Electronics Forums > Current capacity of multi-strand versus single-strand

# Current capacity of multi-strand versus single-strand

Zarbol Tsar
Guest
Posts: n/a

 12-03-2004, 08:33 PM
Would a multi-strand flexible wire have the same current carrying
capacity as a solid single stranded cable of the same cross section.

For example ... a multi-strand with a cross section for its wire
portion of 0.75 mm squared and a single strand cable also of 0.75 mm
squared.

Palindrâ˜»me
Guest
Posts: n/a

 12-03-2004, 09:07 PM
Zarbol Tsar wrote:

> Would a multi-strand flexible wire have the same current carrying
> capacity as a solid single stranded cable of the same cross section.
>
> For example ... a multi-strand with a cross section for its wire
> portion of 0.75 mm squared and a single strand cable also of 0.75 mm
> squared.

Any particular frequency in mind?

Skin effect is the obvious thing that will make a stranded wire better
than a solid of the same csa. But this kicks in at higher frequencies.

Here is a brief simplified guide:

http://www.st-andrews.ac.uk/~www_pa/...ect/page1.html

However, if the actual csa of the solid wire is compared with actual csa
of the stranded wire (rather than the sum of the csa of individual
strands), then a solid wire will have more copper and hence lower
resistance. This is frequency independent.

So, at low frequencies or DC, a solid wire of the same core diameter
will be better than stranded. As frequency rises, a point will come
where stranded wire will be better; it will be able to take a higher
current than a solid core of the same diameter.

There are obviously exceptions.

Hope that is what you were after...

--

Sue

Tim Williams
Guest
Posts: n/a

 12-03-2004, 09:09 PM
Ya, until you get into higher frequencies where the slightly greater surface
area of the stranded wire represents more cross section. Probably above
10MHz...

Tim

--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

"Zarbol Tsar" <(E-Mail Removed)> wrote in message
news:95B4D128678B651D7E@130.133.1.4...
> Would a multi-strand flexible wire have the same current carrying
> capacity as a solid single stranded cable of the same cross section.
>
> For example ... a multi-strand with a cross section for its wire
> portion of 0.75 mm squared and a single strand cable also of 0.75 mm
> squared.

Andrew Gabriel
Guest
Posts: n/a

 12-04-2004, 12:18 AM
In article <QI4sd.429\$(E-Mail Removed)>,
"Tim Williams" <(E-Mail Removed)> writes:
> Ya, until you get into higher frequencies where the slightly greater surface
> area of the stranded wire represents more cross section. Probably above
> 10MHz...

Skin effect is easily measurable at a few tens of kHz.
It comes into effect at regular mains frequences for very large
conductors. An alternative to making them stranded is to pick
a different shape which avoids significant depth of metal,
such as a flat strip (thin rectangular profile), or tubular.

--
Andrew Gabriel
Consultant Software Engineer

John Popelish
Guest
Posts: n/a

 12-04-2004, 12:40 AM
Zarbol Tsar wrote:
>
> Would a multi-strand flexible wire have the same current carrying
> capacity as a solid single stranded cable of the same cross section.
>
> For example ... a multi-strand with a cross section for its wire
> portion of 0.75 mm squared and a single strand cable also of 0.75 mm
> squared.

In most cases, they are exactly the same. When the frequency gets
high enough that skin effect starts to crowd the current toward the
surface, stranded wire has a slight advantage, but this is greatly
enhanced if all the strands are insulated from each other (look up
litz wire). This effect can also be used ot advantage when winding
magnetic devices with enameled wire, and two or more parallel strands
can be used in place of an equivalent cross section single wire. But
for ordinary hook up wire, they are usually assumed to have an
ampacity based on their cross sectional area, not the number of
strands.
--
John Popelish

Gerald Newton3
Guest
Posts: n/a

 12-04-2004, 04:04 AM

"Zarbol Tsar" <(E-Mail Removed)> wrote in message
news:95B4D128678B651D7E@130.133.1.4...
> Would a multi-strand flexible wire have the same current carrying
> capacity as a solid single stranded cable of the same cross section.
>
> For example ... a multi-strand with a cross section for its wire
> portion of 0.75 mm squared and a single strand cable also of 0.75 mm
> squared.

A No, 12 AWG wire has a diameter of 0.78 mm.

