar and 40KW of battery storage in to augment the mains. Currently the house averages 12Kwh per day, and not much happening in the shed till I get it fitted out and get back on my feet, but it will potentially use a similar amount, hence the oversize solar and battery system. But we were lucky and able to make provision while planning our retirement home from scratch, it's much harder doing things as a retrofit.

Originally Posted by malb

Running a mid sized saw and some dust extraction from a 16A circuit in a location a moderate distance from the house is definitely doable, I had a barn in my yard in Melb that was fed from a 15external GPO via a 40m long 4mm² 3 core extension cord, and that worked fine for 10 years. But it is a juggle to do so and also cope with other moderate loads like lighting, recharging batteries for portable tools, drill presses, mains portable tools, jointers and thicknessers, drill presses etc simultaneously.

New barn is 1m wider, closer to the house but further from the street. I have 63A 3 phase to the house with 40A 3 phase cable capacity from house to shed, and are putting in 20KW of 3 phase solar and 40KW of battery storage in to augment the mains. Currently the house averages 12Kwh per day, and not much happening in the shed till I get it fitted out and get back on my feet, but it will potentially use a similar amount, hence the oversize solar and battery system. But we were lucky and able to make provision while planning our retirement home from scratch, it's much harder doing things as a retrofit.

This is a good reference point, thanks. I think I will just get a second cable run to the shed. Means it's done and I don't have to have a false start!

Originally Posted by lemerv

Running a new cable from the main switchboard is probably not an option just based on the layout of our property and the fact that there is a big slab of open driveway between the main board and the shed, so the cable can't be buried. I think it would probably have to be strung from the main house to the shed? I'm not sure, but either way, too much faff.

However, upgrading the cable from the granny flat to the shed is possible. The existing cable is buried under concrete in conduit. So maybe a sparky can thread a new cable? Otherwise there is a solid structure to run conduit along. But I'm just considering the cost of doing that.

If I can get away with rewiring the shed with a new sub-board and gpos, then that is ideal. But it does sound like it's borderline, so probably best to just bite the bullet and upgrade the line from the granny to the shed.

Just some thoughts ...
the number of conductors, and the resulting current, you can run through an underground conduit is limited by things like cross section (mm2) of the conductor, heat dissipation and cable length.

From what you have said, the supply to the granny flat is 32Amps.
This implies a 3 wire bundle (active, neutral, ground) in an appropriately sized conduit -- what size is the conduit?

A 16A breaker on the Granny flat sub-board would seem to imply that your existing conduit -- between the granny flat and the shed -- is too narrow to run a larger cross section (greater current capacity) wire inside it.

If your existing sparky doesn't have access to the current Aust Wiring Guide, find one who does.


I recall another Forum member who was stymied when looking to upgrade the main supply to 3 phase, because the "wiring rules" wouldn't allow him to pull the required 4th conductor through the existing (32 mm?) underground conduit between the street supply and his main board.

Originally Posted by ian

Just some thoughts ...
the number of conductors, and the resulting current, you can run through an underground conduit is limited by things like cross section (mm2) of the conductor, heat dissipation and cable length.

From what you have said, the supply to the granny flat is 32Amps.
This implies a 3 wire bundle (active, neutral, ground) in an appropriately sized conduit -- what size is the conduit?

A 16A breaker on the Granny flat sub-board would seem to imply that your existing conduit -- between the granny flat and the shed -- is too narrow to run a larger cross section (greater current capacity) wire inside it.

If your existing sparky doesn't have access to the current Aust Wiring Guide, find one who does.


I recall another Forum member who was stymied when looking to upgrade the main supply to 3 phase, because the "wiring rules" wouldn't allow him to pull the required 4th conductor through the existing (32 mm?) underground conduit between the street supply and his main board.

Very good advice.

Unfortunately, there are some less scrupulous tradesman about, either through ignorance, indifference etc. We discovered a rather "non-compliant" wiring installation when we replaced our two large split system AC's, (installed some 10years earlier). The licensed electrician who installed the originals came recommended via a major electrical retailer.

The new installer became suspicious when removing the old units and asked to access the "whole installation." His investigations revealed some very dodgy use of multiple connectors on a 12m run for a 15A dedicated circuit including the use of low voltage terminal strips, stating "you are lucky the place hasn't burnt to the ground!"

Needless to say the whole of the orig install was removed and replaced. More cost, but very reassuring to know it had been found. When attempting to report the orig "lecky" we found he no longer held registration. I wonder why???

Originally Posted by ian

Just some thoughts ...
the number of conductors, and the resulting current, you can run through an underground conduit is limited by things like cross section (mm2) of the conductor, heat dissipation and cable length.

From what you have said, the supply to the granny flat is 32Amps.
This implies a 3 wire bundle (active, neutral, ground) in an appropriately sized conduit -- what size is the conduit?

A 16A breaker on the Granny flat sub-board would seem to imply that your existing conduit -- between the granny flat and the shed -- is too narrow to run a larger cross section (greater current capacity) wire inside it.

If your existing sparky doesn't have access to the current Aust Wiring Guide, find one who does.


I recall another Forum member who was stymied when looking to upgrade the main supply to 3 phase, because the "wiring rules" wouldn't allow him to pull the required 4th conductor through the existing (32 mm?) underground conduit between the street supply and his main board.

Will have to see what he says. I think the conduit is small though. It’s also very old (mid 80s) so I would rather just run new conduit if it comes to that than mess with the existing one.

I have no idea of the wiring to my shed except for the ciruit breakers on the main switchboard:

• lighting circuit - 10 amps,
• power point circuit - 20 amp.

The latter is adequate on the rationale that I never use more than two tools at a time. The exception is in winter when I may also have a heater switched on. Then I occasionally pop the circuit breaker.

Originally Posted by GraemeCook

I have no idea of the wiring to my shed except for the circuit breakers on the main switchboard:

• lighting circuit - 10 amps,
• power point circuit - 20 amp.

The latter is adequate on the rationale that I never use more than two tools at a time. The exception is in winter when I may also have a heater switched on. Then I occasionally pop the circuit breaker.

Hi Graeme

from what you describe your shed is wired with two separate circuits -- 10A lighting, 20A for the GPOs
I'll not suggest the likely cross section of each wire because a google search would bring up the result and I'm very mindful of the consequences should I state the wrong wire sizes.

Your response also implies that the wiring is run "bare" -- stapled to ceiling joists, or run through the studs -- and not in conduit as the number of wires (6) would require a fairly large diameter (perhaps 50mm) conduit -- if my memory from when I last looked at the Aus Wiring Rules is accurate.


however, you may wish to have an electrician check the heat generation / voltage drop of the power point circuit wiring. "Occasionally" popping the breaker when you also have the heater switched on could imply that the steady state current draw through the GPOs (with only two machines running) exceeds that intended for the power circuit.

Originally Posted by ian

... Your response also implies that the wiring is run "bare" -- stapled to ceiling joists, or run through the studs -- and not in conduit as the number of wires (6) ...

You've got your wires crossed, Ian I don't think I mentioned conduit or its absense ...

The wiring runs in 1" plastic conduit to the shed and to exposed powerpoints and light switches. The only "bare runs" are in the ceiling cavity.