For a 220v 100amp subpanel located 100′ from the main panel – the run is through an open basement ceiling and then up through a sheetrocked wall…
The electrician is planning to use #4 3 wire BX. The capacity chart at Home Depot says #4 is good to er, 75amps? What am I missing?
The only thing I can think of is the hot is on two wires, so perhaps the capacity of the cable is really 75a + 75a which is greater than the load the panel needs?
If this wire IS too light, what is the right cable to use?
Thanks in advance,
Replies
I am NOT a electrican.. I think # 4 is what is used for 100 Amp. ... COPPER wire only.. Aluminum you askin fer trouble in the long haul for anything...
Not sure what the run length can be..
If you look at typical ampacity tables for building wire, they would say you need #3 wire for 100 amps. However, there is an exception - according to article 310-15(6) and table 310-15(b)(6) of the National Electric Code (this is from the 1999 edition, the number may be slightly different in other editions), #4 wire can be used for 100 amps with 120/240 volt, three wire, single phase services and feeders in dwellings. A quick check of the voltage drop calculation for 240 volts at 100 amps over 100 feet indicates #4 will have a drop of 2.6%, which is within limits (you want to keep voltage drop less than 3%.)
Without re-typing all the text from the code, it appears this article applies mainly to service to the main panel, but I think it also applies for sub-panels. I don't have expertise with residential wiring, so hopefully one of the guys who do will chime in to confirm or deny. Personally, I'd like to see #3 wire used instead, but I only work on commercial projects where I know that's what would be required.
Edited to add: this is for copper wire, the ampacities are different for aluminum.
Edited 6/19/2005 5:30 pm ET by Stuart
There are a couple problems with using table.First it is restricted to SE or USE cables or certian indiviual wire specs. So it would not cover NM or BX or MC.And I see a lot of people quote that section for sub-panel feeders. But it says "service entrance conductors, serice lateral conductors, and feeder conductors that serve as THE MAIN POWER FEEDER".I don't see anyway that the typical sub-panel instalation falls under that section.Like you I am an EE, but my experience is with electronics and control systems.
I have a 100 amp, 240v sub panel for an addition, run from main to sub panel is about 90', across a basement, up inside of a sheetrock wall.
My electrician ran #2 AWG aluminum Romex (3 full sized conductors + downsized ground, #2-3 w/ground). Passed inspection without any hesitation or comment.
100 amp breaker in main panel and the lugs in the sub panel are all listed / rated for CU/AL. Used Alnox on all aluminum connections. In DE most residential circuits over 40 amps are run in aluminum due to cost factors.
In service 13 years with 0 problems.
Why BX?? Is this cable going to be subject to some sort of abuse?
Jim
Never underestimate the value of a sharp pencil or good light.
Thanks for the feedback!
So #4 3-wire BX is acceptable, but #3 would be better - is what I get from the comments. I'm not sure why the BX - its just an ordinary basement. Maybe a local rule? Substitution for conduit?
Don't know if this is the case in your area, but some areas require wiring in basements or garages that is below 8' to be sheathed in some fashion. I think in Columbus BX might not qualify, but I'm not positive. I know it doesn't qualify as the ground. That's my guess as to why BX.As long as the breaker will accept #3, I'd go up a gauge. And I always specify copper, even for SEC.
Siemens breakers will accept #2.My 60 amp 130 foot run is #2,#2,#4 direct bury aluminum. The #2 goes right into the breakers...
Are we there yet ?
I was planning on putting in a 100A subpanel, but inspector required #2 cable (#1 if aluminium). I went with 100A subpanel on 60A breaker, I get off with #6 cable. After getting the cable up through one closed stud bay and some twisting solid conduit, I am so glad I did not try this with #2....