retrofit subpanel - neutral sizing

Reply

  #1  
Old 01-06-15, 10:40 PM
Member
Thread Starter
Join Date: Jan 2015
Posts: 3
retrofit subpanel - neutral sizing

I could not find a code section that addresses this clearly so hope to get some advice here...

I have a subpanel fed by two AWG#2 AL conductors from a 100A breaker in the main panel. Conductors are in FMT conduit along with a bare stranded ground wire. I believe that the original use for this panel was to feed an electric resistance furnace and water heater. Previous owner replaced these with gas appliances and installed 120V circuits in the panel using the bare ground as neutral. This is not particularly to my liking so would like to install a reliable ground and neutral conductor.

I would like to run a neutral conductor in a separate 1/2" EMT conduit (is this legal?) using the conduit as equipment ground. I am not clear on the neutral sizing and the work will be very simple if it will fit in the 1/2" EMT (which is already partially installed). I don't have much in the way of unbalanced load as the circuits I intend for this panel are:

30A One mini-split heat pump @ 23A 240V
15A One freezer outlet and light in the freezer closet @ 2A 120V
15A One refrigerator outlet and light in the refrigerator closet @ 7A 120V
20A One outdoor lighting and receptacle circuit @ 120V

future growth may include:
20A Second smaller heat pump @ 15A 240V
and - maybe a spa (not electrically heated)

Thanks.
 
Sponsored Links
  #2  
Old 01-06-15, 11:34 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,587
The flex cannot be used as a ground at the ampacity of the panel. Secondly, all conductors need to be in the same raceway or conduit so you cannot ad a second conduit for the neutral. A new 4 wire feeder needs to be run.
 
  #3  
Old 01-07-15, 06:33 AM
Member
Join Date: Jun 2012
Location: USA
Posts: 1,024
Lastly - #2 AL is only rated for 90amps as a feeder.
 
  #4  
Old 01-07-15, 06:46 AM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,502
What size is the flex conduit? There may be room to pull in a ground and neutral. Can you identify the insulation type on the existing hots, that will nail down what the final ampacity could be.
 
  #5  
Old 01-07-15, 07:40 AM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: USA
Posts: 9,379
Conductors are in FMT conduit along with a bare stranded ground wire.
Could this have been a typo? Did you really mean EMT?
 
  #6  
Old 01-07-15, 09:23 AM
Member
Thread Starter
Join Date: Jan 2015
Posts: 3
This is the answer I did not want to hear... But thanks, a new feeder will be more work since I will need to cut into walls. -and a bit more money- This was actually my original thought but sought to save some work. Requiring all conductors in the same raceway seems to me to be without electrical consequence but if there is one I would be interested.

Secondly, if I abandon the original feeder do I need to remove it or just cut them off at the entrance to the main panel?

Thanks.
 
  #7  
Old 01-07-15, 09:47 AM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 33,041
All the current carrying conductors in the same raceway reduces heating because the electromagnetic fields cancel out. Because the ground only carries current during a fault situation it does not have to be in the same raceway.
 
  #8  
Old 01-07-15, 09:57 AM
Member
Thread Starter
Join Date: Jan 2015
Posts: 3
Well since the art of replying to a single post escapes me for the moment, here are the rest of my replies:

FMT - Should have said flexible metal tubing but on closer inspection appears to be common helical flexible metal conduit and is 1.5" OD. The conduit is inside the wall and the geometry is such that at least three 90 deg bends would be needed (but more likely four). Also, the conduit had been damaged at one point and repaired where it enters the wall so I couldn't get a fish tape in there anyway. - nor do I believe that I can pull the conductors out easily. I can install a new surface mounted conduit run faster than salvaging the old one.

Ampacity - Conductors are AWG#2 aluminum THW. Total conductor length from the 100A breaker to the panel is probably 18'. The county signed off on this in the mid seventies.
 
  #9  
Old 01-07-15, 12:17 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,502
Theoretically you could reuse the 1.5" flex to pull the new feeder, but it sounds like that may be more trouble than it's worth. Under modern code your panel should be 90A max so the breaker would need to be replaced too. If you want to reuse the 100A breaker, your hot and neutral wires need to be #1 aluminum or #3 copper in conduit with #8 copper green insulated ground (req'd for future spa usage). Conduit size should be 1-1/4" if you go with aluminum wire option or 1" if you go with copper wire option.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes