What's new
What's new

Question about wire ampacity and intended use

atomarc

Diamond
Joined
Mar 16, 2009
Location
Eureka, CA
I'm not new to wire, cords and electricity but am confused as of late on this issue. All this info was gleaned from the McMaster-Carr website which I consider to be quite accurate.

A 600 volt 10/4 SO cord is rated at 25 amps at 86 degrees F. This is the standard neoprene cord you would use for an extension cord, etc.

A 600 volt 10/4 cord designated 'harsh environment cable' is rated for 49 amps at 86 degrees. Granted, this isn't your typical extension cord duty cable but I'm amazed at the ampacity difference between the two that seems only related to how the wire group is gathered together and insulated and I guess how much heat that will produce.

For my particular project, which I have fused for 25 amps, I would love to use the harsh environment cable as the specs say I could get away with 14/4..yikes. Traditional cord would dictate 10/4 for 25 amps. There is no particular point to this post other than the fact that I'm amazed at the difference between these two types of wire.

Stuart
 
I'm not new to wire, cords and electricity but am confused as of late on this issue. All this info was gleaned from the McMaster-Carr website which I consider to be quite accurate.

A 600 volt 10/4 SO cord is rated at 25 amps at 86 degrees F. This is the standard neoprene cord you would use for an extension cord, etc.

A 600 volt 10/4 cord designated 'harsh environment cable' is rated for 49 amps at 86 degrees. Granted, this isn't your typical extension cord duty cable but I'm amazed at the ampacity difference between the two that seems only related to how the wire group is gathered together and insulated and I guess how much heat that will produce.

For my particular project, which I have fused for 25 amps, I would love to use the harsh environment cable as the specs say I could get away with 14/4..yikes. Traditional cord would dictate 10/4 for 25 amps. There is no particular point to this post other than the fact that I'm amazed at the difference between these two types of wire.

Stuart

You may consider the McMaster-Carr site quite accurate, but in this case, I'm dubious.

They don't show a spec for the "Harsh Environment Cable". I'd call them and ask for more info. What is it?
 
The rating for wire is really completely dependent on the insulation. The wire itself can carry far more current than the ratings allow, although it will get heated by the current. The actual melting temperature of the wire can be reached only at currents far higher than any considered by standards. I found, back in my benighted youth, that one strand taken out of regular lamp cord wire was a fuse at about 30 amperes, when I hoped for a much lower current. This was a strand of perhaps 10 or 15 thou diameter.

But the insulation over the wire may become softened by heat, and not be able to keep its shape long term at elevated temperatures. That is particularly the case with solid wire, which is stiff and can apply force to insulation where it passes around a corner, etc, but also for large cable which may be stranded, but with relatively large, stiff, strands.

So, the ratings in this case are presumably based on the temperature which the wire will reach in the specified ambient, vs the allowable temperature of the insulation material without softening too much.. The standard rubber insulation will "take" a certain temp, and the insulation in the other case will evidently take a higher temperature.

Then also, extension cords are allowed a somewhat higher current for their size than fixed wiring. As plug-in devices, they fall under UL, and not the NEC, which pretty much stops at any receptacle in the fixed wiring system of a structure. And, they are always stranded cable, and have padding and protection in two layers over the wires.

Whether the actual rating of the cable under UL is correctly given, or not, I cannot tell without seeing the UL information about the cable. But there is no specific reason why it COULD not be correct, even though it appears to be rather high.

.
 
There are two things to consider when choosing an extension cord. The first is the Voltage drop in the wire. This depends on three factors, the wire alloy, the wire gauge, and the length of the cord. These three factors determine the amount of resistance in the cord and that, with the current determines how much Voltage is dropped (lost) in the wire. The physical construction of the cord type of insulation, outside diameter, etc.) does not come into this. Only Ohm's law (V = IR). So a 10 gauge cord will have less resistance and less Voltage drop than a 14 gauge one of the same length.

