Wiring Standards

[mttee2]

I am given a task to convert a Machine's Switchboard from Relays to PLC and improve it's performance. I am still new in this field and facing some high amps motors.
  I searched google and only received brief explaination on wiring standards not how to choose the right wire size. What is the safety factor practiced? At what temperature the copper wire will melt? What is the resistivity on standard copper wire?  

[GeekGirl]

Any decent rule book (in Au we use the AS3000) for wiring in structural systems eg house, office etc.

For in equipment wiring you really need to look at the cable charts for the cable you wish to use, as the current capacity is not derated (as it is for structural).

All suppliers have a brochure (some even have pdf's on their site) that lists the ratings of the cable, it may be able to handle say 100A but if you need UL compliance then it may only do 80A etc.

If you need a very low voltage drop then you will need a larger cable than otherwise needed for the current capacity.

[Erik]

Perhaps this site can give you some information:

http://www.powerstream.com/Wire_Size.htm

[Simon]

if your lookig to play very safe and have it last years go for 5-8 A/mm2

[Zero999]

Deciding the crossectional area for a high current cable isn't always easy.

It depends on many factors: current, maximum acceptable voltage drop, conductor material, ambient temperature, maximum temperature rating of electrical insulation and surrounding materials, the number of neighbouring conductors in the conduit/sheath and the thermal insulation of the surrounding material.

For a start answer the following questions:

How much current are you drawing? Give the typical DC/RMS figure, not the surge.

What's the maximum acceptable voltage drop?

Are the cables well ventilated?

Low voltage high current cables are, in many ways  more dangerous than high voltage low current cables because the amount of heat given off can easily start a fire. It's very easy to get it wrong, for example you buy a 25m length of PVC insulated 1.25mm² mains cable which is rated to 13A. You put a  plug on one end and a socket on the other end to make your own extension lead. It's too long for some applications so you coil it, you then use it to power a heater rated at 13A and after a couple of hours of being left on the cable melts and catches fire. The cable should have been derated when coiled because the heat couldn't escape causing it to become dangerous. You should have used a heavier duty cable with a higher temperature rating and larger crossectional area if you wanted to coil it and power a 13A heater.

A good section of the IEE wiring regulations is devoted to choosing the right type of cable for the job: you should look at the regulations relevant to your jurisdiction.

[Simon]

I bought a 2 KW fan heater with a 0.75mm2 cable ! needless to say it gets warm and the fuse is naturally a 13 A fuse in the plug, if its warm at 8.3 amps I'd not like to see what happens when it passses 13 A before the fuse even blows.... always remember that thicker is much better than thinner always best to play safe and you should always look ahead, as Hero999 said you can make up a lead for one thing and next you know your overloading it. I always make extensions from at least 1.5mm2 for example, that way I know I've got it well covered.

[mttee2]

Deciding the crossectional area for a high current cable isn't always easy.

It depends on many factors: current, maximum acceptable voltage drop, conductor material, ambient temperature, maximum temperature rating of electrical insulation and surrounding materials, the number of neighbouring conductors in the conduit/sheath and the thermal insulation of the surrounding material.

For a start answer the following questions:

How much current are you drawing? Give the typical DC/RMS figure, not the surge.

What's the maximum acceptable voltage drop?

Are the cables well ventilated?

Low voltage high current cables are, in many ways  more dangerous than high voltage low current cables because the amount of heat given off can easily start a fire. It's very easy to get it wrong, for example you buy a 25m length of PVC insulated 1.25mm² mains cable which is rated to 13A. You put a  plug on one end and a socket on the other end to make your own extension lead. It's too long for some applications so you coil it, you then use it to power a heater rated at 13A and after a couple of hours of being left on the cable melts and catches fire. The cable should have been derated when coiled because the heat couldn't escape causing it to become dangerous. You should have used a heavier duty cable with a higher temperature rating and larger crossectional area if you wanted to coil it and power a 13A heater.

A good section of the IEE wiring regulations is devoted to choosing the right type of cable for the job: you should look at the regulations relevant to your jurisdiction.

