Electrician questions

[Rich Webb]

On Wed, 30 Mar 2011 19:27:23 -0400, ehsjr <ehsjr@nospamverizon.net>
wrote:

default wrote:
On Mon, 28 Mar 2011 22:29:35 -0400, ehsjr <ehsjr@nospamverizon.net
wrote:


The rule of thumb is 3% voltage drop. With 14 AWG that would be about
17 amps, so 12 AWG is already one size larger.

Not quite. #14 wire is 3.14 ohms per 1000 feet. The computation:
17 * 3.14/1000 * 160 = 8.5408 volts; 8.5408/120 = .0711 or a bit
over 7%

Using #12 wire at 1.98 ohms per 1000 feet:
17* 1.98/1000 * 160 = 5.3856 volts; 5.3856/120 = .0448 or
4.48%.

My guess is that you used 80 feet in the calculation instead
of 160. You have to use 160 because there are two wires in
the 80 foot run.


That is exactly what I did. Thanks for pointing out the error.

Guess why I am familiar with that particular error. :-(


My ARRL handbook puts 14 AWG at 2.575 ohms per thousand

and 12 at 1.619 ohms/1000.

There are several different wire tables floating around, so
it's not unusual to find different values for the same wire.
For house wiring, I figure it's best to use the table in the
NEC (National Electrical Code). The table in the ARRL book
may be more accurate - I don't know.

A great thumb rule that I wish I'd seen (or noticed) long ago:
Start from the easy to remember value for AWG 10 at 1 ohm per 1000 feet,
the other gauges are all sized in dB. AWG 20 is 10/1000, 24 is 25/1000,
16 is 4/1000, 0 is 0.1/1000 and so on. The "real" numbers vary by
manufacturer and specific part numbers, of course, but the thumb rule
comes in handy for estimating.

--
Rich Webb Norfolk, VA

 

On Wed, 30 Mar 2011 20:05:52 -0400, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

ehsjr wrote:

default wrote:
On Mon, 28 Mar 2011 22:29:35 -0400, ehsjr <ehsjr@nospamverizon.net
wrote:


The rule of thumb is 3% voltage drop. With 14 AWG that would be about
17 amps, so 12 AWG is already one size larger.

Not quite. #14 wire is 3.14 ohms per 1000 feet. The computation:
17 * 3.14/1000 * 160 = 8.5408 volts; 8.5408/120 = .0711 or a bit
over 7%

Using #12 wire at 1.98 ohms per 1000 feet:
17* 1.98/1000 * 160 = 5.3856 volts; 5.3856/120 = .0448 or
4.48%.

My guess is that you used 80 feet in the calculation instead
of 160. You have to use 160 because there are two wires in
the 80 foot run.


That is exactly what I did. Thanks for pointing out the error.

Guess why I am familiar with that particular error. :-(


My ARRL handbook puts 14 AWG at 2.575 ohms per thousand

and 12 at 1.619 ohms/1000.

There are several different wire tables floating around, so
it's not unusual to find different values for the same wire.
For house wiring, I figure it's best to use the table in the
NEC (National Electrical Code). The table in the ARRL book
may be more accurate - I don't know.


The ARRL table is for winding transformers.

That's what I thought too, the actual ampacity for wire in air would
be higher - and the NEC is probably more concerned with wire runs in
hot attics with insulation and lots of dry combustibles around.

Not much of a concern when the wire is underground maybe.

 

[Don Klipstein]

In article <bpl7p6l4k2upu13ad1jnak70pd0595b1is@4ax.com>, default wrote:

On Wed, 30 Mar 2011 20:05:52 -0400, "Michael A. Terrell"
mike.terrell@earthlink.net> wrote:

<SNIP a lot to edit for space>

The ARRL table is for winding transformers.

That's what I thought too, the actual ampacity for wire in air would
be higher - and the NEC is probably more concerned with wire runs in
hot attics with insulation and lots of dry combustibles around.

Not much of a concern when the wire is underground maybe.

As far as I know, NEC mentions 2 different figures for copper wire
of each wire guage, likely with one of these figures varying with
temperature and insulation type.

The one figure which varies with temperature and insulation type is
ampacity. The other is maximum allowable overcurrent protection.
--
- Don Klipstein (don@misty.com)

 

[amdx]

"Rich Webb" <bbew.ar@mapson.nozirev.ten> wrote in message
news:slh7p6hiinocev870l5sp5foi8t4p563hp@4ax.com...

