Thread: 3-phase power

I believe it is quite expensive to have 3 phase power brought to your site but I am sure others will be able to give you a more accurate idea of the cost. You can pick up some really cheap bargain, used 3 phase machinery and it is usually industrial quality machines you get. Table saws, Jointers, Lathes, Drill presses etc

depends what max current circuit you want to put in, and how far the run is from your existing 3phase termination. the killer cost is the cables. Mine were 16mm STI and that's a LOT of copper. Given that the price of copper has gone up significantly in the last 12 months, the price of STI cables has increased a lot.

approx 2.5 years back I got 3phase run from the meter box at the front of the house, up to the garage at the rear, and that cost me about $1200 to the subboard in the shed. That gets me 63A per phase RCD protected. Not including 3phase circuits run within the shed that is. They were done later.

If you don't already have 3 phase at your meter box, and you're running it in from the street, then i dare say you'll be up for $3-4K by the time you're done, for a residential block, and more if a rural property, and the cable runs are longer.

That said, I'd do it again in a second. the grunt from 3 phase machines is worth it. And most industrial saws/bandsaws etc are 3 phase, so you'll get your savings back on buying 2nd hand 3 phase gear.

You will, however, have to save all your pennies to pay your electricity bills....

My experience in Brissie about 4 years ago.

Upgraded to 3 phase insulators on the house. Cabled down to new meter box. Cabled to separate upgraded switchboard. Cabled to shed, 7 or 8 metres to back of house and then underground for 4.5 metres, into shed to subboard ex house, 2 double gpo's in shed. $3500. The sparky left enough cable loop to raise the house 1.5 metres.

About 2 years ago I had various GPO's and 3 phase outlets and lights fitted, as well as a sub meter. That cost me about the same. No shortage of power or light in my shed.

Cheers

Originally Posted by jackypunker

After numerous conversions in the distribution and transmission network, the power is transformed into the standard mains voltage. At this point, the power may have already been split into single phases or into three phases. With three phase power, the output of the transformer is usually star connected with the mains voltage, 230 volts in Europe and 120 volts in North America.

I'm not quite sure what that means......

3 phase power in Australia is 400V, single phase is 230V (in fact although Australia changed to a theoretical 230V/400V system, we allowed +10% and therefore most states didn't actually change from the old 240V/415V standard). It comprises 3 "wires" each carrying 230V, but with their AC waveforms offset by 120 degrees. The disadvantage with single phase is that with every cycle (i.e. 50 cycles per second) the voltage twice falls to 0V. With 3 phase power this never happens, as (in simplistic terms) the phases cover each other so when one is at zero, the other two are not.

A single phase 230V supply is easily extracted by using any of the 3 phases and neutral.

Things to look for:
Make sure a neutral cable of the same size as the live cables is installed - a 3 phase device can run without any "neutral" connection as the phases balance each other, but if a phase fails the balance is lost and the neutral is used. Also a neutral is required to extract 230V single phase.

Make sure the cables are suitably sized. This depends entirely on load and length, and your electrician can calculate the required size if you tell him your estimated loadings. Remember that copper is expensive BUT the cost of digging it all up to upgrade the cable because you unexpectedly bought a bigger table saw (or whatever) is greater again! Also note that electricians normally calculate cable size to allow a certain "voltage drop", and minimise the installation cost (especially for quoted jobs where competitive pricing is important!). This is, in reality, a power loss and because it is likely to be on your side of the meter (from the meter to the shed) you will be paying for power you do not use, not much but it adds up over the years. If your cable run is short (domestic block?), ask the electrician to install bigger cables than the minmum he has calculated. Pay the extra for bigger cables and be safe in the knowledge that the cicuit is bigger than you'll ever need, and has minimal power/voltage loss.

Because the 3 phase load is spread across 3 cables rather than 2 for single phase, you may find the electrican specifies smaller cables (x4) than for a single phase installation. However it is important to note that this assumes a 3 phase load - running a large single phase load will still only use 2 of the cables (1 live + neutral), which is why your electrician will need to know about large single phase loads, and you will need to consider multiple single phase circuits to spread such loads across the 3 phases.

If your existing meter is single phase, it will need to be upgraded.

In order to keep the 3 phases reasonably balanced, it is wise not to load up one single phase to excess. To achieve this, if you intend to run multiple single phase items concurrently get your electrician to run 2 or 3 single phase circuits around your shed, one from each of the 3 phases. Then distribute your large single phase equipment between the three circuits. Obviously if you only use one single phase item at a time, this doesn't apply, but then think 3HP 240V table saw +1.5HP 240V dust extractor!

