Also, the number of wires by size in conduit should help the cost equation
also. 3 ea #6 conductor requires 1" conduit, while 3 ea, #12 can be put in
1/2" conduit.
Mel, K6KBE
From: Lewis Phelps <l...@n6lew.us>
To: Elecraft Reflector Reflector <elecraft@mailman.qth.net>
Sent: Thursday, August 11, 2016 12:03 PM
Subject: Re: [Elecraft] [OT] NEC wire size requirements (was: Zip Wire
Misrepresentation?)
I’ve changed the subject line because this discussion has strayed from the
original question.
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
> On Aug 11, 2016, at 11:12 AM, Kevin - K4VD <ke...@k4vd.net> wrote:
>
> The added cost of a 3x over-build seems like it would be awfully high. Is
> this good engineering practice (seriously, I don't know)?
>
> Kevin K4VD
>
> On Thu, Aug 11, 2016 at 12:38 PM, Clay Autery <caut...@montac.com
> <mailto:caut...@montac.com>> wrote:
>
>> How do you define "necessary".
>>
>> Paraphrasing K9YC, Jim.... "Big wire is your friend".
>>
>> I agree...
>>
>> I typically use wire at least 2 sizes larger than "required".... Often
>> the wire size is determined by how big of a wire I can FIT in the
>> application and how much money I can part with at the time.
>>
>> For instance, on my current power project... I am running 6 AWG x 4 from
>> the service to the sub-panel @ 50 foot run for a 60 Amp separately
>> derived service where there'll never be more than about a 20 Amp total
>> demand...
>>
>> From the sub-panel to the receptacles about 3-1/2 feet below, I am using
>> 10 AWG.... only because that's the largest wire the receptacle
>> terminals are rated for.
>>
>> From the receptacles to all equipment, minimum 10 AWG...
>>
>> Bigger and shorter the wire, the better...
>>
>> ______________________
>> Clay Autery, KY5G
>> MONTAC Enterprises
>> (318) 518-1389
>>
>> On 8/11/2016 10:43 AM, Ronnie Hull wrote:
>>> Is 10 gauge necessary?
>>>
>>> Sent from my iPhone
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Lew Phelps N6LEW
Pasadena, CA DM04wd
Elecraft K3-10 / KXV144 / XV432
Yaesu FT-7800
l...@n6lew.us
www.n6lew.us
Generalized Law of Entropy: Sooner or later, everything that has been put
together will fall apart.
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