For wire sizes 2.47 mm and under (No. 2 AWG and under) the DC resistance
nearly equals AC resistance. Therefore the ampacity for stranded conductors
is nearly equal to that for solid conductors for these small conductors
(also demonstrated using the N-M equations.) For building wire types,
conductors size No. 8 AWG and larger that are installed in raceways are
required to be stranded (NEC Section 310.3.) Therefore, we seldom see
conductors larger than No. 8 that are solid, except for the No. 4 bare
grounding conductors carried on line trucks and sold for services by supply
houses that are used as grounding electrode conductors so that no additional
protection is required. The standard ampacity table in the NEC used for
building wires, 310.16, does not distinguish between solid and stranded
conductors. This is probably because almost all building wire No. 8 and
larger is stranded.
The N-M equations also do not distinguish between solid and stranded, but
uses DC resistance multiplied by (1+YC) where YC becomes measurable for
wire sizes above No. 2 because of skin effect.

Gerald Newton3
Guest
Posts: n/a

 12-04-2004, 12:13 PM

"Zarbol Tsar" <(E-Mail Removed)> wrote in message
news:95B4D128678B651D7E@130.133.1.4...
> Would a multi-strand flexible wire have the same current carrying
> capacity as a solid single stranded cable of the same cross section.
>
> For example ... a multi-strand with a cross section for its wire
> portion of 0.75 mm squared and a single strand cable also of 0.75 mm
> squared.

For No. 12 AWG Table 8 of the NEC does list two different DC resistances for
stranded and solid copper.

DC resistance per 1000 feet for solid is 1.93 ohms

For stranded the DC resistance is given as 1.98 ohms per 1000 feet.

So solid should have a slightly higher ampacity.

If we use Table 310.16 of the NEC to determine RCA and substitute into the
Ampere calculation we can find the approximate differences in ampacity.

From Table 310.16 using 75 degrees C as the ambient.

I = 25 amperes, TC = 75 degrees C, and TA = 30 degrees C and RDC = 1.98 ohms
per 1000 feet or 0.00198 ohms per foot.

This converts to 1980 microhms.

From I (in kiloamperes) = SQRT(( TC-TA)/RDC*RCA))

Or

RCA=(TC-TA)/RDC*I*I

Or RCA = (75-30)/1980*0.025*0.025

RCA = 36 thermal ohm feet

For stranded, I = 0.025 kiloamperes from the table

For solid No. 12 copper

I (in kiloamperes) = SQRT ((75-30)/1930*36)

or I = 0.0254 kiloamperes

Then the solid No. 12 copper would have a 0 .4 ampere increase in ampacity.

This is a 0.4/25 *100 or only a 1.6 per cent increase.

Considering that ampacity tables are approximations, this increase in
ampacity does not exceed the error of approximation.

Tom Biasi
Guest
Posts: n/a

 12-04-2004, 12:30 PM

"Zarbol Tsar" <(E-Mail Removed)> wrote in message
news:95B4D128678B651D7E@130.133.1.4...
> Would a multi-strand flexible wire have the same current carrying
> capacity as a solid single stranded cable of the same cross section.
>
> For example ... a multi-strand with a cross section for its wire
> portion of 0.75 mm squared and a single strand cable also of 0.75 mm
> squared.

If indeed the conducting areas are the same the capacity will be the same
for all practical purposes.
The AWG (American Wire Gauge) numbers take into account the actual total
cross-sectional area.
Some charts will compare stranded vs. solid for actual wire outer diameter
though.
If you start talking about impedance in some applications the story will
change.
Regards,
Tom

Rowbotth
Guest
Posts: n/a

 12-05-2004, 02:17 AM
As far as I am aware, the amount of copper being the same should mean
the same current flow - but the more strands should mean it is easier to
work with, albeit more expensive?

H.

In article <41b1a9de\$(E-Mail Removed)>,
"Gerald Newton3" <(E-Mail Removed)> wrote:

> "Zarbol Tsar" <(E-Mail Removed)> wrote in message
> news:95B4D128678B651D7E@130.133.1.4...
> > Would a multi-strand flexible wire have the same current carrying
> > capacity as a solid single stranded cable of the same cross section.
> >
> > For example ... a multi-strand with a cross section for its wire
> > portion of 0.75 mm squared and a single strand cable also of 0.75 mm
> > squared.

