wisniewski
New member
Can you explain to me why the ICEA 440 and NEC power cable ampacities in
trays are so different. For example the NEC LV small power cable
ampacity , with the tray filled to 42 sq. inches (36 inch tray) is so much
greater than the ICEA power cable ampacity. I am very familiar with the
Stolpe method and accounting for the different fills cannot explain it, by
a longshot. I am beginning to believe that ICEA is way too conservative.
The NEMA standard dealing with the cable trays is ICEA P-54-440/WC 51.
Let me provide an example by determining the ampacity of #2 AWG , 90C,
0-600V class, 1.13 inches OD - 3/c cables in an open ladder bottom 36 inch
tray in a 40 C ambient. All cables are of the 3/c type with an overall
jacket .
NEC ampacity (2002 Code)
Per 392.9 A(2) and table 392.9 the maximum allowable fill area for
multiconductor cables in a 36 inch open ladder bottom tray is 42 sq.
inches. Considering that my 3/c cable has OD of 1.13 inches (area of 1.003
sq. inches) we can place 41 such cables in the tray.
The ampacity , according to Section 392.11 (A) is given in Table 310.16.
The ampacity at a 30C ambient is 130 amperes. Adjusting for a 40C ambient
using correction factors from Table 310.16 the ampacity is 118 amperes.
NEMA WC 51, ICEA P-54-440
This standard identifies the cable ampacities depending on a depth of fill.
Assuming 41 cables as established above this gives us a "NEMA depth of
fill" (section 2.2 of the NEMA std.) of 1.45 inches. Table 3.6 indicates the
ampacity of 85 amperes for the same cable at 1.5 inches depth of fill. The
same 40C ambient.
The difference is clear and I have a hard time explaining it except that
possibly that NEMA values are very conservative. I know that they were
originally developed around 1970 as a result of tests by Mr. Stolpe at
Southern California Edison and the methodology which until today bears his
name . Mr. Stolpe's tests were conducted as a part of the investigation
into cable failures at the SCE's nuclear power plant. Maybe the tests and
the analytical method which ensued are conservative due their nuclear
pedigree.
[ February 10, 2005, 07:38 PM: Message edited by: wisniewski ]
trays are so different. For example the NEC LV small power cable
ampacity , with the tray filled to 42 sq. inches (36 inch tray) is so much
greater than the ICEA power cable ampacity. I am very familiar with the
Stolpe method and accounting for the different fills cannot explain it, by
a longshot. I am beginning to believe that ICEA is way too conservative.
The NEMA standard dealing with the cable trays is ICEA P-54-440/WC 51.
Let me provide an example by determining the ampacity of #2 AWG , 90C,
0-600V class, 1.13 inches OD - 3/c cables in an open ladder bottom 36 inch
tray in a 40 C ambient. All cables are of the 3/c type with an overall
jacket .
NEC ampacity (2002 Code)
Per 392.9 A(2) and table 392.9 the maximum allowable fill area for
multiconductor cables in a 36 inch open ladder bottom tray is 42 sq.
inches. Considering that my 3/c cable has OD of 1.13 inches (area of 1.003
sq. inches) we can place 41 such cables in the tray.
The ampacity , according to Section 392.11 (A) is given in Table 310.16.
The ampacity at a 30C ambient is 130 amperes. Adjusting for a 40C ambient
using correction factors from Table 310.16 the ampacity is 118 amperes.
NEMA WC 51, ICEA P-54-440
This standard identifies the cable ampacities depending on a depth of fill.
Assuming 41 cables as established above this gives us a "NEMA depth of
fill" (section 2.2 of the NEMA std.) of 1.45 inches. Table 3.6 indicates the
ampacity of 85 amperes for the same cable at 1.5 inches depth of fill. The
same 40C ambient.
The difference is clear and I have a hard time explaining it except that
possibly that NEMA values are very conservative. I know that they were
originally developed around 1970 as a result of tests by Mr. Stolpe at
Southern California Edison and the methodology which until today bears his
name . Mr. Stolpe's tests were conducted as a part of the investigation
into cable failures at the SCE's nuclear power plant. Maybe the tests and
the analytical method which ensued are conservative due their nuclear
pedigree.
[ February 10, 2005, 07:38 PM: Message edited by: wisniewski ]
