Box fill

Rufino

Member
Location
Florida
Occupation
electrician
IN A NEW JOB WE ARE PULLING 16 AWG 10 + GROUND NUMBER 10. IN A 1INCH CONDUIT. THE BOX WE USE IS 8X8X4.. 8 12/2 mc cable are comming to the box to splice .. how do you size the box because IN those boxes there is no cubic inches and i cant find it IN the nec..

Because of derating factor the wire ampacity is less than 18 amps, is that ok to put that wire in 20 amps braker..
 

tank728

Member
Location
New Jersey
Occupation
Electrician
If memory serves correctly you should be okay on derating assuming thhn. But you are over on conduit fill. The egc counts towards fill but not derating.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I would use math and get close 8x8x4 = 256 cu.in

16 #12== 16 x 2.25 = 36
17 #10 = 17 * 2.5 = 42.5

42.5 + 36 = 78.5 so you should be good.



17 # 10 = .0211==.3631 in^2

1 1/4"
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
(a) simply do the math 8x8x4 = 256 cu in good for 100+ #10s
Your derating for 16 current-carry #10 THWN-2 should be 20 amps I believe
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
IN A NEW JOB WE ARE PULLING 16 AWG 10 + GROUND NUMBER 10. IN A 1INCH CONDUIT. THE BOX WE USE IS 8X8X4.. 8 12/2 mc cable are comming to the box to splice .. how do you size the box because IN those boxes there is no cubic inches and i cant find it IN the nec..

Because of derating factor the wire ampacity is less than 18 amps, is that ok to put that wire in 20 amps braker..
How did you come up with less than 18 amps after derating? Also what kind of raceway, for 1" EMT your tubing is too small.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
IN A NEW JOB WE ARE PULLING 16 AWG 10 + GROUND NUMBER 10. IN A 1INCH CONDUIT. THE BOX WE USE IS 8X8X4.. 8 12/2 mc cable are comming to the box to splice .. how do you size the box because IN those boxes there is no cubic inches and i cant find it IN the nec..

Because of derating factor the wire ampacity is less than 18 amps, is that ok to put that wire in 20 amps braker..
It would be okay to put on a 20 if the load was equal to or less than 18 amps but the fact is the de-rating would be 20 amps as Gus stated.

#10 at 90C is 40 amps. 50% of 40 amps is 20 amps
 

Rufino

Member
Location
Florida
Occupation
electrician
(a) simply do the math 8x8x4 = 256 cu in good for 100+ #10s
Your derating for 16 current-carry #10 THWN-2 should be 20 amps I believe
If you the math thats correct .. but is that what nec permit... because when you look at the boxes fill in the nec you have to use less than the actual cubic inches
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
Are the NEC box fill tables based on internal box dimensions instead of external? A 4x4x1.5 box is 21 cu inches in the table and just multiplying you would think it is 24. Boxes not listed in the NEC table and plastic boxes need to be marked unless they are over 100 cubic inches. The 8x8x4 box is well over 100 cu in, so you just multiply it out. Perhaps you need to use interior dimensions, but when they get that large the difference isn't that much.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Are the NEC box fill tables based on internal box dimensions instead of external? A 4x4x1.5 box is 21 cu inches in the table and just multiplying you would think it is 24. Boxes not listed in the NEC table and plastic boxes need to be marked unless they are over 100 cubic inches. The 8x8x4 box is well over 100 cu in, so you just multiply it out. Perhaps you need to use interior dimensions, but when they get that large the difference isn't that much.
I started to mention that earlier but I realized it didn't matter in this case. Yes the dimensions in the table is interior dimension
 
Top