>
> For No. 12 AWG Table 8 of the NEC does list two different DC resistances for
> stranded and solid copper.
>
> DC resistance per 1000 feet for solid is 1.93 ohms
>
> For stranded the DC resistance is given as 1.98 ohms per 1000 feet.
>
> So solid should have a slightly higher ampacity.
>
> If we use Table 310.16 of the NEC to determine RCA and substitute into the
> Ampere calculation we can find the approximate differences in ampacity.
>
> From Table 310.16 using 75 degrees C as the ambient.
>
> I = 25 amperes, TC = 75 degrees C, and TA = 30 degrees C and RDC = 1.98 ohms
> per 1000 feet or 0.00198 ohms per foot.
>
> This converts to 1980 microhms.
>
> From I (in kiloamperes) = SQRT(( TC-TA)/RDC*RCA))
>
> Or
>
> RCA=(TC-TA)/RDC*I*I
>
> Or RCA = (75-30)/1980*0.025*0.025
>
> RCA = 36 thermal ohm feet
>
> For stranded, I = 0.025 kiloamperes from the table
>
> For solid No. 12 copper
>
> I (in kiloamperes) = SQRT ((75-30)/1930*36)
>
> or I = 0.0254 kiloamperes
>
> Then the solid No. 12 copper would have a 0 .4 ampere increase in ampacity.
>
> This is a 0.4/25 *100 or only a 1.6 per cent increase.
>
> Considering that ampacity tables are approximations, this increase in
> ampacity does not exceed the error of approximation.

Gerald Newton3
Guest
Posts: n/a

 12-05-2004, 02:54 AM

"Gerald Newton3" <(E-Mail Removed)> wrote in message
news:41b1a9de\$(E-Mail Removed)...
>
> "Zarbol Tsar" <(E-Mail Removed)> wrote in message
> news:95B4D128678B651D7E@130.133.1.4...
>> Would a multi-strand flexible wire have the same current carrying
>> capacity as a solid single stranded cable of the same cross section.
>>
>> For example ... a multi-strand with a cross section for its wire
>> portion of 0.75 mm squared and a single strand cable also of 0.75 mm
>> squared.

>
> For No. 12 AWG Table 8 of the NEC does list two different DC resistances
> for stranded and solid copper.
>
> DC resistance per 1000 feet for solid is 1.93 ohms
>
> For stranded the DC resistance is given as 1.98 ohms per 1000 feet.
>
> So solid should have a slightly higher ampacity.
>
> If we use Table 310.16 of the NEC to determine RCA and substitute into the
> Ampere calculation we can find the approximate differences in ampacity.
>
> From Table 310.16 using 75 degrees C as the ambient.
>
> I = 25 amperes, TC = 75 degrees C, and TA = 30 degrees C and RDC = 1.98
> ohms per 1000 feet or 0.00198 ohms per foot.
>
> This converts to 1980 microhms.
>
> From I (in kiloamperes) = SQRT(( TC-TA)/RDC*RCA))
>
> Or
>
> RCA=(TC-TA)/RDC*I*I
>
> Or RCA = (75-30)/1980*0.025*0.025
>
> RCA = 36 thermal ohm feet
>
> For stranded, I = 0.025 kiloamperes from the table
>
> For solid No. 12 copper
>
> I (in kiloamperes) = SQRT ((75-30)/1930*36)
>
> or I = 0.0254 kiloamperes
>
> Then the solid No. 12 copper would have a 0 .4 ampere increase in
> ampacity.
>
> This is a 0.4/25 *100 or only a 1.6 per cent increase.
>
> Considering that ampacity tables are approximations, this increase in
> ampacity does not exceed the error of approximation.
>

I just reviewed the Samuel Rosch Paper from 1938, "The Current Carrying
Capacity of Rubber-Insulated Conductors."
He used a slightly different formula where N represented the number of
current carrying conductors. Since Table 310.16 is for three current
carrying conductors in a raceway to find RCA the following should be used:

I (in kiloamperes) = SQRT(( TC-TA)/N*RDC*RCA))
RCA=(TC-TA)/N*RDC*I*I
RCA = (75-30)/3*1980*0.025*0.025
RCA= 12 Thermal Ohm Feet

Then for solid No. 12 AWG copper:
I (in kiloamperes) = SQRT ((75-30)/3*1930*12)
I = 0.0254 kiloamperes

And there is no difference in the answer.

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