Electrical Engineering - Actual Ampacity Rating ROMEX SIMPull Type NM-B in a residential

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Actual Ampacity Rating ROMEX SIMPull Type NM-B in a residential

**THE-RFI-EMI-GUY**

2006-05-14, 12:21 am

Please help me keep from tearing my hair out.

I am installing a subpanel in an existing, small 22 by 8 foot (176 sqft)
addition to my home. The original wiring was not done to code and has
since been removed. I am dividing this area in half, one half will
enclose a Jacuzzi whirlpool tub adjacent to existing bathroom. The
subpanel will be located in the other dry partitioned space.

My loads are:

Jacuzzi motor 9.7A 115VAC 1116 VA
Jacuzzi heater 12A 115VAC 1380 VA
Small 5000 BTU A/C 5.2A 115 VAC 598 VA
Lighting 4.6A 115VAC (based on code 3VA*176') 529 VA
Branch circuit 12A 115 VAC (derated one branch 80%, this is a very
small area) 1380 VA

Assuming the above, I have calculated abou 5282 VA load, 22A per leg,
including addl, 25% for the Jacuzzi motor. I have not reduced by a
"demand factor" because conceivably all the above might be running,
although the AC and Jacuzzi heater will cycle. Are these good assumptions?

Because the distance across the house from the main disconnect/load
panel is 124 feet, allowing 3% (3.6V) voltage drop from main to subpanel
I have calculated that the conducters need to be 18,187 CMA which works
out to be just a bit too much for #8 CU, so I think I need to go with #6
CU. Is #6 CU the correct size?

Now is where I get fuzzy. Most of this cable needs to go through my
attic (in Florida) which gets quite hot, and will be insulated soon
(hence the push to get this cable run). The cable will lay on rafters
and will eventualy be buried in as much as R30 insulation. When reading
the Ampacity tables 310-13 and the correction facters, I am led to
beleive that I need at least a 75C rated #6 CU cable which would derate
to 43A. Off I go to Home Depot and Lowes, where I find a cable labeled
"Waterson/Hammock E18679 (UL) ROMEX SIMPull AWG 6 CU 3 with AWG 10
ground Type NM-B 600 volts".

Nowhere does this cable have any marking as to the temperature rating or
type of insulation, so off I go to the website where I find this
statement in a product sheet:

"Southwire's Romex ® SIMpull TM Type NM-B (nonmetallic-sheathed cable)
may be used for both exposed and concealed
work in normally dry locations at temperatures not to exceed 90°C (with
ampacity limited to that for 60°C conductors) as
specified in the National Electrical Code".

Does this mean that the cable is actually rated to 90C? Is it good to
55A under such an installation without derating? If I derate it, per
310-13 the numbers come out to 22A which pose a problem as my loads
exceed a 20A breaker size, and as a practical matter, I need at least a
40A breaker to accomodate a #6 CU conductor.

Given my loads, how would you do this?

Thanks

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

**THE-RFI-EMI-GUY**

2006-05-14, 12:21 am

**THE-RFI-EMI-GUY** wrote:

> Please help me keep from tearing my hair out.
>
> I am installing a subpanel in an existing, small 22 by 8 foot (176
> sqft) addition to my home. The original wiring was not done to code
> and has since been removed. I am dividing this area in half, one half
> will enclose a Jacuzzi whirlpool tub adjacent to existing bathroom.
> The subpanel will be located in the other dry partitioned space.
> My loads are:
>
> Jacuzzi motor 9.7A 115VAC 1116 VA
> Jacuzzi heater 12A 115VAC 1380 VA
> Small 5000 BTU A/C 5.2A 115 VAC 598 VA
> Lighting 4.6A 115VAC (based on code 3VA*176') 529 VA
> Branch circuit 12A 115 VAC (derated one branch 80%, this is a very
> small area) 1380 VA
>
> Assuming the above, I have calculated abou 5282 VA load, 22A per leg,
> including addl, 25% for the Jacuzzi motor. I have not reduced by a
> "demand factor" because conceivably all the above might be running,
> although the AC and Jacuzzi heater will cycle. Are these good
> assumptions?
>
> Because the distance across the house from the main disconnect/load
> panel is 124 feet, allowing 3% (3.6V) voltage drop from main to
> subpanel I have calculated that the conducters need to be 18,187 CMA
> which works out to be just a bit too much for #8 CU, so I think I need
> to go with #6 CU. Is #6 CU the correct size?
>
> Now is where I get fuzzy. Most of this cable needs to go through my
> attic (in Florida) which gets quite hot, and will be insulated soon
> (hence the push to get this cable run). The cable will lay on rafters
> and will eventualy be buried in as much as R30 insulation. When
> reading the Ampacity tables 310-13 and the correction facters, I am
> led to beleive that I need at least a 75C rated #6 CU cable which
> would derate to 43A. Off I go to Home Depot and Lowes, where I find a
> cable labeled "Waterson/Hammock E18679 (UL) ROMEX SIMPull AWG 6 CU 3
> with AWG 10 ground Type NM-B 600 volts".
> Nowhere does this cable have any marking as to the temperature rating
> or type of insulation, so off I go to the website where I find this
> statement in a product sheet:
>
> "Southwire's Romex ® SIMpull TM Type NM-B (nonmetallic-sheathed cable)
> may be used for both exposed and concealed
> work in normally dry locations at temperatures not to exceed 90°C
> (with ampacity limited to that for 60°C conductors) as
> specified in the National Electrical Code".
>
> Does this mean that the cable is actually rated to 90C? Is it good to
> 55A under such an installation without derating? If I derate it, per
> 310-13 the numbers come out to 22A which pose a problem as my loads
> exceed a 20A breaker size, and as a practical matter, I need at least
> a 40A breaker to accomodate a #6 CU conductor.
>
> Given my loads, how would you do this?
>
> Thanks
>
I forgot to add this info ffrom the manufacturer:

"CONSTRUCTION
Southwire's Romex® SIMpull TM Type NM-B cable is manufactured as 2, 3,
or 4 conductor cable, with a ground wire. Copper
conductors are annealed (soft) copper. Stranded conductors are
compressed stranded. Conductor insulation is 90°C-rated
polyvinyl chloride (PVC), nylon jacketed. Southwire's SIMpull TM
Designed for Easier Pulling, Resulting in Easier installation.
The cable jacket is color-coded for quick size identification; White -
14 AWG, Yellow - 12 AWG, Orange - 10 AWG, and Black -8
AWG and 6 AWG."

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

**THE-RFI-EMI-GUY**

2006-05-14, 1:21 am

**THE-RFI-EMI-GUY**

2006-05-14, 3:21 am

gfretwell@aol.com wrote:

>On Sun, 14 May 2006 03:10:33 GMT, **THE-RFI-EMI-GUY**
><rhyolite@nettally.com> wrote:
>
>
>
>NM-b uses a 90c conductor and you use the 90c column for derating.
>Since this is a spa your AHJ may want wire in pipe (insulated ground
>wire)./ Check before you buy anything.
>
>
Thanks; Its an indoor Jacuzzi, the manufacturer requires two 15A
receptacles one each for the pump and heater to be installed in the
enclosed base. Those will come from the subpanel in an adjacent room. I
will check local codes about pipe under the Jacuzzi.

Here in FL, romex is used for just about everything. My old 1947 house
had mettallic conduit throughout which I feel is safer. We rewired that
house simply by pulling the new wire in by pulling out the old (rubber)
wire.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

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