Electrical wire rating question

[Furu]

Why do the charts show smaller gauge wire sizes for 12 volts systems than you do for higher volt. Amps are amps. That is why transmission lines are bumped up to much higher voltages to keep the wire size down for the power company as well as voltage drop. Is it an AC vs DC thing?

Also why is it so hard to find waterproof bullet connectors? I would think that they would make them as the first choice. I keep finding these cheap POJ at the auto parts store.

 

[Little_Grizzly]

Do you have a reference or example? Ampacity is ampacity but sometimes there is mechanical considerations too. For example in a high vibration environment.

 

[beppington]

Why do the charts show smaller gauge wire sizes for 12 volts systems than you do for higher volt. Amps are amps. That is why transmission lines are bumped up to much higher voltages to keep the wire size down for the power company as well as voltage drop. Is it an AC vs DC thing?

I think your answer is because for a given power, the lower the voltage the higher the resistance.

And power co's use higher voltage because it has lower resistance, therefore less loss over distance, & also can be carried on thinner wire.

Also, I think that rather than "amps are amps" it's actually "watts are watts", or more basically "power is power" ... since P (power (watts)) = V (potential (volts)) x I (current (amps))

I think I have all this correct - It's late!

 

[Monster5601]

Also, I think that rather than "amps are amps" it's actually "watts are watts", or more basically "power is power" ... since P (power (watts)) = V (potential (volts)) x I (current (amps))

I think I have all this correct - It's late!

True for DC but for AC, you need to use the RMS value of the AC voltage in the equation. AC voltage is often referenced as peek or peek to peek. The RMS value (peek AC * .707) is the value considered to so the same amount of work as DC.

 

[Eddie89]

There's less current that's why there're fire smaller gauge wire.

 

[chim]

Why do the charts show smaller gauge wire sizes for 12 volts systems than you do for higher volt. Amps are amps. That is why transmission lines are bumped up to much higher voltages to keep the wire size down for the power company as well as voltage drop. Is it an AC vs DC thing?

Also why is it so hard to find waterproof bullet connectors? I would think that they would make them as the first choice. I keep finding these cheap POJ at the auto parts store.

Everything comes back to money. For example, if you design something that will draw a maximum of 3.2 amps and the distance is short and fixed, you don't need very large wire to power it. Determining wire size may depend more on physical performance than electrical current-carrying capacity. A 16 ga wire could be overkill with regard to current, but necessary to make the installation robust enough to withstand damage from vibration. If you're a manufacturer who uses miles of wire daily, you don't want to spend money on copper that isn't really needed.

If you have a situation where the load can change, wiring is sized for the anticipated max. Likewise, adjustments are made for distance to counter voltage drop.

For the connectors, it's much the same story. Stores can sell truckloads of "POJ's" and have most customers ignore the better (and more expensive) stuff. They see stocking slow movers as a waste of shelves.

Peak, RMS and Average AC voltages make for interesting discussions in class, but any test instruments available for normal use take all that into consideration. When you connect your meter to an outlet and it reads 117 volts, that's the effective voltage. For calcs we normally use 120 even though the reading might be anywhere between 110 and 125.

What becomes more complicated for AC is when you need to deal with Power Factor, generally with inductive loads such as installations that include a substantial motor load.

 

[JerryK]

There is really two different codes. NEC, National Electrical Code that covers homes... and SAE, Society of Automotive Engineers. The codes that cover home use basically allow no 'heating' of the conductor by current flow of the wires in your wall, and the capacity charts are leaning on the safer side. The wires coming from the pole to your home are a lot smaller than the wires you have to come down thru your meter because these conductors are in 'free air'. The code that covers automobiles is a completely different thing. It is weighted more to cost savings for each vehicle while keeping conductor heating/voltage drop down to resonable levels.

 

[EE_Bota]

Years ago, our substation transformer test station had, instead of wire, copper pipe carrying the current. It looked just like what carries water in many houses.

Transformers are AC, of course, so the reason they did that was the pipe is rigid, and will support itself, and the reason for pipe instead of copper rod is the center of the rod would do very little anyway, since due to the skin effect, the center portion of a solid copper rod carries little current anyway.

It's true. If I give you a 1/4" solid copper rod, it is a solid conductor for DC, but you might as well consider it a hollow pipe for AC, because it is, and you can't do much about that.

I wrote the reason why, but I erased it...if you care, read up on the "skin effect" until your eyes hurt. But in my experience, it is far better to make sure you know what a multi-meter is, how to use one, and what all of the tests mean, and how to troubleshoot the real problems you will encounter in your lives with a good multi-meter. I think I have seven multi-meters, and I never leave home without one in my car. I have clamp on probes for AC and DC current measurement.

Radio Shack has one for around $50 bucks that does most things, and even does clamp on AC and DC current. Heck, I used to have to spend a fortune to get a meter that will do clamp-on ac current measurements. Now I own two personally...one I gave $400 dollars for years ago, and my Radio Shack one. The cheap purchase means I can actually use it any time I like without fear of ruining a $500 meter.

 

[EE_Bota]

I think your answer is because for a given power, the lower the voltage the higher the resistance.

And power co's use higher voltage because it has lower resistance, therefore less loss over distance, & also can be carried on thinner wire.

Also, I think that rather than "amps are amps" it's actually "watts are watts", or more basically "power is power" ... since P (power (watts)) = V (potential (volts)) x I (current (amps))

I think I have all this correct - It's late!

Your are mostly right. Try not to think of it as "the lower the voltage the higher the resistance." Think instead as " for a given power, the higher the voltage, the lower the current, and the lower the current for a given resistance, the lower the power loss in the line."

Folks say that ohm's law is V= IR. But I want you to think of the resistance is an independent thing, and a physics book would say that Ohm's law is R= r l /A (but they would use different symbols.) Resistance equals resistivity times length divided by area.

A raw material will have a given resistivity. Gold has a lower one than copper. Regardless, resistance goes up with the length of the conductor, and goes down with the area of the conductor. So, as you see, the resistance does not change with the voltage, nor with the current. It can change with some other things, such as temperature. For most things, as temperature rises, resistance increase a bit. Carbon and few other things are an exception...their resistivity goes down somewhat with temperature.

 

[LD1]

My guess is that they are just listing what is commonly used and commonly avaliable to buy.

With a little googling you can find charts that list AC ampacity down to 22ga wire. But not many people out there are wiring AC with that small of wire. And if they are, it is usually just for signal wires running from sensors to I/O cars which carry only milli-amps. So ampacity isnt a huge concern.

Why are you asking for??