The second consideration is how much heat the cord can dissipate. Now the type and thickness of the insulation does come into play, perhaps along with other physical factors. This is why different types of wire of the same gauge have different current ratings when installed differently (conduit, open air, cordage, etc) or when it has different insulation type of thickness. The above calculation with Ohms law plus the power equation (P=VI or P = I^2R - they are equivalent) determines how much heat is created in a given length of the wire or cable. And the type and thickness of insulation along with other physical factors determines how quickly that heat can be dissipated into the environment. This in turn determines how hot the wire/cable will get. This operating temperature must be kept low enough to prevent damage to the wire/cable and to it's connections if you don't want smoke.

Notice that the second consideration does depend on the first, but the first does not depend on the second in any great way.

Tables and ratings, like those in McMaster should take both of these factors into consideration. But you must apply those tables and ratings properly. You can not take a table or rating that is intended for open air operation and apply it to a wire or cable in a conduit or raceway or to an extension cord. This is why the ratings can sometimes seem to be somewhat random. But, they really are not.
 
I'm not new to wire, cords and electricity but am confused as of late on this issue. All this info was gleaned from the McMaster-Carr website which I consider to be quite accurate.

A 600 volt 10/4 SO cord is rated at 25 amps at 86 degrees F. This is the standard neoprene cord you would use for an extension cord, etc.

A 600 volt 10/4 cord designated 'harsh environment cable' is rated for 49 amps at 86 degrees. Granted, this isn't your typical extension cord duty cable but I'm amazed at the ampacity difference between the two that seems only related to how the wire group is gathered together and insulated and I guess how much heat that will produce.

For my particular project, which I have fused for 25 amps, I would love to use the harsh environment cable as the specs say I could get away with 14/4..yikes. Traditional cord would dictate 10/4 for 25 amps. There is no particular point to this post other than the fact that I'm amazed at the difference between these two types of wire.

Stuart

I'd suspect it was mostly about the insulation's thermal survivability, not so much any conductivity diff, one 10 Gage Copper conductor to another.
 
Compare that to the 4mm square braid running at 200 amps in a small water cooled tig torch cable!

Power = Current squared times resistance. IE double the current and the heat generated on a given cable goes up by the square.

The hotter the cable runs the easier it is for it to dissipate heat to its surroundings. So long as the insulation holds up. hence even slightly higher temp insulation can gain you one hell of a lot more current rating.

Then you get to were this starts to become real engineering, like here in the UK on house lighting circuits that always use to be wired with 1mm square cable and protected with a 6 amp fuse which was ample to trip before the PVC insulation got any were near hot enough to fail. All well and good in-till modern 1'+ of insulation is added on top!!! Same cable needs to be dropped to a 4 amp fuse to remain safe, thankfully do to CFl and LED bulbs current draws are tiny and hence there’s not the fires this could have resulted in as very few people think to check fuse ratings when they add insulation!
 
Last edited:
Power = Current squared times resistance. IE double the current and the heat generated on a given cable goes up by the square root.

Square, not square root. The resistance of the wire also increases with temperature, exaggerating the effect.

Bill
 
Fixed the typo mind was else were.

Yeah the resistance to temp is surprising, at just circa 200C still bellow the ignition point of most insulations but well past there melting points, your resistance has increased to circa 170% of its starting resistance. This results in a massive voltage drop which if your powering things like old filement light bulbs means there running colder so there resistance goes down hence sucking even more current, rapidly becomes a positive feed back loop and generally ends in the place burning down!
 
Wire TYPE makes a difference...what is commonly called welding cable is also used for sound stage work.

Carries more amps per same size and more expensive.

Reason is the strands are very fine resulting in less air in the cross - sectional area.

There are calculators for voltage drop and they are based on circular mills as a unit of measurement of the cross sectional area.

Welding cable has more in same general size.

Extension cords such as SO as SOO have different strand size as well as different insulation and packing that impact heat dissipation

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 








 
Back
Top