Thread: 3-phase power

I would go with the heavier cable.

It is cheaper to do it first and once than have to have the thinner one replaced later. DAMHIK.

voltage drop only becomes apparent when loading of the cable begins to take place...the more load the more voltage drop

Aluminium conductors are notorious for getting dry joints

we know that bigger is not neccessarliy better, but in single phase supplies it does make one hell of a differnce.

I ran 16mm from my meter box to the new sub-board in the shed. Since then I have 3 x 3ph machines, with 2 more in the pipeline.

Did I plan on that many when I ran 3 phase a couple of years ago. No way. I had one machine I had bought that required 3 phase, hence the upgrade. Once you've got it, your mindset changes and you realise the array of machines both 2nd hand and new that are available to you, that weren't before.

The price difference between the 10mm and 16mm cables is not going to be THAT big. The disappointment you're going to feel, in about 2-3 years when you have to start thinking "damn, I have to turn this machine off, so I can turn that machine on....blah blah blah..."

As was mentioned several times already - you only get 1 shot at running the cables, and unless you want to dig up whatever gets laid over them in the future (which is usually a nice new concrete slab) you might have to limit what you can/can't do in the future.

I recently investigated buying a new house, that happened to have a 9 car brick garage only 2 years old (must have cost the guy a mint to build!), that had 3 phase.
Upon closer investigation, he'd run a smaller 3ph cable run to the shed, then done a brilliant job of landscaping the back yard between the house, and new shed.

End result - the house went for $50K less than it should. Why? Every guy like me who wanted the huge shed with the 3 phase, refused to bid cause the cable was too small, and digging up the landscaping just wasn't worth the pain.

Hope that experience helps justify what everyone's saying - spend the money on the bigger cable now.

Trade prices (Olex) from the end of 2009, per 100m:

SDI (PVC/PVC) 10mm = $307
SDI (PVC/PVC) 16mm = $390
XLPE/PVC (Copper) 16mm = $443
XLPE/PVC (Copper) 25mm = $643


You require 4 * 12m, we'll call that 50m for ease of calculations, so going from 10mm PVC/PVC ($153.50) to utter overkill XLPE at 25mm ($322) will add about $150, plus a touch more for larger conduit, to your pricing.

Or, from another point of view, about 10% of a Festool Domino.............. not much really!

Thankyou for the replies

I only asked my sparky what he was going to do after reading the thread. I'll ask him to get the 16mm. I guess they were just trying to save me a few dollars but seeing those prices makes me think "why bother".

The cable won't go underground but through the roof space of the house. My shed doesn't share any walls with the house but abutts it on one corner.

By the way, out of interest what is the difference between the PVC/PVC and XLPE/PVC cabling in terms of specification and functionality? I know XLPE is an abbreviation for cross-linked polyethylene and PVC is polyvinylchloride. Is the XLPE just a tougher sheath?

Cheers

Horaldic

XLPE is normally used underground. It has a higher temperature rating, PVC is rated at 75C but XLPE is rated at 90C. The "power loss" we have been talking about causes the cable to heat up, at 90C PVC has softened and become much less resistant to physical damage. XLPE does not do this, and so can be rated at a higher temperature.

Cables are rated for a particular current in a particular use. For example, an aerial cable can radiate a fair degree of heat, so for a given power loss (heating effect) it can carry a greater current before it gets hot enough to cause concern. When a cable is buried, it cannot dissipate heat as quickly, so it gets hotter when carrying the same current. If the insulator can withstand a higher temperature before failing or risking physical damage, then the same conductor size can be rated to carry a larger current etc.

However in either case the assumption is being made that the power loss is only relevant to the cable heating safety and acceptable voltage drop. The fact that the customer is paying for the power taken to heat up the cable does not enter the equation, nor that the customer may buy more and bigger toys in the future.

The electrician will try to save you a few dollars by using a smaller cable, especially if he/she thinks you're only looking at the price and might get an alternative quote from someone else. He will also makes his own life easier as the cables are lighter and perceived as easier to handle, and he has been trained only to do what is required to satisfy the regulations. However the electrician won't be footing the bill to upgrade the cable in a few years when new and unexpected machines are added! You need your electrician to understand that you want the job done properly with allowance for any and all future developments, and that you realise that this will cost a bit more.....

Yeah, I only wish my electrician was as quick to show up and return my calls. Really nice guy but very preoccupied. It's a bit hard to get my point across when he is busy working on a massive factory or warehouse job

I'm afraid I'm very "small cheese" and have specific requirements not to mention my job is fiddly.

Thanks for the information,

cheers

Horaldic

Surprised no one has mentioned phase converters yet...

Yes three phase is nice, but it all depends on your location as to the cost of three phase... In my case I was advised $15-20 000 just to get it to the meter board (all they had to do was run one wire to be run 300m plus a transformer)... Then another large sum to get it to the shed and extra for 3 new meters for the box...

I went with a phase converter...

Originally Posted by .RC.

Surprised no one has mentioned phase converters yet...

Yes three phase is nice, but it all depends on your location as to the cost of three phase... In my case I was advised $15-20 000 just to get it to the meter board (all they had to do was run one wire to be run 300m plus a transformer)... Then another large sum to get it to the shed and extra for 3 new meters for the box...

I went with a phase converter...

Three phase is three "lives" plus a neutral, so that's four cables, in conduit in a 600mm trench. If we assume 50mm2 XLPE (about $1100/100m), that's about $13,000 in cable alone, then add conduit and trenching. And whilst I don't know for certain I'd guess 3 phase supply transformers aren't cheap, and if it's installed only for you (so doesn't count as a "shared asset"), you'd be up for a fair chunk of cash. The cables would be smaller if run above ground, but then you need to add the price of poles. I'd say that the price you were quoted was quite reasonable, and probably no more than installing a single phase supply from scratch.

I've never tried a phase converter, but the digital ones and the better rotaries look like they're quite efficient these days. Obviously the supply still needs to be sufficient to provide the required power (so 20 amp 240V for 3hp) and the power bill will be slightly higher due to the power loss in the converter. My major concern would be longevity of the system, however. A 3 phase supply. once installed, is there for life. I have no idea of the reliability or longevity of a phase converter, and it probably depends largely on the quality of unit purchased.

It's really horses for courses however. Because power loss (voltage drop) is dependent on current, distance and resistance, sometimes installing three phase can actually be as cheap as single phase because each cable only carries a third of the current so they can be smaller cables. Also, for short cable runs, the additional cable costs are not really significant compared to the "fixed" costs of trenching etc.

So installing from scratch to a "new" facility, and given 3 phase available at the site (or if a new transformer was required either way), I'd go with a 3 phase service. However as you obviously discovered, given an exisiting single phase supply, no 3 phase available, and a long run of cable, "upgrading" to 3 phase is an expensive process, because it's basically a complete new installation. In that situation, assuming the single phase supply is adequate, then the converter is a valid option as you say.

Originally Posted by Warb

Three phase is three "lives" plus a neutral, so that's four cables, in conduit in a 600mm trench.

I meant with the 300m that is the distance from the closest 3 phase transmission line... Our spur line (300m long) only uses two phases to the main transformer as we only use single phase (rural property)

The phase converter I bought can start/run a 10hp motor and draws about 600w with no load... It would take a lot of electricity to make up the extra 10-$15 000 that I would be out of pocket to get proper three phase installed.

We should have had three phase initially installed back when we built here in the early 1980's but I was but a very young youngster back then and no one in the family was interested in the sort of things that require three phase..

This is what the internals of my phase converter look like.... It has a transformer to bump the power from 240 to 415..

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