Thread: Circuit breaker / Wire Size combo

How big is the shed?

first of all, why so many subboards? I don't do much ordering myself but I'd imagine it'd be cheaper to have one larger board and run circuits seperately. The only advantage I could see is that for items in one area, only one cable needs to be run there initially.

the sparkies are probably basing all their calculations on maximum demand theory... not how much can be drawn at any one time, but how much is likely to be drawn at anyone time. this may explain variation between the two quotes and what you want and they recommend.


for the cable size, 6mm should be adequate for 40amps, but off the top of my head i am unsure if it will cope with 63amps. 10mm is suitable for both. One sparky may only be putting a 40amp breaker on the boards. ALso, 10mm allows for some expansion... you may go 40 amps now, but when you get that dust extraction you may need to go higher.

Individually, prices for breakers and rcd's may not be an issue, as prepacked boards are availabel and are often a lot cheaper than buying seperate breakers... in many cases for a large rcd or breaker it is cheaper to buy a prepacked board and get all the extra breakers, mains switches, terminal strips. You may be able to find one of these that suits your need.

I hope this has been of help and has made sense of what they're telling you

Originally Posted by thatirwinfella

first of all, why so many subboards? I don't do much ordering myself but I'd imagine it'd be cheaper to have one larger board and run circuits seperately. The only advantage I could see is that for items in one area, only one cable needs to be run there initially.

Workshop is 27.5m X 9m. Designed to be able to be divided into 3 bays in order to seperate work areas. Mains comes in at front of building and with the cost of cable now (eg: 4mm 3C+E @ $4.00.mtr) it all gets a bit expensive running cables that distance. Using one sub board would only mean 3 less c/b's. and surface enclosures are relativly cheap. The availiability of space for running all those cables was also an issue. The only viable option for cabling was accross the edges of the floor in mezzanine area. Although they would not be in the way I dont like the idea of cables being in an area of easy access which is used regularly.

the sparkies are probably basing all their calculations on maximum demand theory... not how much can be drawn at any one time, but how much is likely to be drawn at anyone time. this may explain variation between the two quotes and what you want and they recommend.

Both lecco's were given the same breifing on requirements. Put it this way .... I will dim the light in town if I used the machines all at once.
It is a little difficult to calculate total load at any one time as the combination of machines used varies so much. Some of the main machinery is ...
Dust colection system is 5HP which would be on for most woodworking machinery (9A) so needs to be added to load of individual machines

Tennoner, 2 X 5hp + 2 X 2hp (415V 25A)
Wide Belt sander, 15hp (415V 27A) ( I still tossing up whether to buy this one yet, but knowing me I will)
Belt Sander (4hp 415V 7A)
Spindle, 1 X 5hp + 1hp Auto Feed (415V 11A)
Spindle, 1 X 3hp + 3/4hp Auto feed (415V 7A)
14" Jointer 5hp (415V 9A)
Thicknesser 1 X 7hp Cutter head+ 1 X 2hp Auto feed (415V 13A)
Lathe, 2hp (415V 4A)
3ph list goes on with cpl of other 2 ->3hp machines. Then there is the usual collection of 2-> 3 hp 240v machines, cabinet saw, 8" jointer, 14" RAS, morticer etc. We're onlly about half way down the shed yet. List continues with similar sized machines as we get into the engineering department.

for the cable size, 6mm should be adequate for 40amps, but off the top of my head i am unsure if it will cope with 63amps. 10mm is suitable for both. One sparky may only be putting a 40amp breaker on the boards. ALso, 10mm allows for some expansion... you may go 40 amps now, but when you get that dust extraction you may need to go higher.

I think I am there now. As a general rule I work alone but at times there will be two of us working at the same time and using a cpl of machines at the same time.

Tool Horder,
sounds like a serious workshop! I've had bugger-all sleep so I can't check your maths, but I assume you have been dividing your amperage by three for your 3 phase machines. ie your 9hp dusty = 3 hp per phase = 9.375 amps per phase. Sorry if you've already got this worked out, just trying to save you some grief if you hadn't.