On Wed, 30 Mar 2011 19:27:23 -0400, ehsjr <ehsjr@nospamverizon.net
wrote:

default wrote:
On Mon, 28 Mar 2011 22:29:35 -0400, ehsjr <ehsjr@nospamverizon.net
wrote:


The rule of thumb is 3% voltage drop. With 14 AWG that would be about
17 amps, so 12 AWG is already one size larger.

Not quite. #14 wire is 3.14 ohms per 1000 feet. The computation:
17 * 3.14/1000 * 160 = 8.5408 volts; 8.5408/120 = .0711 or a bit
over 7%

Using #12 wire at 1.98 ohms per 1000 feet:
17* 1.98/1000 * 160 = 5.3856 volts; 5.3856/120 = .0448 or
4.48%.

My guess is that you used 80 feet in the calculation instead
of 160. You have to use 160 because there are two wires in
the 80 foot run.


That is exactly what I did. Thanks for pointing out the error.

Guess why I am familiar with that particular error. :-(


My ARRL handbook puts 14 AWG at 2.575 ohms per thousand

and 12 at 1.619 ohms/1000.

There are several different wire tables floating around, so
it's not unusual to find different values for the same wire.
For house wiring, I figure it's best to use the table in the
NEC (National Electrical Code). The table in the ARRL book
may be more accurate - I don't know.

A great thumb rule that I wish I'd seen (or noticed) long ago:
Start from the easy to remember value for AWG 10 at 1 ohm per 1000 feet,
the other gauges are all sized in dB. AWG 20 is 10/1000, 24 is 25/1000,
16 is 4/1000, 0 is 0.1/1000 and so on. The "real" numbers vary by
manufacturer and specific part numbers, of course, but the thumb rule
comes in handy for estimating.

I recently made a 0.001 ohm resistor using 10 gauge house wire.

I thought I would need 12" to get the proper resistance, it was closer
to 10" than the 12" I expected.
Calibration was good at 75 degrees F.
Mikek

 

[George Herold]

On Mar 30, 8:43 pm, Rich Webb <bbew...@mapson.nozirev.ten> wrote:

On Wed, 30 Mar 2011 19:27:23 -0400, ehsjr <eh...@nospamverizon.net
wrote:





default wrote:
On Mon, 28 Mar 2011 22:29:35 -0400, ehsjr <eh...@nospamverizon.net
wrote:

The rule of thumb is 3% voltage drop.  With 14 AWG that would be about
17 amps, so 12 AWG is already one size larger.

Not quite. #14 wire is 3.14 ohms per 1000 feet. The computation:
17 * 3.14/1000 * 160 = 8.5408 volts; 8.5408/120 = .0711 or a bit
over 7%

Using #12 wire at 1.98 ohms per 1000 feet:
17* 1.98/1000 * 160 = 5.3856 volts; 5.3856/120 = .0448 or
4.48%.

My guess is that you used 80 feet in the calculation instead
of 160. You have to use 160 because there are two wires in
the 80 foot run.

That is exactly what I did.  Thanks for pointing out the error.

Guess why I am familiar with that particular error. :-(

My ARRL handbook puts 14 AWG at 2.575 ohms per thousand

and 12 at 1.619 ohms/1000.

There are several different wire tables floating around, so
it's not unusual to find different values for the same wire.
For house wiring, I figure it's best to use the table in the
NEC (National Electrical Code).  The table in the ARRL book
may be more accurate - I don't know.

A great thumb rule that I wish I'd seen (or noticed) long ago:
Start from the easy to remember value for AWG 10 at 1 ohm per 1000 feet,
the other gauges are all sized in dB. AWG 20 is 10/1000, 24 is 25/1000,
16 is 4/1000, 0 is 0.1/1000 and so on. The "real" numbers vary by
manufacturer and specific part numbers, of course, but the thumb rule
comes in handy for estimating.

--
Rich Webb     Norfolk, VA- Hide quoted text -

- Show quoted text -

That's great! I never noticed.

Thanks Rich.

George H.

 

[m II]

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On 11-03-30 06:43 PM, Rich Webb wrote:

A great thumb rule that I wish I'd seen (or noticed) long ago:
Start from the easy to remember value for AWG 10 at 1 ohm per 1000 feet,
the other gauges are all sized in dB. AWG 20 is 10/1000, 24 is 25/1000,
16 is 4/1000, 0 is 0.1/1000 and so on. The "real" numbers vary by
manufacturer and specific part numbers, of course, but the thumb rule
comes in handy for estimating.

A broader explanation may be found here:

http://en.wikipedia.org/wiki/American_wire_gauge


mike





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