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

[**THE-RFI-EMI-GUY**]

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**]

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

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."

[**THE-RFI-EMI-GUY**]

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

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."

--

[elect...@electrician2.com]

TAble 310.16 of the National Electrical Code and applicable notes and
sections is where you find the ampacity of NM building cable. Although
NM is rated 90 degrees C. you are required to use NM at no greater than
the 60 degree C ampacity given in Table 310.16 (per section 334.80
copied below.) However the 90 degree C. ampacity given in Table 310.16
is allowed to be used for derating purposes.
Table 310.16 has derating factors for high ambient temperatures at the
bottom of the table. For example a No. 12 NM copper has a 90 degree C.
ampacity of 30 amperes. However it cannot be used at greater than the
60 degree C. ampacity rating of 25 amperes. But there is also one more
requirement given in 240.4(D) (also quoted below) that No. 12 be
protected at no greater than 20 amperes. So what does all this mean.
In short a No. 12 in an ambient of 132 degrees F to 140 degrees F. has
a derating factor of .71. Since .71 x 30 is greater than 20 amperes
this No.. 12 can be protected by a 20 ampere circuit breaker. However
for a continnous loads that lasts for 3 hours or more branch-circuit
conductors shall have an ampacity not less than the maximum load to be
served. Where a branch circuit supplies continuous loads or any
combination of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any adjustment
or correction factors, shall have an allowable ampacity not less than
the noncontinuous load plus 125 percent of the continuous load..

If you don't understand all this then hire a competent electrician ,
master electrician, or electrical engineer.

References:
334.80 Ampacity. The ampacity of Types NM, NMC, and
NMS cable shall be determined in accordance with 310.15.
The ampacity shall be in accordance with the 60°C (140°F)
conductor temperature rating. The 90°C (194°F) rating
shall be permitted to be used for ampacity derating purposes,
provided the final derated ampacity does not exceed
that for a 60°C (140°F) rated conductor. The ampacity of
Types NM, NMC, and NMS cable installed in cable tray
shall be determined in accordance with 392.11.
Where more than two NM cables containing two or
more current-carrying conductors are bundled together and
pass through wood framing that is to be fire- or draftstopped
using thermal insulation or sealing foam, the allowable
ampacity of each conductor shall be adjusted in
accordance with Table 310.15(B)(2)(a).

240.4 (D) Small Conductors. Unless specifically permitted in
240.4(E) or 240.4(G), the overcurrent protection shall not
exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG,
and 30 amperes for 10 AWG copper; or 15 amperes for
12 AWG and 25 amperes for 10 AWG aluminum and
copper-clad aluminum after any correction factors for
ambient temperature and number of conductors have
been applied.

210.19 Conductors - Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.

[**THE-RFI-EMI-GUY**]

elect...@electrician2.com wrote:

Thanks, it looks like I am safe, my #6 CU rated 75A (at 90C), derated x
.71 (at 140F/60C) is good for 53A (closely agreeing with 60C rating of
55A). In case my 22A total Jacuzzi and misc loads all run continuous (>3
hours, mult x 1.25) I will need at least 30A rated breaker, however I
will need to use a 40A for the disconnect breaker as that is minimum for
fitting the #6 CU conductor. The branches are further protected 15A each
at the subpanel.

[**THE-RFI-EMI-GUY**]

elect...@electrician2.com wrote:

Thanks, it looks like I am safe, my #6 CU rated 75A (at 90C), derated x
.71 (at 140F/60C) is good for 53A (closely agreeing with 60C rating of
55A). In case my 22A total Jacuzzi and misc loads all run continuous (>3
hours, mult x 1.25) I will need at least 30A rated breaker, however I
will need to use a 40A for the disconnect breaker as that is minimum for
fitting the #6 CU conductor. The branches are further protected 15A each
at the subpanel.

--

[**THE-RFI-EMI-GUY**]

gfre...@aol.com wrote:

On Sun, 14 May 2006 03:10:33 GMT, **THE-RFI-EMI-GUY**
<rhyo...@nettally.com> wrote:

  

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:
    

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.

[bum...@gmail.com]

Have you heard of Q?