What type of wire for 50A subpanel feeder

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  #1  
Old 01-12-19, 08:48 AM
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What type of wire for 50A subpanel feeder

Hello, I am using STW 6ga. 3-wire w/ 8ga. Ground to supply power to my 50A subpanel. This is a heavy duty outdoor rated cord typically used for RV power supply. I had some laying around and decided to use it instead of SER indoor cable. My problem is that when tightening the lugs on the wire it seems to be damaging and maybe severing some of the strands. This wire has maybe 50 strands each that are very very thin. SER has 7 or so thicker strands for each wire, but I used the STW because it's more flexible. Will this be fine or create potential for problems? Thanks for any help.
 
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  #2  
Old 01-12-19, 09:03 AM
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Cord is not allowed for that type use. You need to use SER or NM-b cable.
 
  #3  
Old 01-12-19, 09:13 AM
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I had some laying around and decided to use it instead of SER indoor cable.

SER cable can be used either indoors or outdoors.


pattenp is correct.
 
  #4  
Old 01-12-19, 09:18 AM
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Against the rules. Switch to Nm-b cable is its all inside. How far of a run?
 
  #5  
Old 01-12-19, 09:40 AM
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Thanks for the responses. When I said SER indoor I meant unprotected (no outdoor conduit). The STW is meant to lay on the ground outdoors for powering RVs. This subpanel is in a tiny house on wheels I'm building myself. I just cut a section off the STW and used it between the through-the-wall power inlet and panel. When you say it's not allowed your referring to code I assume. I just want to know if there's any danger using this thin stranded wire on the terminal lugs. Thanks again.
 
  #6  
Old 01-12-19, 09:43 AM
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The run inside from the panel to the wall is 5 feet. The run from the exterior plug to the RV park supply recep. will be about 15 feet.
 
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Old 01-12-19, 09:49 AM
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Also go to 6 gauge for 50 amps for NM cable.
 
  #8  
Old 01-12-19, 09:51 AM
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Will it work? Answer is yes. Is it the right type of cable? NO. Its such a short run why not go buy the right cable? A short legth of 6-3 NM-b with a ground from the local big box store will only be a few dollars. I say do it the correct way. Just my 2 cents.
 
  #9  
Old 01-12-19, 10:01 AM
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Yes I understand. I noticed when I was going to buy the SER NM-B 6/3 the ground was 10 ga. The 50amp STW 6/3 had a 8 ga. ground. I thought that this mattered so just wanted everything the same size. And the STW is very well insulated. I will change it for sure if there's any safety concern. I want it all to be 100% safe. Thanks for the help.
 
  #10  
Old 01-12-19, 10:09 AM
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National electric codes are in place for a reason. An 8 gauge ground is correct. My sub currently has 6-3 with a 10 gauge ground only because I couldnt find 6-3 with a 8 gauge ground. Is what it is... better off having correct cable with an undersized ground than incorrect cable with correct size ground. Im taking a guess that the reason the cord isnt supposed to be used in a breaker is cause the strands are a lot more fine and you risk them breaking or not being secured like the cable would be.
 
  #11  
Old 01-12-19, 10:21 AM
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Ok that validates my concern. The terminal lugs do seem to be damaging the thin strands and that's why I was inquiring. I know the Codes, just needed some additional input. Theres no inspections required for this type of build, but I had a electrical company look at all the finished work and they said it looked good. I'm about to close up the walls and had this in the back of my mind, but I'll change it out. I dont know why the ground sizes are not the same across the board. Thanks again.
 
  #12  
Old 01-12-19, 11:20 AM
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A #10 copper grounding conductor is fine even for a 60 amp circuit/feeder. NEC table 250.122
 
  #13  
Old 01-12-19, 11:41 AM
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The allowable ampacity of 4 wire cordage is not the same as building wire such as NM-b. #6 four wire cord where 3 conductors carry current is 45A, whereas 6/3 NM-b is 55A.
 
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Old 01-12-19, 12:36 PM
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A #10 copper grounding conductor is fine even for a 60 amp circuit/feeder. NEC table 250.122
cool. Thats some good info. I always thought the ground was undersized.

The allowable ampacity of 4 wire cordage is not the same as building wire such as NM-b. #6 four wire cord where 3 conductors carry current is 45A, whereas 6/3 NM-b is 55A.
it should also be noted that the NEC allows the next size up for the breaker. A 55 amp breaker is not common. So if using a 6 gauge NM-B cable you are allowed to up the breaker size to 60 amp as long as your sub is rated for at keast 60 amp.

 
  #15  
Old 01-12-19, 02:53 PM
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I wished I would have known all that before cutting a 10' random piece of my 50A cord. That stuff is pricey. I will be swapping it out for the NM-b right away. Good to know about the ground size amp ratings. Thanks for all the info.
 
  #16  
Old 01-12-19, 06:25 PM
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My sub currently has 6-3 with a 10 gauge ground only because I couldnt find 6-3 with a 8 gauge ground.

You couldn't find it because no one makes it. Both 6-3 NM-B and 6-3 UF-B cables only have a #10 ground as that is all that is required.
 
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