workshop electrics

[Jacob]

I have 16 amp supply to my garage via buried armoured cable 35 metres.
Would that be enough to run my 3HP 3 phase machine via an inverter, plus single phase 750w dust extractor, plus low energy LED light strips?
Not likely to ever need more power on at same time.
If not, am I right in thinking that I could lay a bigger armoured cable overground as long as it is away from traffic etc, e.g.close alongside a wall?
Or what would minimum trench spec be?

 

[Sideways]

Can you see printed on the cable sheath or anywhere alse, what is the size of the cable ?
1.5mm2 or 2.5mm2 conductors.
We need to know that to do some math and work out how much the voltage will drop over a 35m round trip.
Ignore the breaker, what matters is the detail of the Steel Wire Armored cable.

 

[Jacob]

Can you see printed on the cable sheath or anywhere alse, what is the size of the cable ?
1.5mm2 or 2.5mm2 conductors.
We need to know that to do some math and work out how much the voltage will drop over a 35m round trip.
Ignore the breaker, what matters is the detail of the Steel Wire Armored cable.

Just says 600/1000v BICC etc.
Would 2.5mm2 do it? I guess 1.5 definitely not.
I can have a look in the fuse box and measure cable size but am away until next week

 

[Sideways]

Measure it for us when you get back Jacob, that'll be easiest.
Cheers

 

[Tortion-Stephen]

Always great to see the knowledge on here, I would agree it's likely 2.5mm (or imperial equivalent like 7-29 etc...) as a 16A radial. I did my A- Level CDT Industrial Study at BICC PalCo in 1995. I remember some of the guys there making some beer tokens when the colours were harmonised, and people realised that 2 or 3 core and CPC, as well as separates in red/black were handy to have for folk wanting to add circuits/rewire and avoid Building Control/Regs...

Cheers

Stephen

 

[Gordon Tarling]

There's a cable size calculator here - Cable calculator - looks like 2.5mmSWA will do the trick.

 

[ChaiLatte]

Maybe another thing to throw into the mix is duty cycle. How many minutes per hour will the draw on the supply be at the high level? The cable size calculators have to assume some value for this and if your own use case is way below that, it is in your favour.

 

[HamsterJam]

Just says 600/1000v BICC etc.
Would 2.5mm2 do it? I guess 1.5 definitely not.
I can have a look in the fuse box and measure cable size but am away until next week

Probably worth looking closer to see if you can find more markings on it.
However, knowing it’s 600/1000v SWA cable and assuming it’s 3-core, we can probably have a good stab at the conductor size from the diameter of the cable over the sheath which may be easier to measure. Not much in it though - 12.6mm for 1.5mm and 14mm for 2.5mm

For what it’s worth I have a 50m run in 10mm square on a 40A breaker. No big machines but went 10mm to minimise voltage drop over the length.
Cable is in 2” duct buried 2 foot deep with safety tape over it.

 

[Fergie 307]

Hamster Jam makes a good point. Whether buried or not makes a lot of sense to put it in a conduit, you never know what you might want to add later. Good idea to put a pull cord in with it as well.

 

[Jacob]

Thanks for replies. Off to Wales today will follow up when I get back. The cable itself is about 15mm dia. My usage will be very intermittent in terms of full load - no mass production just odds and ends. So maybe it'll do as it is which would be good.

 

[Sideways]

Measure it for us when you get back Jacob, that'll be easiest.
Cheers

And to give you some encouragement:
As long as you have 2.5mm2 conductors minimum, we're not worried about the current capacity of the cable. It will be ample.
What we're interested in is the voltage dropped or lost across the length of the live and neutral wires over a 35metre out and 35m back loop.
If your loads add up to 15Amps, you'll lose 10V off the mains along that cable for example.

There are rules in the wiring regs that state the maximum loss of voltage through the entire wiring from your consumer unit to any socket. 16A circuit, 10V loss, about 4% of 240V is around the borderline condition.

But, your lights are LED and they will be very tolerant of the supply voltage.
Your machine is on a VFD which is the same. If the mains is a bit low, it'll just pull a bit more current to make up for it.
The dust extractor is not critical machinery so ifnit runs a little slow, it shouldn't matter much. Just keep an eye on it.

Make sure the supply cable to the shed is fed from a breaker with an RCD. That provides valuable extra safety in making sure the RCD will trip in case of an earth fault even though the cable is long.

Lastly, whenever your load is mostly motors and electronics, not simple resistive loads like heaters, you can't use simple calculators or rules of thumb based on 4Amps = 1kW.
The full load currents from your motor rating plates are what you need to add up (note @Spectric's point below, the Full Load Amps on a 3 phase emotor are PER PHASE and to get the single phase equivalent you have to multiply that value by SQRT(3) which is 1.73). Then allow just a few amps more for the LEDs, battery chargers and the VFD conversion losses. (VFDs are very efficient at converting single to 3ph but not 100%).

The VFD will do a superb job of reducing the inrush surge to your main motor so I expect all will be fine just so long as the cable is 2.5 mil.

 

[woodieallen]

And if you do find that the initial draw is too much and the voltage drops then a simple buck-boost transformer will get you out of a hole rather than lay in a thicker cable.

 

[flying haggis]

4mm squared cable is about 15mm outside diameter so you might actually be lucky.!

 

[Spectric]

You also need to think about the protective earth and means of earthing, not good practice to take the CPC from a PME system to external buildings, that is if your property is supplied by a PME system. If in the sticks and supply comes overhead then it would be a TT system. Also again not good practice to use the armour of an armoured cable for the CPC but the armour itself must be connected to the main earthing terminal (MET) at the supply.

I have 16 amp supply to my garage via buried armoured cable 35 metres.

That 16 amp supply on a 2.5mm CSA cable is common to provide basic power to a garage for a socket and lighting, you will probably have a two way distribution board in the garage. If that was an installation job then 4mm SWA but I would use 6mm for future proofing and a board with extra capacity for circuits.

The single to three phase invertor to power a 3 Hp motor (2.2Kw), it has to pull more current on the single phase side because this one phase is supplying three phases. This is a case of power factor and 1.73 . The current drawn by the VFD from the supply will be 1.73 times the current drawn by the motor times the power factor.