You can get some bargain secondhand 3 phase equipment, it's true, but equally any 3 phase equipment you buy will be harder to sell because few "hobby" users have 3 phase supplies - that's why it was cheaper to start with! New equipment will likely cost the same price either way, and the running costs will be the same.

Costs? Depends on cable size, run length, trenching requirements etc. If you are installing from scratch then it's the difference in price that is important - so 4 cables rather than 2, but many of the other costs will be the same or similar to single phase.

Is it worth it? If you are installing from scratch then I'd say yes, it covers all future possibilities. If you already have a sufficiently good single phase supply, then unless you are looking at some big machinery it's probably not.....

Be aware that the power outlets and plugs on the machines are very expensive. I have three phase but it is a very old installation and was installed for nothing because we were going to use a minimum amount of power per billing period. Yes you can get good bargains without a doubt, is it worth it, I don't know. it has worked for me but for me to pay for the installation now might be doubtful.

How do they charge for three phase, is it cheaper than single phase?

When I built my new house I paid extra for 3 X 15 amp and 1 X three phase curcuits to be run to the side of the garage for me run to the shed when I get the time. Two years later and the shed is still connected by a long 15 amp extention leed.

Ross

Originally Posted by Ross

How do they charge for three phase, is it cheaper than single phase?
Ross

The energy costs are exactly the same as for single phase. Depending on the tariff, the same time-of-use (peak, off-peak and shoulder) rates are available. My supplier also charges service availability at the same rate for single and 3 phase meters, meaning there is no difference at all in the ongoing costs*. The meter records the energy used, totalled across the three phases, so a 1kW load connected to a single phase will rack up costs at exactly the same rate as a 1kW 3 phase load.

I am not aware of any difference in the cost of obtaining a 3 phase meter vs. a single phase meter, but as the meter is supplied and installed by an electrician (rather than the energy supplier) that may be down to the individual in question!

*Unless you are unfortunate enough to be a business user, in which case whilst your energy costs are likely to be the same, your "service availability" charge - for exactly the same meter - jump to about $3.25 PER DAY........

So much for that myth. The builder told us that the aircon and the hot water booster were on three phase because it was cheaper to run.

Ross

Originally Posted by Ross

So much for that myth. The builder told us that the aircon and the hot water booster were on three phase because it was cheaper to run.

Ross

He may still be correct though, as some aircons and hot water boosters can draw quite a fair bit of current when they kick in especially on single phase whereas a 3 phase unit will start easier and smoother with less current draw.

I had my parents old joint rewired for 3 phase a long time ago and after a few power bills, the old man actually said that it had come down in price, even with me using extra gear. Also rewired my current place from single to 2 phase ( would've gone for 3 but estate isn't set up for it ) so that I could split the loads between the house and the sheds and have noticed that the air compressor starts a lot easier than when the joint was wired for single phase

3 phase motors have several advantages over single phase units. The starting current for any motor is far higher than the current it draws when running normally - a 2.1kW (8.75amp) single phase motor can draw 20+amps at start-up - but 3 phase motors do improve things. I cannot, however, see why a hot water booster, assuming this means a simple heating element, should benefit from being three phase (especially as this would basically be 3 smaller single phase heaters) except by reducing the current in each phase and therefore reducing cabling losses if the cables are undersized to start with.

When re-wiring, it is also entirely possible that cable sizes were increased with respect to the load they were running, therefore reducing the power loss and in turn reducing the power used. It is not unusual for wiring to be under sized, or at least "minimally" sized! When houses are built, they are built to a budget. This means that there is a guesstimate or assumption of the maximum total power draw, and the supply cables are sized to give a maximum of (normally) 5% voltage drop which equates to a 5% power loss. Power loss increases with current, so if the current is increased by adding a table saw and a dust extractor (running at the same time as the air con, oven and kettle!) the power loss increases and the user is paying to heat up the cable...... Unfortunately when houses are built nobody anticipates the owner adding a shed full of machinery!

Edit:
Whilst it is normal practice to spec a supply for a 5% allowable voltage drop, it is interesting to note that cable sizes for a solar (PV) generation system are specified to give less than 1% loss to maximise the financial return.

