[Elecraft] [OT] NEC wire size requirements

[Clay Autery]

You make a lot of assumptions there:

1) It is not a 50 foot run carrying 120...  It's a 50 foot 4 wire run
carrying 240 volts to the sub-panel from the service plus the neutral
and the green wire. Then a 6-gang box containing 8 x 120 and 4 x 220
receptacles wired independently.
2) IF some day I NEED 60 Amps, I will be able to get it because I BUILT
for it...  No climbing back into the attic.  No buying all new wire.  No
pulling new wire.
3) I don't pay retail.
4) There are other considerations other than cost and electrical....
there's RF radiation from and to the wire, etc.  Which is why the wire
is BOTH hand-twisted into pairs AND run in bonded flex-steel conduit.

IF I built for 20 amps now, and then later needed 20+ amps, I would have
to REBUILD...  my time is worth more than the cost differential.  I want
to TALK on the radio and build OTHER stuff... NOT do repeated upgrades
of my power infrastructure.

Bottom Line:  My stuff works and doesn't break.  I don't have to use my
time to do stuff twice.  And because I want to...  :-)

______________________
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389

On 8/11/2016 2:03 PM, Lewis Phelps wrote:
> At retail rates (e.g. price per foot from lower.com) #6 wire is 89 cents per foot, and #12 wire is 8.2 cents per foot;  assuming Clay’s 50 foot run and 3 wires for a 110 VAC circuit with ground, per NEC, the added cost for wire would be $121.20.  
>
> Is it “good engineering practice?”  It seems to be to be OK from an electrical standpoint, albeit unnecessary, and unnecessary from a cost standpoint, albeit not harmful. 
>
>  I can certainly understand “over-specifying” wire size in a 12 volt circuit, and using larger wire size than is required simply from considering ampacity,  because the voltage drop is a much larger change proportionally, but I really don’t see the benefit from the expense and added installation difficulty of using larger-than-required wire for a 120VAC supply circuit.
>
> according to the online calculator at http://www.southwire.com/support/voltage-drop-calculator.htm, which takes into account both resistance and reactance of the wire:
>
> — for a 50 foot run of cable of #6 wire, at 20 amps and 120 volts AC single phase,  the total voltage drop will be 0.884 volt, or 0.74%, for a net voltage at the end of the circuit of 119.1 volts (rounding)
> — for the same run with #12 wire, the total voltage drop will be 3.472 volts, or 2.90 percent, for a net voltage at the end of the circuit of 116.5 volts (rounding). 
>
> The 3.47 volt drop would be intolerable in a 12 volt circuit providing power directly to amateur radio equipment, but seems to me irrelevant if feeding a competently-designed power supply that reduces the 120 volts AC  supply to some lower voltage of DC supply. Any ham radio power supply that is specified to operate on 120 VAC should be able to operate without difficulty from a 116.5 volts supply.
>
> So, why go to the extra expense of #6 wire? While it seems to me to be to be harmless to “over-spec” the wire size, it also seems expensive and not necessary either per requirements of Section of 310-15 of the NEC or from a “good operating practices” perspective. 
>
> Lew N6LEW