Wire sizing question

[wizard69]

Yes, I know all that.  I am actually an licensed electrician, but my license was issued by Japan.  (it's no good here)  I do know few facts in NEC and local code though. 

I think I did mis-speak.  I meant to say #14 and #12 but instead, I said something else.  I blame my drink (that contained a wee bit of Vodka)   

Regulations really don't apply in my case; though, as there is nothing that regulates what DIYers use for their home made equipment.  You can use paper clip if you want.  Unwise, silly, but not illegal.  It may invalidate home owner's insurance but still not illegal.

Yes.  Buying Chinese merchandise from eBay is a challenge.  I had mostly good luck but few bad ones.  Fuse holders were 50/50 in luck gauge.  All I wanted to know was if China had different numbering system for wires.  Thanks!!

I don't know about China, they are a metric country but I'm not sure if or how they standardize things like this.   I can't see them accomplishing what they have without some sort of Electrical policy / standard.

As for the challenge of buying online via E-Bay, I'm not one to go that route often.    In my old age the grief isn't worth it when I can order online with DigiKey or MSC in minutes (seconds) and have the goods the next day.

[LogicalDave]

I've always been curious about this.  I believe NEC indicates 12AWG for carrying up to 20A, but that has always seemed like a rule of thumb rather than a precise engineering calculation.  There are multiple factors involved in actual ampacity including the thermal conductivity/resistance of the insulation and its surroundings.

The calculation of heat generated by passing a current through a wire is easy; for example, 12AWG wire presents 1.6 ohms of resistance per thousand feet (1.6mOhms/ft).  So the power dissipated (heat generated) by passing 20A of current through 1ft of 12AWG wire is I^2 R or 20*20*0.0016 = 0.64W.

The question then is how much the temperature of that wire will rise before insulation fails or other thermal damage.  To know this, we must understand how much resistance there is to transfer of heat from the wire into the environment which depends on the insulation material and thickness, air, earth, other wires, foam insulation, etc..  It also depends on the presence of other conducting wires (often there will be at least two in the same bundle, each dissipating heat.

This site offers a good discussion: http://electrician2.com/articles/ampacity.htm

I'm not an EE, so if I've botched any of the above, please set me straight.  If anyone knows of a good online calculator for ampacity that's more precise, please share it!  At the least, it would be great to have one that lets you calculate ampacity for a variety of common materials (e.g. 12/2 Romex) and environments (air, conduit, wall).

[tkamiya]

There's a lot to do with calculating how many amps a wire can carry.  Romex in free air is considered to have 16 amp and 20 amp capacity, #14 and #12 respectively.  When single wires are in conduit, we were taught to de-rate them either by 70% or 63% depending on how many wires are in conduit.  This has all to do with heat and ability to dissipate it.

In United States, electricians are prohibited from "bundling" wires in attic.  That is, group Romex wires and zip tie them.  This is for the same reason.  Heat build-up cannot be dissipated effectively.

The same wire can be used in appliances.  Then, they can be OVER-rated by quite a bit.  That wasn't part of my training.  But if you look into dryer, very thin wires are used to carry up to 30A.

In Japan, we do not use AWG, or gauge.  We call it 1.6mm and 2.0mm.  (#14 and #12)

By the way, those rating has nothing to do with doing your own calculations.  The limitations are all defined beyond doubt in National Electrical Code.  You must obey then or else, it won't pass inspection.

[TimFox]

The US codes call out different wire requirements for building installation and internal appliance wiring, since the conditions are different.

[themadhippy]

In the uk our  regs  have different current ratings for cables depending on the installation method,along with correction factors for things like ambient temperature, running in thermal insulation,grouping of cables and  method of protection ,so a lump of 2.5mm²might be good for 30A or might not be  big enough for 15A.