The current is about 20 AAC for the AC motors. Acceptable voltage drop is 10 VAC but better keep to 5 VAC. Is inside a switchboard but no hot components around. Hence assume 30 celcius. Cable length less than 2 meter.

   I will buy a engineering handbook when I got the money.  

[Simon]

3 mm square wire should do you and 5 V drop is a lot, normally the voltage drop is specified per set length of cable or as a percentage and should be very low

[Zero999]

I bought a 2 KW fan heater with a 0.75mm2 cable !

I hope it's high temperature rubber cable, not PVC, 0.75mm² PVC cable has a maximum rating of 6A.

If it's PVC cable, then I'd consider replacing it with 1mm².

The current is about 20 AAC for the AC motors. Acceptable voltage drop is 10 VAC but better keep to 5 VAC. Is inside a switchboard but no hot components around. Hence assume 30 celcius. Cable length less than 2 meter.

The voltage drop isn't going to be a problem, 10V@20A is a power dissipation of 200W which will melt any 2 meter length of cable.

I wouldn't normally call 20A a very high current for cables.

I've just checked the 17th edition wiring regulations, assuming you're using ordinary copper conductor, PVC insulated, mains cable with adequate ventilation (i.e. not embedded in foam or coiled up) you need 2.5mm² minimum.

[Simon]

I bought a 2 KW fan heater with a 0.75mm2 cable !

I hope it's high temperature rubber cable, not PVC, 0.75mm² PVC cable has a maximum rating of 6A.

If it's PVC cable, then I'd consider replacing it with 1mm².

The current is about 20 AAC for the AC motors. Acceptable voltage drop is 10 VAC but better keep to 5 VAC. Is inside a switchboard but no hot components around. Hence assume 30 celcius. Cable length less than 2 meter.

The voltage drop isn't going to be a problem, 10V@20A is a power dissipation of 200W which will melt any 2 meter length of cable.

I wouldn't normally call 20A a very high current for cables.

I've just checked the 17th edition wiring regulations, assuming you're using ordinary copper conductor, PVC insulated, mains cable with adequate ventilation (i.e. not embedded in foam or coiled up) you need 2.5mm² minimum.

being a peice of chinese crap (with emphasis on the word crap) I'd guess it is PVC but would like to delve deeper. as it is on a plug with a 13 a fuse it should becapable of carrying 13 amps. I'm in a good mind to investigate and if i find foul play call trading styandards and health and safety

[Brett]

Deciding the crossectional area for a high current cable isn't always easy.

It depends on many factors: current, maximum acceptable voltage drop, conductor material, ambient temperature, maximum temperature rating of electrical insulation and surrounding materials, the number of neighbouring conductors in the conduit/sheath and the thermal insulation of the surrounding material.

Don't forget duty cycle of current flow.  "current" sums that up if you think about it the right way... but not everyone does.

http://www.milspecwire.com/  has a lot of nice charts on the home page (scroll down), and it's an easy site to remember.

A nice design guide courtesy of the US Department of Defense is MIL-HDBK-508.  Check out section 100 on Wire and Cable.
You can get the guide here free:
http://www.assistdocs.com/search/search_basic.cfm

Just type in "MIL-HDBK-508" in the document ID.  Click through a couple times and then select the Base Document.  The PDF will load for your viewing pleasure.

Also consider the circuit breaker rating, and choose the wire based on the available current from the breaker when the wire is shorted at the motor.  You could also design for the right wire for the motor, and then set the breaker rating appropriately.  Something to consider.

[Neilm]

If you are after a guide for how much current will go down cables have a look at this programme. http://miscel.dk/MiscEl/miscel.html. One of the functions gives rough ideas on how much current you can put down a cable, what losses you could expect and so on. I tend to use this at work as it has some other very useful features.

[Brett]

If you are after a guide for how much current will go down cables have a look at this programme. http://miscel.dk/MiscEl/miscel.html. One of the functions gives rough ideas on how much current you can put down a cable, what losses you could expect and so on. I tend to use this at work as it has some other very useful features.

Whoa, where has that program been all my life?  Nice!

[mttee2]

Wow. I should search more on US defense department website. They really provide some good technical datas. Thanks guys for making my live a lot easier or lazier.