2nd Gen Non-Engine/Transmission Batteries & Cables

[Willys]

I'm not sure about how the wire can carry more load(current aka amperage?) depending on the insulation alone. This just doesn't make much sense to me... Also, are we talking about a change in insulation compound, thickness of insulation, or both?



I'd also like to better understand what you mean by the ampacity of wires being affected by being inside conduit. Would this also mean that split loom or spiral wrap can affect ampacity? I'm learning here... so please keep it coming. If this is more than you have time to type, could you recommend some sites/books to read? Are you an EE?

 

[rbattelle]

I'm not sure about how the wire can carry more load(current aka amperage?) depending on the insulation alone. This just doesn't make much sense to me... Also, are we talking about a change in insulation compound, thickness of insulation, or both?

As I understand it, current flowing along a wire generates heat in direct proportion to the amount of current. Then a cable with insulation capable of sustaining a higher temperature continuously without breaking down will be of higher ampacity than a cable of the same size and makeup but with lower-temperature insulation. The cable's ability to transfer heat to the atmosphere is compromised by being inside a conduit or enclosed area, so that will effect the ampacity of the cable. Of course, we're assuming here that the copper itself that makes up the wire is capable of handling the current without melting.



I am not an EE, but that's what I've been lead to believe. Someone will correct me if I'm wrong.



-Ryan

 

[Ncostello]

I'm not an EE either. I'm a CE. Which could be even more dangerous here . But my EE friends have given me advice in the past.



Rbattelle... you are right on. Failure of a conductor (cable) is not solely based on the ability of the copper to handle amps. For example... if the insulation fails at 200 amps then the wire will ground out and destroy itself and its surroundings. If the insulation holds at 200 amps... all is well and everything keeps working. The wire is considered good for the job then and able to handle the amp load.



Your power company will use a larger gauge wire in an underground run to your house than they will for an overhead drop. Reason is the overhead wire can vent heat to the air quickly and handle the same load without burning in half from high heat generation.



I consider split loom a closed conduit. It does hinder the wires ability to conduct heat to the outside air. But alot of fire truck builders... who employ EE's... pack as many wires as possible into polyloom. Thus they don't seem to mind in an automotive application as much as I do. In buildings... it becomes more critical to follow such codes on wire size because of the value of the loss and the possible loss of life.



In general... I use a chart from the internet to get a wire size based on load. Then I go up one size. Works well.

 

[K5IP]

May I chime in here.

They main reason we have such large wires connecting our two batteries is to eliminate excessive voltage drop. When a starter motor draws, as ours do, 450 to 700

amps, any voltage drop, caused by resistance in the cables causes an imbalance resulting in the battery closest to the starter providing most of the power to the load.

Very large (ie. low resistance cables) tend to equalize the load to both batteries.

At lower current levels, charging and lighter loads, the cable size is not signifigant.

The cables we have are capable of currents far in excess of anything they will ever see, as far as load capacity is concerned. Insulation is also unimportant in this (intermittant) configuration since temperature will never be a constraint. Insulation here is mostly a matter of eliminating short circuits due to mechanical wear.

To explain further, the 1/0 battery cables have resistance (over 5 ft of about 5x10-4 ohms) (Times wire data sheet) This results in an imbalance by ohms law, (E=IR) of about . 2 vdc between batteries during staring.

A . 2Vdc difference does not sound like much, but it does result in one battery (assuming they are identical) sourcing a significantly larger part of the starting load. o

Yes, they are both primary batteries, but one is slightly more dominant than the other under starting conditions.

Talk about mudding the water!

Rog

 

[rbattelle]

In general... I use a chart from the internet to get a wire size based on load. Then I go up one size. Works well.

When it comes to battery or jumper cables I do the same thing: I buy the biggest cable I can afford that is at least 1 size bigger than the maximum amperage I intend to carry. With smaller wires (12ga through 22ga) I tend to "push" the ampacity a little and buy wiring with higher quality insulation (SXL cross-linked, for example, instead of PVC) and use the smallest gauge I feel I can get away with.



I think the length of the wire comes into play somehow too, although I don't fully understand how. Some ampacity charts (like the one here ) rate ampacity by length as well as size. That chart indicates, for example, that a 3/0 welding cable can safely carry 500 amps 50 ft. At 200 ft that same cable is only rated to carry 150 amp. It stands to reason, then, that at 10 ft it will carry around 950 amp and at 5 ft around 1100 amp.



-Ryan

 

[Willys]

Ok, this pretty well clears up most of my questions. Thanks for the responses.



But... K5IP, you've brought up another point. Ok, I understand that the larger positive wire(2/0) between the batteries is to prevent a difference in voltage between the batteries during startup, and that the startup current draw would be short and not likely to create heat problems within that short time frame(less than 30 seconds). But suppose I added something to either battery, that could create a heavy draw(for an extended period). Say, a winch? From what I've read a winch doesn't draw quite as much current as our ginormous starters, but I could be wrong. At any rate, a winch can pull quite a load. So is the 2/0 cable going to be sized large enough without a potential for overheating the insulation/copper? Is this a situation where upgrading to a better insulation, higher strand count, or larger gauge is warranted? While we're discussing winches, which battery would be best to install a winch cable to?



BTW, the winch in question is a Warn 8274...

 

[K5IP]

Ryan,

Again, voltage drop, not cuurrent capacity is the limiting factor. In your case the #3 cable was rated for 500A at 50 ft. The same wire could be used at 250A for 100ft or 125A at 200 ft and still provide the same voltage drop between the welder and the work. (each time the cable length doubles, so does the voltage drop at a given current, E=IR). In welding, This voltage drop is important because of the voltage change, and resultant current change that occurs when an arc is first established. Lower drop-better regulation and easier welding.

 

[rbattelle]

Ryan,

Again, voltage drop, not cuurrent capacity is the limiting factor. In your case the #3 cable was rated for 500A at 50 ft. The same wire could be used at 250A for 100ft or 125A at 200 ft and still provide the same voltage drop between the welder and the work.

OH! Okay, I understand now. Thanks K5IP!



-Ryan