Tutorial: Getting Hot / Ampacity!! AWG, Wire, Connectors and Fuses

[MJLorton]

A tutorial on the importance of selecting the correct gauge wire to carry the current required.  A little look at AWG (American Wire Gauge), resistance in wire, connectors and fuses getting Hot!!

Link to video here: http://youtu.be/q4GLrbJEDkI

Add your pearls of wisdom to the conversation....I know there's a lot more for me to learn on this topic! 

Attached (thanks to a great viewer and forum member sending it my way), a very handy reference on how to crimp wire connectors correctly by NASA:

[SeanB]

Just note that wire temperatures are so high because they are rated to run in conduit ( steel or plastic) buried in walls. Thus they will get quite hot when doing so, especially if there are multiple conductors adding heat to the conduit.

For an insane cable look at mineral insulated cables, RS http://za.rs-online.com/web/p/lighting-electrical-cable/2909690/ has some, rated at 250C. A small 1.0mm cable is capable of handling 15A continuously. This is designed to survive fire and still work in a fire until the building collapses.

Interesting that the connections are so cool, must be varying areas per connection to dissipate heat, and thus each runs at a cool temperature. If you did thermal cycling though the copper/plastic crimp will eventually suffer from flowing as the plastic insulation creeps ( the silicone will not creep as it is a thermoset cured plastic and not a meltable plastic) and with tension on the wire from the loom it will pull loose after time.

[Stephan_T]

Hi Martin,

wow, you are pretty wrong this time! You seem to regard the connectors as heat sources, while they here actually act as temperature connectors to a heat sink. You could have  recognized this due to the fact that the cable is much warmer than each of the connectors. The reason why the cable gets so hot is, because it is thermally isolated by the silicone and the poor thermal conductivity of the air. Meanwhile the connectors are mounted metal to metal to the contacts of the battery and the (electrical) power sink. Both are acting also as thermal heat  sinks.

So the coldest of the connectors is actually the best thermal connector. Usually we may expect a poor thermal connector to be also poor electrical one in our situation here, so this would lead us to the opposite conclusion:
The colder the connector, the stronger is the (thermal) connection between the copper of the cable and the metal of the heat sink.

It's funny, how you trust the arbitrary small readings of a multimeter (0.02 vs 0.01) more than math

13.7 Ω/km =  0,0137 Ω/m

How long is the cable that you are testing. Just calculate what its expected resistance should be.

But what you really should do here is to make good use of Ohm's law. You can measure the voltage drops on all of the related items fairly easy. Remember that when you measure Volts, your digital multimeters all have a high impedance. So they do not significantly add to the current in the circuit. Also the test leads of the voltmeters will only have to carry a tiny little current. You could for example add test point behind (seen from the contact side) the connectors by sticking a needle through the cable. As our test currents will be tiny, you don't have to worry too much about their connectivity. It just has to be much smaller than the impedance of the volt meters. You can also remove a little bit more of the insulation and put crocodiles on the wire.


[Contact A = Test Point 1]-<Connector A>-[Test Point 2]------------<cable>-------------[Test Point 3]-<Connector B>-[Contact B = Test Point 4]

With such test points in the cable right after each connector you can directly measure the voltage drop of each part of the circuit.
You may take a look at the Wikipedia page "Four-terminal sensing"

When you do this with a current of let's say 10A, you can get very precise numbers for the small resistor values by an exact measurement of a small voltage.
At 10A you should get 137mV along one meter of your cable. Just adjust it to your actual length and you can verify the spec. As you ability of measuring volts can get much lower, you can even detect much lower resistance values than your ohms meter. Just trust Ohm's law and math

With two extra test  points (2 and 3) sticking into the cable, you can measure all necessary voltages along all relevant parts of the circuit:

Voltage between 1 and 2: voltage drop of connector A
Voltage between 2 and 3: voltage drop along the cable
Voltage between 3 and 4: voltage drop of connector B

This would give you much more meaningful results than your temperature values. You may also be able to observe how the resistance values change with temperature.
An other thing that you may want to give a try is to measure the resistance of a connection while you are crimping.
But wait with switching on the load until you have some initial connection. Otherwise you may weld the connection with your current

[John Eckert]

Hello everyone:
Properly sized wire can make the difference between inadequate and full charging of a battery system, between dim and bright lights, and between feeble and full performance of tools and appliances. Designers of low voltage power circuits are often unaware of the implications of voltage drop and wire size.  In conventional home electrical systems (120/240 volts ac), wire is sized primarily for safe amperage carrying capacity (ampacity).  The overriding concern is fire safety.  In low voltage systems (12, 24, 48VDC) the overriding concern is power loss.  Wire must not be sized merely for the ampacity, because there is less tolerance for voltage drop (except for very short runs).  For example, a 1 Volt drop from 12 Volt causes 10 times the power loss of 1 Volt drop from 120 Volts.

A general rule is to size the wire for approximately 2 or 3% drop at typical loads, However, different electrical circuits have different tolerances for voltage drop.  A good example of this is in lighting circuits.  INCANDESCENT AND QUARTZ HALOGEN lighting CANNOT tolerate voltage drop as a 5% voltage drop causes an approximate 10% loss in light output.  This is because the bulb not only receives less power, but the cooler filament drops from white-hot towards red-hot, emitting much less visible light.

In addition, when a wire goes from cold, to warm, to hot, the resistance of the wire increases exacerbating the problem with using a wire undersized for the load being placed on it.

Stranded wire Verses Solid conductor wire of the same gauge have properties all of their own.  Now you would think that a good, heavy solid conductor wire would be better BUT THAT IS NOT THE CASE.  When electricity flows through a wire, it mostly flows on the surface of the wire, not through the middle.  This effect is more pronounced on high frequency AC than it is on DC or low frequency AC like the 60 or 400-cycle power used on military and commercial aircraft, but this means that a "wire" of a given size that made up of many smaller strands can carry more power than a solid wire.

[LightAges]

You are getting a little carried way for the skin effect. Skin effect for 60Hz is around 10mm. So unless you have a conductor more than 20mm in diameter the skin effect does not come into any consideration. At 400Hz it is around 4mm, so unless your conductor is more than 8mm dia. again it is pointless to consider this effect.

Multistrand wires are made for their flexibility mainly. A solid wire will be more reliable in a low flex environment because of less surface area exposed to oxidation.

[MJLorton]

Just note that wire temperatures are so high because they are rated to run in conduit ( steel or plastic) buried in walls. Thus they will get quite hot when doing so, especially if there are multiple conductors adding heat to the conduit.

For an insane cable look at mineral insulated cables, RS http://za.rs-online.com/web/p/lighting-electrical-cable/2909690/ has some, rated at 250C. A small 1.0mm cable is capable of handling 15A continuously. This is designed to survive fire and still work in a fire until the building collapses.

Interesting that the connections are so cool, must be varying areas per connection to dissipate heat, and thus each runs at a cool temperature. If you did thermal cycling though the copper/plastic crimp will eventually suffer from flowing as the plastic insulation creeps ( the silicone will not creep as it is a thermoset cured plastic and not a meltable plastic) and with tension on the wire from the loom it will pull loose after time.

Very interesting Sean, thanks. Impressive cable...at an impressive price!! Appreciate it has a very specific use though.

[MJLorton]

Hi Martin,

wow, you are pretty wrong this time!

Stephan, your input would be appreciated if you were not condescending. Please take note of the common sense message at the start of the video and what I had to say at the introduction of this video.
If you don't appreciate the spirit in which I create the videos..don't watch.

Cheers,
Martin.