Originally Posted by Warb

3 phase motors have several advantages over single phase units. The starting current for any motor is far higher than the current it draws when running normally - a 2.1kW (8.75amp) single phase motor can draw 20+amps at start-up - but 3 phase motors do improve things. I cannot, however, see why a hot water booster, assuming this means a simple heating element, should benefit from being three phase (especially as this would basically be 3 smaller single phase heaters) except by reducing the current in each phase and therefore reducing cabling losses if the cables are undersized to start with.

It was spec'ed as part of a solar hot water system.

When re-wiring, it is also entirely possible that cable sizes were increased with respect to the load they were running, therefore reducing the power loss and in turn reducing the power used. It is not unusual for wiring to be under sized, or at least "minimally" sized! When houses are built, they are built to a budget. This means that there is a guesstimate or assumption of the maximum total power draw, and the supply cables are sized to give a maximum of (normally) 5% voltage drop which equates to a 5% power loss. Power loss increases with current, so if the current is increased by adding a table saw and a dust extractor (running at the same time as the air con, oven and kettle!) the power loss increases and the user is paying to heat up the cable...... Unfortunately when houses are built nobody anticipates the owner adding a shed full of machinery!

The shed was built before the house and power requirements planned for which is why they insalled 3 x 15 amp circuit breakers for me. I was not planning to run 3 phase.

What size 240 volt cables should I run to the shed?

Ross

Originally Posted by Ross

The shed was built before the house and power requirements planned for which is why they insalled 3 x 15 amp circuit breakers for me. I was not planning to run 3 phase.

What size 240 volt cables should I run to the shed?

Ross

It is standard practice to use a 16amp circuit breaker for a "normal" 10amp circuit. It allows for several appliances to be run at the same time. It is likely that the cables have been sized with that in mind. However this creates issues when large machines are used - a 12" table saw would be recommended to be installed on a slow blow 20amp breaker, but this may well be pushing the limits of what the cabling was designed to supply, and installing a breaker of higher rating than the cable it supplies is a real no-no!

The "old" regulations in Australia called for sockets to be matched by cabling and breakers, but this is no longer the case. Now (more or less) a cable/breaker designed to support 15amp can have multiple 15amp sockets, and it's up to the user to ensure that the circuit is not overloaded. This creates a problem, as the user can decide to simply fit a larger breaker (especially if it's an old style plug-in fuse box) which means the weak link becomes the cabling.....and then the fires start!

If your electrician has presented you with 3 x 15amp breakers, it is likely he intended for standard 10amp circuits. You need to work out what you really need (like that 12" table saw!), then upgrade the breakers and their supply first.

If the breakers are on the house, are they all connected to the same phase? If so, disregard two of them and upgrade the other to supply the total load - there is no point in running three parallel single phase cables to the shed! If they are on different phases, then by all means use all three. If you intend to run 3 phase (or it's a possibility later) then keep all three, but install 4 cables to the shed (i.e. 3 single phase lives, plus a spare to act as the neutral for the 3 phase if you choose to upgrade later!).

As for cable size, it entirely depends on load, length and resistance. Assuming you will be burying the cables (in conduit, obviously) you will be using XLPE cable. This is sometimes described as "direct bury", but NEVER bury it without conduit (600mm deep with a warning tape buried half way to the surface)! It comes in 2 types, either aluminium or copper conductors, and is a single condutor cable meaning you need two length (live and neutral) for single phase, plus a smaller earth cable for safety. Aluminium is a poorer conductor of electricity, so whilst it is cheaper cable you often need to go up a size which removes at least some of the saving, especially if it then means larger size of conduit as well!

Work out your total load, and whether you be installing 1 or three circuits (as described above). If three circuits, then work out the biggest single circuit load. Then double it to allow for the inevitable upgrades! Then measure the total distance. Then we can work out what cable size you need. Then (if I were you) I'd go up at least one size again.......

So, for example:
1 x 10amp lighting circuit
3 x "10amp" power circuits (assume 10 amp load on each)
3 x 15amp circuits (assume full 15 amp load on each)

Worst case scenario is 85amp BUT domestic single phase supplies are normally 60amps, so in reality that is your limiting factor, unless you have more than one phase to play with......

Then calculate (or look up in the AS tables) the cable size required, e.g. a 25m run of single phase 240V would require a 12mm+ cable to give a 2% voltage drop, or 6mm+ at 5% drop, at 85amps. Note that for a single phase circuit the conductor length is twice the measured length (live and neutral cables total length), and lastly check that the cable thus selected is rated for the total load (which is in fact almost inevitable, given a 5% or less voltage drop!).

