Anybody ever use 250mcm or 350mcm or 500mcm cable?

softdown

4/0 welding weighs about .8/lbs per foot.
250 mcm weighs about 1 lb per foot. 
350 mcm weighs about 1.4 lbs per foot. Getting close to double the weight of 4/0.
500 mcm is pretty heavy......about 2 lbs per foot.

softdown

I would *think* that one might be able to use two 4/0s rather than...say....350 mcm. 350 mcm is rare. 

Vic

"Anybody ever use 250mcm or 350mcm or 500mcm cable?"

YES!!

Vic

softdown said:

I would *think* that one might be able to use two 4/0s rather than...say....350 mcm. 350 mcm is rare. 

The rarity of cables larger than 4/0  depends mostly on the application.

As has been noted here,   in the past:

Using parallel conductors that are relatively large is permitted in the NEC.
You will notice,  when looking at Ampacity tables,   that large parallel conductors will have a greater Amapcity per pound of Copper than does the same weight of copper in a single large cable,   probably due to a larger surface area for the parallel conductors (and therefore greater ability to lose heat resulting from voltage drop due to cable resistance*current flow).

FWIW,   Vic

BB.

One big "but" about using parallel cables for handling large currents...

If you have one connection/cable fail, then the rest of the current flows in the remaining conductors.

And another caveat that I have experienced multiple times in high end test equipment...

Many times, the test equipment sent (for example) +5 volts on 4-6 parallel wires (and another set for ground/return current). plus other signal cables using one large 24 conductor (for example) mating pairs of connectors.

What I saw happen was not a somewhat higher resistance current path (say a connector pin+socket connection) that failed.... But the "perfect" pin+socket+crimps+wiring run that had the lowest resistance... The lowest current path would take the most current (I=V/R) and would heat up the most (P=I*2R). Notice that heating goes with the square of the current... And I would pop open a piece of test gear and I would see the connectors "unzipping" from overheating pin+socket connection. First one at the end, then it got hot and caused the next one to fail, which got the next pin+socket hot then in failed... Until the last cables (good connections) had to fail because they were too small to carry the whole current by themselves.

What should have been done? First, the loads should not have been paralleled, but divided into sections (each less than the rated capacity of the cable+connector). Sometimes that cannot be done--Next, would have suggested longer wire run/smaller diameter cable--You want the resistance of the cable to be the "major" ballasting factor (i.e., if the connector went a little bit low resistance than the cable, it would not matter since the resistance of the cable is >> than connector resistance, or lack of resistance).

And/or more parallel connections... I used 1/1 = rated current for one cable. For two parallel cables, I used 1/1 + 1/2 = 1.5x single conductor rated current. For three, 1/1+1/2+1/3 and etc. for more parallel connections. I never did have any failures in my designs (that I know of from following that rule--And as you see, 1/N gets very small for lots of parallel paths, and why I suggest more than 3 parallel battery strings is not recommended by me--if at all avoidable).

Using heavier/higher rated contacts+cables is obviously another answer--But not always possible (costs, space, dealing with >0.5" diameter copper cable, etc.).

Remember that many times we are paralleling cable to lower voltage drop--And a single cable is "capable" of carrying the current without failure--But would have "excessive" voltage drop--Another issue.

-Bill

mcgivor

Running parallel conductors is often done for ease of pulling through conduit,  pulling 500 mcm through 90° is tough, one 50' pull I did had 4 × 500 mcm conductors feeding a distribution in a 6 inch conduit, there were 3 × 90's, it took a chain hoist to pull and 4 men to feed with lots of lube, it was like pulling rebar, in retrospect it would have been easier pulling parallel 250 mcm. 

Where parallel runs are used, it's good practice to check the terminals periodically, in industrial installations thermal imaging is used to check for hot spots as part of regular maintenance. In the solar world it is also good to check all terminals periodically, in leu of a thermal camera, a milli volt meter can be used to check for volt drop. Doing this I've noticed that occasionally a screw terminal needs to be re torqued, especially after the initial installation, this is most important when using aluminum. Some may be under the impression that because it's low voltage there is less potential for things to go wrong, but remember a welder uses ~24V to melt and fuse steel.

Ethan Brush

softdown said:

4/0 welding weighs about .8/lbs per foot.
250 mcm weighs about 1 lb per foot. 
350 mcm weighs about 1.4 lbs per foot. Getting close to double the weight of 4/0.
500 mcm is pretty heavy......about 2 lbs per foot.

As an electrician I use those sizes all the time.  Mostly I use aluminum though as it is lighter and 1/5 the cost.  The largest I have used is 750.   The ampacity really starts to tank when you get above 250-350 size so above that you start thinking parallel. 

softdown

Ethan Brush said:

softdown said:

4/0 welding weighs about .8/lbs per foot.
250 mcm weighs about 1 lb per foot. 
350 mcm weighs about 1.4 lbs per foot. Getting close to double the weight of 4/0.
500 mcm is pretty heavy......about 2 lbs per foot.

As an electrician I use those sizes all the time.  Mostly I use aluminum though as it is lighter and 1/5 the cost.  The largest I have used is 750.   The ampacity really starts to tank when you get above 250-350 size so above that you start thinking parallel. 

Al is poorly regarded in solar applications. I keep wondering what to do with about 500' of 4/0 Al.

250 Al  should be pretty close to the capacity of 4/0 Cu. The grid is carried on Al for numerous reasons. I would also suspect a Cu grid would sag horribly under heaviest load. 

mike95490

softdown said:

Al is poorly regarded in solar applications. ......

Which makes no sense at all, because proper terminations are "proper".   It could be conduit fill factor and needing copper for that, but the cable cares not about AC or DC, only if it's prepped/treated and terminated properly.

  I've got a thousand feet of #6 aluminum, for the cost and ease of pulling 3 cables uphill in 1.5" conduit (but it is an AC cable hooked to my inverter)

Dave Angelini

Pulling cables uphill Mike?  Downhill is much easier

mike95490

I know that, and knew it then, but the layout and topography wasn't well suited for the easy way.  When is it ever ?

Dave Angelini

I do not do things the hard way anymore. Getting too old/wise for that. Surgeons charge and promise too much also...
Are you done with your rehab?  My torn meniscus took 7 months but it is a success. 
Rain is coming!!

mike95490

Shoulder rehab is 12+ months. Really complicated joint, hips and knees are easy.  But I'm pretty much on schedule

Saggys

Yes, as an electrician we use this for large feeders. 750's rarely, usually use parallel cables as much easier to handle and terminate.