Attached Structure Subpanel - wire sizing (attic & ground) complications

[vtsoundman]

Have a quick question regarding ground wire size.

Subpanel installed in attached structure in a residential application.

Running #2 AL XHHW-2 in conduit (in attic) which will be spliced to connect to SER for a temp feed. Eventually (within the months or so )all the SE will be ripped out and replaced with all #2 XHHW (don't have a heartattack...)

Panel needs a 90 or 100A breaker.

Can the NEC experts help with this? I'm in Cali (NEC 2014)

1) Will #2 Al wire be sized for a 90A breaker?
2) Any concerns about derating for attic?
(Wire has 130A/90C mfr rating; OR 100A @ 90C per NEC)
3) Ground Wire size? Assume Copper - #6 or #8 for 90/100A breaker.

Thx!

 

[wyliesdiesels]

Have a quick question regarding ground wire size.

Subpanel installed in attached structure in a residential application.

Running #2 AL XHHW-2 in conduit (in attic) which will be spliced to connect to SER for a temp feed. Eventually (within the months or so )all the SE will be ripped out and replaced with all #2 XHHW (don't have a heartattack...)

Panel needs a 90 or 100A breaker.

Can the NEC experts help with this? I'm in Cali (NEC 2014)

1) Will #2 Al wire be sized for a 90A breaker?
2) Any concerns about derating for attic?
(Wire has 130A/90C mfr rating; OR 100A @ 90C per NEC)
3) Ground Wire size? Assume Copper - #6 or #8 for 90/100A breaker.

Thx!

1) #2 is max 90a in this application so yes.

2) Yes derating should be done if attic temps reach higher than 30* c. Refer to T310.15(B)(2)(a)

The 90* c amapcity values in T310.15(B)(16) are for derating purposes only- such as derating conductors for ambient temperatures. If no derating is needed, then you must use the 75*c ampacity values.

3) EGC would be #6 al or #8 cu for up to 100a breaker.

 

[vtsoundman]

1) #2 is max 90a in this application so yes.

2) Yes derating should be done if attic temps reach higher than 30* c. Refer to T310.15(B)(2)(a)

The 90* c amapcity values in T310.15(B)(16) are for derating purposes only- such as derating conductors for ambient temperatures. If no derating is needed, then you must use the 75*c ampacity values.

3) EGC would be #6 al or #8 cu for up to 100a breaker.

Thx Wylie... This is why I like this forum.

The trick with the attic would be to know what temp to assume. As far as I can tell, there is no guidelines to assume for temp rise in an attic space. External ambient temps around here exceed 90F only a few times/year...and if I am reasonable about the rise in the attic, then I would conservatively assume a 30deg rise (~120degF). A derating of 0.82 against the 90C column yields 82A. Rounding up to the next breaker size -> 90A breaker.

Makes sense?