What wire to feed sub panel?

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  #1  
Old 02-21-15, 08:40 AM
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What wire to feed sub panel?

Might sound like an easy question, but I'm getting all kinds of conflicting information.

I want to feed a 100 amp sub panel box in a detached garage from our house panel. Actually, I've already done it, but I want to replace what I've done to bring it up to code.

The garage is only 8 feet away from the house and I have PVC running underground from the block wall of the house to the block wall of the garage...once again...only 8 feet.

The total run is 100 feet and about 80 feet of it is above a drop ceiling in the basement. (The house panel is at the complete opposite end of the house compared to where the garage is.)

I WAS just going to run 1/0-1/0-1/0-2 SER from a 100 amp breaker in the house all the way to the garage going to a breaker box with a 100 amp main breaker. Problem is...because of that darn 8 foot underground in conduit section...I guess SER is not allowed.

I have been told to just use URD for the entire distance, but the problem with that is I've been told that it has to be run inside conduit for the 80 feet inside the house...and that's a lot of wire going through a lot of conduit.

I was also told to use the SER wire from the house panel to a junction box right before going into the conduit that goes to the garage...then transition in the junction box to URD wire using split bolts.

Just wondered what you guys would recommend. It's a shame that the measly 8 feet of underground conduit is creating all of these problems.

Thanks!
 
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  #2  
Old 02-21-15, 10:10 AM
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I was also told to use the SER wire from the house panel to a junction box right before going into the conduit that goes to the garage...then transition in the junction box to URD wire using split bolts.
Door #3, IMO. Others will chime in with their take on it.
 
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Old 02-21-15, 10:36 AM
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I've been told that it has to be run inside conduit for the 80 feet inside the house
Not allowed even in conduit unless it is dual rated.
I want to feed a 100 amp sub panel box in a detached garage
You do not need to feed a 100 amp subpanel with cable or wire rated for 100 amps so long as the breaker in the main panel is large enough to protect the wire you do use. What loads do you anticipate in the garage?
 
  #4  
Old 02-21-15, 11:51 AM
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Well as for the load...and they won't all be running at one time...

My biggest power hogs would be...

An 220 oven running about 9000 watts...so about 40 amps.

A 220 10kw Electric heater running on and off in the winter...45 amps.

A 220 7.5hp compressor...it's rated at about 40 amps but Ingersoll Rand recommends an 80 amp breaker due to start up amperage.

Now...the compressor and oven would never be running at the same time. So it would be either oven+heater...or compressor+heater...and even that would only be in winter months.



What I have now (and I know it's not good) is (3) 4awg THHN/THWN coming off of a 90amp breaker going to the garage. I know this is wrong because...

I don't have a dedicated ground going from box to box...only a neutral.
I do not have the wire in conduit.
I chose the 4 awg because I can handle 95 amps at 90 deg C...more than the breaker I have it on...but I've heard that you should chose wire based on the amps you need at the LOWEST temperature...not the highest?

Oh...and even the 90 amp breaker I am using now has not tripped even once.
 

Last edited by MatadorMkV; 02-21-15 at 12:07 PM.
  #5  
Old 02-21-15, 12:33 PM
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Okay. Not even sure 100 amps is enough but for now we will go with that. I would run 1/0 copper (2/0 aluminum) THWN or XHHW in conduit all the way (#6 for ground). For now use a 100 amp breaker in the main panel but this would allow you to easily increase the feed to 150 amps in the future assuming your main panel is rated for an 150 amp branch circuit. If you did increase in the future the subpanel would have to be changed out but the wiring wouldn't have to be. This also gives you good insurance against voltage drop due to distance.
 
  #6  
Old 02-21-15, 03:58 PM
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Would this work as well?

2/0,2/0,2/0 & #1 Syracuse Underground Secondary Distribution Cable

If so, what size conduit would you recommend?
 
  #7  
Old 02-21-15, 04:30 PM
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The pros will have to comment on if that can be used inside.
 
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Old 02-21-15, 04:43 PM
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The URD would need another rating besides URD in order to be used inside the house or in conduit.

The 90 amp rating can be used for derating, but the ampacity cannot be more than the lowest rated terminal. You will not find 90 degree rated terminals in a residence. You will need to use the 60 or 75 degree column for ampacity.
 
  #9  
Old 02-21-15, 07:44 PM
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Well,
How about SER in the house to XHHW-2 through conduit going to the garage?

Reason I'm trying to stay away from conduit in the the house is because of all of the bends I would have to make.
 
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Old 02-22-15, 08:46 AM
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Well,
How about SER in the house to XHHW-2 through conduit going to the garage?
That would work. I'd use #1 aluminum. I don't see a reason to go any larger for a 100 amp feeder.
 