Mick

Originally Posted by journeyman Mick

Tool Horder,
sounds like a serious workshop! I've had bugger-all sleep so I can't check your maths, but I assume you have been dividing your amperage by three for your 3 phase machines. ie your 9hp dusty = 3 hp per phase = 9.375 amps per phase. Sorry if you've already got this worked out, just trying to save you some grief if you hadn't.

Mick

Thanks Mick. You have had bugger all sleep, dusty is only 5hp not 9. But AHHHHH the light goes on. I forgot about splitting amperage across 3 phases. Amperages calculated are total amps. Can I blame lack of sleep too? Na just getting old and too many figures running around in my head and developing a major distrust in tradies. All going to plan this will be the last workshop I build and I want to make sure everything is right from the start.

Originally Posted by journeyman Mick

I assume you have been dividing your amperage by three for your 3 phase machines.

I think it's more like 1.73 or the square root of 3, but don't trust me on that.

3 phase ratings on motors are amps/phase. A 5 HP 415V 3 ph motor (usually rated between 7.7 and 8 amps) will draw 8 amps per phase when up to full load speed.(will draw between 40 and 60 amps/phase while running up to speed unless it has a current limiting starter of some description) Do not divide by three.
The current drawn by single phase equipment can be spread across the 3 phases to balance the load. For example 3x 1HP 240V motors can be connected to 1 phase each to give a load of 6.3 amps/phase.
I hope that makes sense. It is getting close to knock off time.
TH I'm a sparkie and i would recommend a minimum of 10mm for the sub mains. I would even lean towards 16mm depending on the distribution of the equipment.

Originally Posted by NCArcher

3 phase ratings on motors are amps/phase. A 5 HP 415V 3 ph motor (usually rated between 7.7 and 8 amps) will draw 8 amps per phase when up to full load speed.(will draw between 40 and 60 amps/phase while running up to speed unless it has a current limiting starter of some description) Do not divide by three.
The current drawn by single phase equipment can be spread across the 3 phases to balance the load. For example 3x 1HP 240V motors can be connected to 1 phase each to give a load of 6.3 amps/phase.
I hope that makes sense. It is getting close to knock off time.
TH I'm a sparkie and i would recommend a minimum of 10mm for the sub mains. I would even lean towards 16mm depending on the distribution of the equipment.

Now I am getting more confused. I was always under of the understanding that when calculating wire size for lets say a 20A circuit you allow for a draw of 20A per phase. IE each wire capable of carrying 20A and cb will (theoreticly) activate at a load of 20A per phase. But when calculating equiptment loads that the total load was split equally across all three phases.

Originally Posted by Dan

I think it's more like 1.73 or the square root of 3, but don't trust me on that

That's all right Dan I don't trust anybody except me .... and I don't know nuttin'. I have never heard of that before, but it does make some sense. ( 3 X 240V / 1.732 = 415V ) Maybe the loads are distributed the same as voltage. Now you have taken me from 2 possibilities to 3 possibilities.

Originally Posted by Tool-Horder

Now I am getting more confused. I was always under of the understanding that when calculating wire size for lets say a 20A circuit you allow for a draw of 20A per phase. IE each wire capable of carrying 20A and cb will (theoreticly) activate at a load of 20A per phase. But when calculating equiptment loads that the total load was split equally across all three phases.

CB selection and wire size calculation are different animals. The CB is selected to best protect the equipment or in the case of a sub mains, the cable.
The cable is selected to safely carry the maximum demand expected of the circuit with consideration given to the method of installation and the length of the run. The considerations of installation basically have to do with heat dissipation. The length of the run affects voltage drop.

