240 volt feed subpanel

Reply

  #1  
Old 05-03-13, 04:39 AM
Member
Thread Starter
Join Date: May 2013
Location: usa
Posts: 6
240 volt feed subpanel

I'm feeding subpanel from main panel with aluminum direct buried cable. Each feed 120 volts with neutral cable. Worked for many years. Had a short to the circuit box and now volts drop to 60 V on one side and 180 v. on other line everytime I place a load on it. Any thoughts on what happened? Short was on minor 14 wire feed far from the circuit box.(20 amp circuits)
 
Sponsored Links
  #2  
Old 05-03-13, 04:58 AM
Member
Join Date: Jun 2008
Location: VA.
Posts: 813
You have an open neutral on the feed to the panel. Whether it is a loose connection at your panels or burned up underground.
 
  #3  
Old 05-03-13, 07:25 AM
Member
Thread Starter
Join Date: May 2013
Location: usa
Posts: 6
Thank you. Could a #4 aluminum neutral wire have burned up with just a short in a small circuit 100's of feet away beyond a sub panel with breakers that didn't trip?
 
  #4  
Old 05-03-13, 07:34 AM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 32,629
Each feed 120 volts with neutral cable.
No that is incorect. You are feeding the panel with the two legs of the 240 volts the electric company provides your house plus a neutral from which 120 volts is derived using one of the two 240 volt lines to your subpanel.

Short was on minor 14 wire feed far from the circuit box.(20 amp circuits)
A #14 wire is never used on a 20 amp circuit. A 20 amp circuit is always #12 or larger.

What are the following voltage readings at the subpanel?
  • Ground to 240 leg A.
  • Ground to 240 leg B.
  • Neutral to 240 leg A.
  • Neutral to 240 leg B.

If both 240 volt legs are 120 volts to ground but both 240 volt legs measure ~0 to neutral then you have an open neutral as suggested by Wirenut.

Above assumes a code compliant 4 wire circuit with isolated neutral and bonded ground.

Could a #4 aluminum neutral wire have burned up with just a short in a small circuit 100's of feet
What is the actual distance and size of the breaker at the supplying panel? What sort of cable?
 

Last edited by ray2047; 05-03-13 at 09:22 AM. Reason: Correct voltage mistake.
  #5  
Old 05-03-13, 08:41 AM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
If both 240 volt legs are 240 volts to ground but both 240 volt legs measure ~0 to neutral then you have an open neutral as suggested by Wirenut.
Since each leg of the 120/240V single-phase service should have a potential of 120V to either neutral or ground, the reading of 180V on one leg points to a neutral that is not fully connected. The observation that the other leg shows 60V under the same condition, so that the two together add to 240V, suggests that the neutral is damaged but not missing.

Pesiatri, please give us the readings that Ray requested - each leg to neutral and each leg to ground. Since you say you observed this under load, please give us those readings twice - under load and without load. In addition, please measure the voltage across the two hot legs, also under load and without load.
 
  #6  
Old 05-03-13, 10:20 AM
Member
Thread Starter
Join Date: May 2013
Location: usa
Posts: 6
without a load and reset double breaker at feed panel, each line measures 120 v to neutral and ground. Once I place a load on, then it drops to 60 V on one side and 180 volts to other. Can I make feed line a 120 V system? not anything fed requires 240 volts now. Just feeding freezer in out building and maybe a table saw.The sub panel is 150 ft from main feeder panel.Thanks for all your help.
 
  #7  
Old 05-03-13, 11:49 AM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 32,629
Once I place a load on, then it drops to 60 V on one side and 180 volts to other.
Is that to both neutral and ground? You do have four wires to the panel with the ground bonded and the neutral isolated don't you? If not post back and we will have to give you different instruction for testing a three wire feed.
 
  #8  
Old 05-03-13, 01:03 PM
Member
Thread Starter
Join Date: May 2013
Location: usa
Posts: 6
I have a 3 wire set up. Two hots and a neutral.
 
  #9  
Old 05-03-13, 01:32 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 32,629
I have a 3 wire set up. Two hots and a neutral.
That is no longer code compliant but probably grandfathered. To test you will need an extension cord with ground plugged into a receptacle with a known good ground (not on the shed circuit). With one probe of your multimeter measure from the extension cord's ground to each hot wire. Then repeat measuring from the wide blade slot of the extension cord to each hot. Give us the results.
 
  #10  
Old 05-03-13, 02:49 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
without a load and reset double breaker at feed panel, each line measures 120 v to neutral and ground. Once I place a load on, then it drops to 60 V on one side and 180 volts to other. Can I make feed line a 120 V system? not anything fed requires 240 volts now.
No, not with a neutral that may be less than adequate. Running a pure 240V load would probably be safer, because those loads don't use the neutral.

What voltage did you measure between the two hot legs?

I would kill the power at the main panel and take everything to do with neutral in the subpanel apart and check for problems. One common one is that the feeder may have some strands either missing or deeply scored, if an insulated wire was pulled and had to be stripped to be terminated.
 
  #11  
Old 05-04-13, 05:43 AM
Member
Thread Starter
Join Date: May 2013
Location: usa
Posts: 6
thanks

thanks again, I had to go to work before I could try the extension cord readings. I'm working 7p-7a for the next 3 days and won't have a chance to check until mon. evening. If you could check back on tuesday, that would be very appreciated. Thanks again and have a good w/e.
 
  #12  
Old 05-04-13, 06:05 AM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,349
I got the impression that the Op wanted to re-purpose one of the hots to replace the failed neutral.
 

Last edited by pcboss; 05-05-13 at 02:19 PM.
  #13  
Old 05-05-13, 01:25 PM
Member
Thread Starter
Join Date: May 2013
Location: usa
Posts: 6
That is what I would like to do, if possible, because to bury new cable is a huge project in my situation. Can I get rid of failed neutral and use one leg as new neutral and the other hot for 120 service?
 
  #14  
Old 05-05-13, 02:17 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,349
What size conductors are feeding the panel? Electrically this could be done, however it could be a code violation. The size matters.

If this is changed, only every other breaker will be hot at the outbuilding.
 
  #15  
Old 05-05-13, 07:10 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Can I get rid of failed neutral and use one leg as new neutral and the other hot for 120 service?
Probably, but that won't solve the problem if it's in the subpanel. For that reason, before doing that,
Originally Posted by Nashkat1
I would kill the power at the main panel and take everything to do with neutral in the subpanel apart and check for problems. One common one is that the feeder may have some strands either missing or deeply scored, if an insulated wire was pulled and had to be stripped to be terminated.
 
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
'