Know your "circular mils"

[graham-xrf]

OK - I know a "mil" is 0.001", a thousandth of an inch, also called "thou", which for me is already a bit of a discomfort, because here (UK), "mil" is just short slang for millimetres (mm). But now, I have also found "circular mils" (cmil), and additionally, "square mils" even, which appear to equal 1.273 of the circular kind.

In inch-land, a square inch is about 1.273 million of them! Yep - it's an AREA.
So one circular mil is 0.7854 square mils. It's the area of a circle 0.001" in diameter.

1 cmil = (π/4)*(0.001 in)^2
There is hardly any point going metric with this, given the alternative meaning of mils, but for those who care, try 5.067E-04 square millimetres.

I get it that Canadian and US wire gauge tables ran out of steam at 0000 AWG, but then, for the yet bigger fatter wires, they chose this very tiny unit, so now they can know the sizes in many hundreds, or some thousands of these things. Other than that 1000 of them gets you a 1.000 inch diameter, so hardly still a "wire", WHY they thought this was a good idea is, no doubt, some whole other story!

 

[RJSakowski]

Circular mils are used with electrical wiring because conductivity is proportional to circular mils. Easy to calculate because you just square the diameter in mils, No pie involved. If you know the current capacity per mil of your conductor, it's an easy back of the envelope calculation.

Then there is the angle mil = .360 degrees.

 

[rabler]

Adding on to RJ's comment, and to have fun:
At least in the U.S., wire gauge larger than 4/0 is measured in MCM, meaning a thousand circular mils, one thousand circular thousandths. That really strikes me as a confusing use of units AND abbreviations. Other than that 1000 MCM = 1 inch diameter, I think.

 

[rabler]

I'm going to speculate that the advent of modern technology (computers and the Internet) have made the use of this type of convenience (conductivity proportional to area) much less of a useful thing. Most people are quick to look this type of thing up in an online chart than actually crunch numbers, and if so can do it on a calculator so pi * r^2 isn't tough.

A similar issue, how many people care memorized for small angles:
the tan(x) is approx the same as the sin(x) which is approx x/60? (for an angle x measured in degrees)
Why bother, won't a calculator do that exactly? (@graham-xrf might know why I picked this up)

 

[Jake M]

Circular mils are area, not length. So a thousand thousandths doesn't add up to a whole wire. 1MCM or 1KCMIL is about 0.78 square inches.

This does go back to early days of electrical wiring, as that's how you figure out current carrying capacity back when you did such things by hand. The same era when it was a good idea to have wire gauge drills labeled with the wire gauge size. Before we standardized on using exgra "oughts", they were gonna give out letters. Or maybe it was the same time in different areas before standardization... Anyhow, switching areas (and all gauge sizes really) made it a LOT easier, faster, more efficisnt, and less prone to gross errors wen you had to work out your electrical distribution on paper. Just like inches and feet (and number and letter drills), It made a LOT of sense back then. And that's all we need to hang onto it forever.

We gave up at four "oughts" because we ran out of room on stamping or marking machines or something like that. I have to presume that the "oughts" we talk about (sometimes seen as "aughts") is a degenerated rendering of "naught", like we've done with so many words where we new better than the originators did. But I'm making that last part up as I really don't know.

 

[RJSakowski]

My favorite regarding electrical wire is that 10AWG copper has a resistance of about 1 ohm/1000 ft and the resistance doubles for every three wire gauges so 13AWG has a resistance of 2 ohms/1000ft and 16AWG has a resistance of 4 ohms/1000 ft. Easy to remember' no need to look anything up. If you want to go a bit further, commit 11AWG and 12AWG to memory as well and you have the entire table stored in cranial memory.

Another good one that I figured out long ago was that -40ºF and -40ºC are the same temperature. If you add 40 to either temperature then multiply or divide by 1.8 followed by subtracting 40, you get the other temperature. It's easy to commit to memory and oftentimes I can do the math in my head.

 

[RJSakowski]

My British wife frequently uses "nowt" meaning nothing. The usage traces back to medieval England. Whether nought or nowt came first, I don't know. Ought was probably a colloquial expression for nought although I don't know where the custom originated. Here, we refer the the .30-06 caliber as the "thirty ought six"

 

[tq60]

Brings back memories of switchroom power plant design.

DC power is such a pain when you have distance and tight voltage loss spec.

Running 4 1000 MCM cables as we needed redundant feeds costs a bit.

Sent from my SM-G781V using Tapatalk

 

[graham-xrf]

My favorite regarding electrical wire is that 10AWG copper has a resistance of about 1 ohm/1000 ft and the resistance doubles for every three wire gauges so 13AWG has a resistance of 2 ohms/1000ft and 16AWG has a resistance of 4 ohms/1000 ft. Easy to remember' no need to look anything up. If you want to go a bit further, commit 11AWG and 12AWG to memory as well and you have the entire table stored in cranial memory.

Another good one that I figured out long ago was that -40ºF and -40ºC are the same temperature. If you add 40 to either temperature then multiply or divide by 1.8 followed by subtracting 40, you get the other temperature. It's easy to commit to memory and oftentimes I can do the math in my head.

I have nothing against these handy relationships. Being as I started out in a British colony, there was °F as well as °C, and I always knew that to get to °C, one had to subtract 32, then find 5/9ths of what was left. On the world map, it kind of looks like the USA and Liberia are the remaining users, though there be a smattering of little island places that might bring the total up to around 20.

Then too, there are inches. That my lathe(s), when together, are South Bend, amid a modern UK gone metric means I have a whole set of inch micrometers, and spanners, AND also a whole collection of the same stuff in metric. Feet and inches have the advantage being based around 12, which is actually far more composite than based on 10. They easily divide into halves, and thirds, and quarters and sixths, whereas metric has to settle for 5 and zero.

That said, inches force fractions. I can't really handle the fractions as easily as metric decimals. The nearest response from my digital caliper moving off 7/16ths is 55/128ths followed by 27/64ths. So I switch to decimal inches. That's OK, but there are no bolts anywhere around me that are other than metric. When someone in this forum says "quarter-20", I have no idea, except maybe it looks a bit like M6, but will never fit anything here.

While not as crazy as the Indiana Legislature trying to mess with the value of pi (1897), I can see why circular mils happened.

 

[graham-xrf]

Brings back memories of switchroom power plant design.

DC power is such a pain when you have distance and tight voltage loss spec.

Running 4 1000 MCM cables as we needed redundant feeds costs a bit.

Sent from my SM-G781V using Tapatalk

I have been there as well. I had 67m of 95mm^2 cable as 5-conductors 3-phase SWA armoured delivered to a site where we had unwind it directly off the reels on the back of the truck, then payed out on those little guide boards with roller skates. We had to drag the end with a fork-lift truck to put it all through the side of the building at the first floor level switch room, and then lay it in Unistrut tray all through the building to the cabinets that were to use it. The 2m offcut, (about 79"), I tried to pick up. I did manage to pick that bit up, but only just!