Reminder: You have to consider BOTH voltage drop and ampacity when sizing wire.

[Dzl]

A misunderstanding I see waaaay too often, by newbies and non-newbies alike (even some very seasoned/experienced folks) is not understanding that they must take into consideration both Ampacity (current carrying capacity of the wire) AND Voltage drop when sizing wire. Too often I see people giving advice where they only mention, voltage drop, or refer people to voltage drop calculators without warning/acknowledging that Ampacity is equally if not more important (a safety concern). And that voltage drop will depend on length of the wire among other things, while ampacity does not depend on distance, and is affected by variables like insulation temperature rating and ambient temperature.

I'm not an expert on either voltage drop or wire ampacity (or anything else for that matter ?), so I hope i'm not misrepresenting anything, but the oversimplified explanation:

Voltage Drop = the drop in voltage across a conductor resulting from the resistance in the circuit. It can be expressed as an absolute value xx volts or a %. The formula = Vdrop = Current * Resistance. We care about it because it (1) is an inefficiency (2) can lead to mismeasurement or in some cases problems if its too high.

Amapacity = the safe current carrying capacity of a conductor. Different variables affect this, including current, wire insulation temperature rating, ambient temperature, and so on.

 

[Diysolar123]

voltage drop represents the lost power on the wire, or wasted energy...for solar given how inefficient things are already, this matters a lot as its a direct driver on your long term ROI or system sizing.

I "think" what you are trying to bring up is that wires have current limits that must always be respected.
Typically this is written on the cable and is the safe limit of the wire before it is in danger of actually degrading. Much of the NEC code is focused on this; the point is to make sure nothing lets out the magic smoke and starts a fire. This is why there are many rules in the NEC code about multiple wires in confined areas, the wires MUST be able to radiate heat and bundles do not do that well...
Ampacity limits are about NOT letting your wiring get to hot so the insulation melts...this is why you see different wire types such as the easy to use low end NM-b cable, THHN "good stuff", and the "really good stuff" XHHW-2.

If you run a hot air blower for a while in your house you will notice that the wires WILL get quite hot, this is ok as the NEC is worried about dangerous failures(melting insulation) and not efficiencies
fyi, the NEC does revisit the rules ever few years so what was ok 10yrs ago could be a violation today!!

You should never ever, not ever...approach the current capacity limits of a cable!!!
It is not safe, also, it is NOT a good design for any system where you are trying to harvest energy...heat is waste...

 

[Dzl]

I "think" what you are trying to bring up is that wires have current limits that must always be respected.

Precisely!

Typically this is written on the cable and is the safe limit of the wire before it is in danger of actually degrading. Much of the NEC code is focused on this; the point is to make sure nothing lets out the magic smoke and starts a fire. This is why there are many rules in the NEC code about multiple wires in confined areas, the wires MUST be able to radiate heat and bundles do not do that well..

The NEC rules on ampacity can be quite difficult for a beginner (or non-beginner) to wrap their head around as they are somewhat complicated and sometimes hard to understand or interpret and not all found in one place (this is why most people hire professionals for electrical work).

But the main broad points to be aware of are to know that there are safe limits to what a wire can carry that are separate from any voltage drop calculations, when in doubt oversize (and of course is properly sized overcurrent protection), and know that various factors can effect/change the ampacity of a wire (ambient temp for instance, or whether the wire is in free air or bundled in conduit, or on a hot roof), and this last bit is where all the NEC multipliers come into play.

Ampacity limits are about NOT letting your wiring get to hot so the insulation melts...

Precisely (again)! Which is why I think its such an important point to underscore. Its not uncommon to see the person giving the advice on wire sizing to consider the former (voltage drop) but not understand or consider the latter (which is by far the bigger issue from a safety perspective). That is what inspired this post.