As I said earlier, whilst copper is expensive, for normal domestic situations it is normally only a small part of the total cost so I don't think it's wise to skimp! In an exposed frame shed, installing another circuit is cheap and easy but digging up a 25m underground run because the cable is too small for a new machine is far more costly!

Normal disclaimers apply: The above information is only an example and may not be suited to your needs. Please consult your own electrician and do not attempt to carry out this work yourself.

Originally Posted by Warb

The starting current for any motor is far higher than the current it draws when running normally - a 2.1kW (8.75amp) single phase motor can draw 20+amps at start-up -

??? all motors require about 6-7 times full load amps at start up

ie if the full load current (FLC) running amps of a motor is say 10amps then it requires 60-70amps at start up whether or not it is a single phase or 3 phase (excludes VFD stuff or the like)

in short,
a FLC 10amp single phase can draw upto 10amps before it starts to go into an overload condition

A 3 phase FLC 10amp motor can draw upto 10amps per phase before it goes into an overload condition

A 3 hp single phase motor has the same hp as a 3ph 3 hp motor.

However, a 3hp single phase motor draws approx 13.5 amps while the equivalent 3hp 3 phase motor draws approx 4.3amps (not taking into acount motor efficiencies etc) and this gives one less light flicker (voltage drop causes light flicker) at start up. 4.3 amps

Assuming the kWh consumption is the same for both motors, then the running costs of both motors are the same

Originally Posted by eskimo

??? all motors require about 6-7 times full load amps at start up

The current draw on startup is much higher than when running normally, as I said. The actual draw and duration of the spike is dependent on the inertia to be overcome - a motor with a bare shaft requires far less than the same motor spinning up a 12" saw blade. Startup current for an electric motor is normally taken to be 5 times the full load current, though for most purposes this is still a gross overestimation.

Originally Posted by eskimo

ie if the full load current (FLC) running amps of a motor is say 10amps then it requires 60-70amps at start up whether or not it is a single phase or 3 phase (excludes VFD stuff or the like)

The reality is that whilst there may be a large peak draw, it is so short lived that it is almost (?) irrelevant. Which is why a 2kW (8.3amp) table saw will happily start and run on a 16amp circuit breaker, or (more suitable) a 20amp slow blow breaker.

Originally Posted by eskimo

in short,
a FLC 10amp single phase can draw upto 10amps before it starts to go into an overload condition

A 3 phase FLC 10amp motor can draw upto 10amps per phase before it goes into an overload condition

A 3 hp single phase motor has the same hp as a 3ph 3 hp motor.

All true, though not particularly relevant.

Originally Posted by eskimo

However, a 3hp single phase motor draws approx 13.5 amps while the equivalent 3hp 3 phase motor draws approx 4.3amps (not taking into acount motor efficiencies etc) and this gives one less light flicker (voltage drop causes light flicker) at start up. 4.3 amps

It is true that the current draw per phase is lower for a 3 phase motor of the same power, this is obvious (the same total power but across 3 phases).

We are sizing the circuits to allow a 2% voltage drop with everything running at full circuit capacity which is, in turn, far more than the full load current of the devices in question (2kW motor = 8.3 amp FLC but we have allowed a 15amp circuit and used that to calculate the load and cable size). It is unlikely with such cables that the startup current would create a sufficient voltage drop to cause lights to flicker!

This is also why we have not yet bothered with the other important calculation, which is that the cables selected will give a maximum of 10% voltage drop under startup - FLCx5 - loads - though even for that calculation we would never assume that ALL appliances would be started simultaneously!.

Edit (as I have a few minutes to kill...)

Light flicker caused by voltage fluctuation is largely limited to incandescent lighting, and these are now more or less history as (from memory) their importation was halted in about February 2009. Older style fluoros with magnetic ballasts are very rarely affected, but it does happen. They normally flicker because they are starting to fail. Modern electronic ballasted fluoros (including compact fluoros) are fairly well immune to voltage fluctuations. It would be fairly safe to assume that any new workshop or rewiring would include replacing any incandescent lighting with fluoro's (preferably T5 linear units), so lighting flicker should not be an issue!

It is also worth noting that many power tools and motor driven devices these days have a "soft start" system built in, such that the startup is controlled to prevent the large current draws and mechanical strain. This has long been the case with larger 3 phase motors (I have a 60kW 3 phase motor, imagine that without a soft-start!), but is now also appearing on many of the better brands of smaller machine, even down to hand tools - Festool have built a soft start system in to many of their tools, for example.