[TowerTalk] ampacity, overcurrent protection, etc.

[Jim Lux]

Folks..

The rules in the NEC for conductor sizing and overcurrent protection are 
sort of complex, and one does not necessarily depend on the other.  For 
code fiends, 240-3 has the OCP rules. Once you get away from conventional 
residential construction, the rules allow all sorts of things, assuming 
there's a rational basis for what you want to do.

Conductors get sized based on
1) Expected loads
2) Permissible Voltage Drop
3) Temperature ratings

You can use a smaller wire for a given current, if it's rated for the 
temperature it will rise to in use, for instance.  (Last 10 years have 
added requirements that the devices the wire is connected to has to also be 
rated for the higher temperature).

Don't discount the possibility that the heating limit for wires varies 
depending on the environment.  Jamming 10 wires in a conduit will result in 
a lower ampacity rating than single wires strung in open air.

Overcurrent Protection is based on:
1) Expected size and type of load  (in general, 125% of continuous+100% of 
noncontinuous load)
2) Type of wiring and its ampacity

More specifically: "
Overcurrent protection for conductors and equipment is provided to open the 
circuit if the current reaches a value that will cause an excessive or 
dangerous temperature in conductors or conductor insulation.
"

So, if you can make a case that your AWG 20 wire won't be excessively or 
dangerously heated by that 400 service, you can protect it 
accordingly.  Maybe you've got your wires cooled by liquid nitrogen (which 
would also help with the voltage drop problem).

There's a whole raft of exceptions to the ampacity/OCP correlation..
For instance:
Motor circuits
Phase Converters
Devices rated 800A or less (just in case you were running that 100kW 
transmitter)
transformer secondary conductors
etc.

Article 225 covers the rules for "outside branch circuits and feeders" 
which has different rules than normal interior wiring.

Then, there's probably a raft of special stuff in the 500s  Certainly, Art 
530 (Motion picture and Television Studios) has all sorts of weird stuff 
permitted.

Finally, your local AHJ (Authority Having Jurisdiction) might allow all 
sorts of odd things for certain uses (i.e. farm buildings).