When calculating loading, read the FLC (Full Load Current) rating directly from the motor name plate. For a 3 phase motor this is given in Amps per phase. Do not divide by 3.
For single phase loads - single phase motors, 240V outlets, lighting etc. the total can be divided by three to roughly calculate loading PER PHASE.
The single phase equipment then needs to be installed with equal loading on each phase or as equal as possible to keep the phases balanced.

Maximum demand calculations are performed using guidelines in the AS3000 wiring rules. They basically state that for a given installation you can expect to use 'x' outlets, 'x' machines and have 'x' lights on at any given time. The result of these calculations are used to size wiring for an installation.

Originally Posted by NCArcher

CB selection and wire size calculation are different animals. The CB is selected to best protect the equipment or in the case of a sub mains, the cable.

That much I do understand and am confident of.

The cable is selected to safely carry the maximum demand expected of the circuit with consideration given to the method of installation and the length of the run. The considerations of installation basically have to do with heat dissipation. The length of the run affects voltage drop.

That I do understand and am confident of. However from personal experience I feel that the sparkie is cutting it fine on the sub board mains and feel that 10mm would be a minimum. 6mm as sugested is too small.

When calculating loading, read the FLC (Full Load Current) rating directly from the motor name plate. For a 3 phase motor this is given in Amps per phase. Do not divide by 3.

Maybe the sparkies I have used are not listening to what I am telling them with regards to what I am running or think that I am dreaming. After all not every home workshop runs the size of equiptment that I run, but being a cabinet maker by trade, a father who was a wood machinist and a brother who is a fitter & machinist, I have learnt to appreciate the quality of 'man sized' machinery. I had a similar problem with real estate agents when i told them I was looking for 1 -> 1.5 acres in town. I found they thought i was dreaming and had to do all the legwork myself.

For single phase loads - single phase motors, 240V outlets, lighting etc. the total can be divided by three to roughly calculate loading PER PHASE.
The single phase equipment then needs to be installed with equal loading on each phase or as equal as possible to keep the phases balanced.

That I understand and am confident with. Single phase loads will be distributed as 1 X 10A + 1 X 15A on each phase.

Maximum demand calculations are performed using guidelines in the AS3000 wiring rules. They basically state that for a given installation you can expect to use 'x' outlets, 'x' machines and have 'x' lights on at any given time. The result of these calculations are used to size wiring for an installation.

The sparkie who started this job actually did sit down with a calculator and calculate loads. I was very impressed with him and 99% happy with the quality of his work. THe only problem with him was that I could not get him to come back and finish the job. After 5 months I gave up and used the one I am currently using. However, his attitude seems to be she'll be right.

I think I will do a list of machinery / anticipated loads and go back to the sparkie and give it too him in black and white just in case he is not hearing what I am saying or is speaking a different language than me . It will be interesting to see if he comes back with the same answer.

Unfortunately that's always the way with good tradesmen. Once you find them it's hard to get them when you need them because they do a good job and have plenty of work.
Write out exactly what you want connected to each sub board and give the list to your sparkie. Ask him to do MD calcs and then recommend cable sizes, in writing. If it's not written down it didn't happen. I have given copies of MD calcs to customers before to assure them everything is up to scratch. There is special software available to do MD calcs and cable sizing. Although i don't have a copy. It was usually purchased by my employers.

NCArcher,

The software you are thinking of was called PowerPac. We use it at work (in the Electrical Section) and it does a good job of calcs. However if the sparkie knows his stuff (and he should if he is licensed) then he can still calcculate it using the wiring rules. Which it seems he has.

Just for the record I am also a sparkie and wired my own shed - in 16mm cable (single phase). I don't have anywhere near the maximum demand that Tool Horder requires but to account for future expansion I figured 16mm from the main board to the sub board was a better option than running 10mm now and find out later that I need that bit extra.

Tool Horder if money is no major factor then get him to run 16mm from the main board. Cost isn't that much more and you'll have plenty of room to move.

Of course all this work has probably been sorted out by now and as such would be a waste of time for me now to sit down and work the final figures to double check the contractor sparkies.

Good luck with it TH.