  #11  
Old 02-22-15, 09:20 AM
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Joe Replied:
I don't see a reason to go any larger for a 100 amp feeder.
Yes but in post #4 the O/P wrote:
My biggest power hogs would be...An 220 oven running about 9000 watts...so about 40 amps.... A 220 10kw Electric heater running on and off in the winter...45 amps. A 220 7.5hp compressor...it's rated at about 40 amps but Ingersoll Rand recommends an 80 amp breaker due to start up amperage.
So when I suggested a wire size I was building in a cushion if the 100 amp had to be increased.
 
  #12  
Old 02-22-15, 06:19 PM
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What do you guys think about those Polaris connectors?

I've watched videos of guys wrapping split bolts the proper way, and they end up looking about the size of a fist!

The Polaris connectors are kind of pricey, but they look soooo clean.
 
  #13  
Old 02-22-15, 06:57 PM
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They are great. Easy to redo a connection also if you need to.
 
  #14  
Old 02-23-15, 03:41 AM
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Ray,
Our home has two large panels that each have 150A main breakers (300A total entrance). Would I be able to put let's say a 125 amp breaker in one of the cabinets to feed the garage...or should I just try to get by with a 100 amp?

Also...kind of confused about what temp range to go with...

The 2/0-2/0-2/0-1 Aluminum SER is rated...
115A at 60 deg
135A at 75 deg
150A at 90 deg

Would this wire be acceptable with a 125A breaker feeding it?
 

Last edited by MatadorMkV; 02-23-15 at 03:57 AM.
  #15  
Old 02-23-15, 05:13 AM
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The issue you will have is the bus stab ratings. The fingers are only allowed to supply so much per connection so you will need to see if the 125 can even be used or if the spots opposite will need to be blank.

SER needs to be used at the 60 degree ampacity.
 
  #16  
Old 02-23-15, 09:37 AM
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Wow...so even the 2/0 aluminum won't be heavy enough to support a 125 amp feed?

I'll probably just end up going with a 100 amp breaker. Weird that I've been feeding everything with a 90 and it hasn't tripped once.

Like I said though...the oven and compressor will never be running together...so far it's been one of these combinations...

Compressor + heater + misc.

or

Oven + heater + misc.
 
  #17  
Old 02-24-15, 07:12 AM
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You are allowed to "round-up" to the next standard breaker size when the wire ampacity falls between breaker sizes. For that reason the #2/0 aluminum SER can be used at 125A. However I don't think you need a 125A feed based on the loads you described. If the 90A breaker you already own isn't tripping, there's no point in buying another one. Just reuse the one you have.
 
  #18  
Old 02-24-15, 11:35 AM
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Thanks! I think unless someone here says otherwise, I'm going to do this...

Stay with my existing 90 amp breaker

Use 1/0-1/0-1/0-2 Aluminum SER from the house panel to a junction box at the underground conduit point inside of the house.

Use (3) 1/0 XHHW (two hots/one neutral) and (1) 2 awg XHHW (ground) out of the junction box, directly into the conduit, going into the garage.

I will probably be using a connector similar to the Polaris but sold by Ilsco called a Clear Tap to make the connections inside the junction box...pretty much the same thing...it's what the supply house sells in my area.

PCT MULTI TAP INS AL/CU

I will be using an antioxidant on all connections.

I may or may not replace the 90 amp breaker with a 100 amp in the future, but as of right now, I don't see the need.

Well? What do you guys think? Did I forget anything?

Thank you as always...reading "what size wire" posts can drive a person crazy! I've read anywhere between #2 Al to 2/0 for 100 amps...that's a lot of variance...so I settled on the 1/0.
 
  #19  
Old 02-24-15, 12:07 PM
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Along with a properly sized junction box be sure to use the correct color insulation on the wires or tape them the proper colors.
 
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Old 02-24-15, 12:13 PM
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  #21  
Old 02-24-15, 12:37 PM
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As long as that junction box is located in an accessible location inside the house it is ok. It can't be used outdoors. Since it is metal, the EGCs (ground wires) will need to be bonded to the box. Best way to do that is to buy a ground lug kit and bolt it right to the box:

https://platt.com/platt-electric-sup...spx?zpid=29129
 
  #22  
Old 02-24-15, 12:57 PM
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Ok...so I wouldn't need a connector for the ground wires because this would serve as the ground connector?...I would bolt the ground lug to the box...the incoming ground from the SER would go in one hole...and the outgoing XHHW would go into the other.

Yes...the box would be under the house, in a crawl space easily accessible from the basement. It would be mounted to an inside block wall.
 
  #23  
Old 02-24-15, 02:40 PM
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Absolutely no need to use a #2 conductor for the equipment ground, #6 aluminum is sufficient for a 100 ampere circuit breaker. If you think you may someday want to up the circuit breaker to 125 amperes then use a #4 aluminum equipment grounding conductor.
 
  #24  
Old 02-24-15, 03:36 PM
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Ok....I just figured #2 just because it's what comes as part of the SER cable.

Thanks
 
  #25  
Old 02-24-15, 04:38 PM
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If it is part of the SER cable that is fine but when you transition to individual conductors you can go to the smaller size.
 
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