NEC, 6 Ga THWN, And An Outbuilding - Electrical - DIY Chatroom Home Improvement Forum


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Electrical

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Display Modes
Old 12-28-2010, 09:38 PM   #1
Newbie
 
Join Date: Dec 2010
Posts: 8
Rewards Points: 10
Default

NEC, 6 Ga THWN, and an outbuilding


Hello all,

First post here, so be kind!

I am looking for some confirmation of what I think I "already know". I have an outbuilding on my property that was here when I purchased. For what it is worth, the building is about 25 x 40 and built in 2001. I have installed a furnace, fluorescent HO lighting, and recently a 5 HP compressor. There are also other circuits for receps, exterior lighting / garage door opener, and a septic tank warning system. The service panel (sub panel) for the building has been upgraded to a traditional 100 AMP service panel due to all of the above. I had an electrician install the sub panel - I was pretty confident I knew what needed to be done, but, just to be safe. Previously, it was merely a junction type box with 4 breakers in it.

The sub-panel is fed by 6 GA THWN from the main panel in the house. The total run is no more than 125 feet +/-. The THWN is burried in a plastic conduit from buiding to house.

In the main service panel, there is a 50 amp breaker than feeds the sub panel. I can not find anything in NEC that indicates feeding a 100 amp sub panel with a 50 amp breaker is not "ok". Further, I have read in NEC (table 310-16) that the maximum amperage for 6 Ga THWN is 65 Amp.

Questions:

1) Are there any code issues with having a smaller breaker (50 Amp) in the main panel feed the 100 Amp sub panel in the outbuilding?

2) Since THWN is allowed to carry 65 amp, is there any reason why I cant pull the 50 amp breaker and install a 60 amp?

I ask #2 as I have visions (delusions?) of putting a central AC unit on the furnace this spring.

Thanks!

-lower32

Advertisement

lower32 is offline   Reply With Quote
Old 12-28-2010, 09:45 PM   #2
Mad Scientist
 
Join Date: Dec 2009
Location: Minnesota
Posts: 775
Rewards Points: 518
Default

NEC, 6 Ga THWN, and an outbuilding


You could go to a 60A breaker. However, at that distance, especially with a load like an A/C, voltage drop could be a problem. I don't know exactly how to calculate voltage drop, but I imagine that the electrician used a 50A breaker because that may be all the load the circuit can handle without the voltage dropping below acceptable levels.

Advertisement

McSteve is offline   Reply With Quote
Old 12-28-2010, 09:51 PM   #3
Mad Scientist
 
Join Date: Dec 2009
Location: Minnesota
Posts: 775
Rewards Points: 518
Default

NEC, 6 Ga THWN, and an outbuilding


Actually, having quickly played with a couple voltage drop calculators, I'm seeing a drop of only 3.1% at that distance, which should be acceptable. There could be other considerations I'm not familiar with though, so you might as well wait for one of the pros here to chime in.
McSteve is offline   Reply With Quote
Old 12-28-2010, 09:53 PM   #4
Newbie
 
Join Date: Dec 2010
Posts: 8
Rewards Points: 10
Default

NEC, 6 Ga THWN, and an outbuilding


Quote:
Originally Posted by McSteve View Post
You could go to a 60A breaker. However, at that distance, especially with a load like an A/C, voltage drop could be a problem. I don't know exactly how to calculate voltage drop, but I imagine that the electrician used a 50A breaker because that may be all the load the circuit can handle without the voltage dropping below acceptable levels.
Hey McSteve - thanks for the blazing rapid response.

I might add the 50 amp breaker was here when I got here. I have found a whole hose of questionable things in the house. I still talk to the previous owner and he explained his son in law did the electrical work (house built in 61, was gutted and renovated in 2001). When I say "questionable" I mean like - hot neutral in the outbuilding, live 12/2 run going across the attic with fully exposed wires at the end, polarity swapped in outlets, etc etc.

So, I would not assume there was any rhyme or reason for the 50 amp breaker.

Before everyone panics, I have been going through the house top to bottom checking everything and have actually mapped the entire electrical, so I feel I am getting on top of it and the house wont burn down anytime soon!

-lower32
lower32 is offline   Reply With Quote
Old 12-28-2010, 09:54 PM   #5
Member
 
Join Date: Jan 2009
Location: South of Boston, MA
Posts: 17,248
Rewards Points: 2,000
Default

NEC, 6 Ga THWN, and an outbuilding


Voltage drop for #6 wire with a full 60a load = 7.1v @240v
That's 3%, which is acceptable
So I don't see a problem with a 60A breaker
50a of power may have been all they needed
I ran a 60a sub to my pool cabana...may never need all that power
But not that much of a price difference between 50a & 60a breaker
Scuba_Dave is offline   Reply With Quote
Old 12-28-2010, 09:57 PM   #6
Member
 
Join Date: Dec 2010
Posts: 65
Rewards Points: 75
Default

NEC, 6 Ga THWN, and an outbuilding


You're fine feeding big with small. More expensive, but fine. Going the other way is bad, and the coordination of the overcurrent is a bit touchy becasue you'll blow the one in the basement when your AC and compressor both go on at the same time, maybe, instead of the one in the garage. But there's nothing really dangerous about it, just a pain.

Hot neutral? What do you mean by that?
JPraski is offline   Reply With Quote
Old 12-28-2010, 10:03 PM   #7
Newbie
 
Join Date: Dec 2010
Posts: 8
Rewards Points: 10
Default

NEC, 6 Ga THWN, and an outbuilding


Quote:
Originally Posted by JPraski View Post
You're fine feeding big with small. More expensive, but fine. Going the other way is bad, and the coordination of the overcurrent is a bit touchy becasue you'll blow the one in the basement when your AC and compressor both go on at the same time, maybe, instead of the one in the garage. But there's nothing really dangerous about it, just a pain.

Hot neutral? What do you mean by that?
"Hot neutral"... I think thats the proper term - as in 110 V from hot (black) to ground *and* 110 V from neutral (white) to ground. Isnt that technically 220? I didnt even think to check it, but began to wonder why I all my ballasts were failing.

-lower32
lower32 is offline   Reply With Quote
Old 12-28-2010, 10:17 PM   #8
Member
 
Join Date: Dec 2010
Posts: 65
Rewards Points: 75
Default

NEC, 6 Ga THWN, and an outbuilding


That's what I thought you meant. No, you're backfeeding the neutral, it doesn't really have 120V on it. It has the potential of the other bus, or phase, of the panel being read through the wire, basically, although you can't really use that potential. Kind of a 'ghost voltage' if you want to call it that.

Well.. unless you actually do have 220V from black to white. Then, yeah, I could see ballasts fail a lot. Make sure your 3 way switches are only wired from one side. (I mean, don't feed power to both switches. Someone I know did that, happened to get it on the 2 opposite phases, and turned 220v on his stairway lights when he threw both switches, but 110v hen he only threw one.)
JPraski is offline   Reply With Quote
Old 12-28-2010, 10:34 PM   #9
Newbie
 
Join Date: Dec 2010
Posts: 8
Rewards Points: 10
Default

NEC, 6 Ga THWN, and an outbuilding


Quote:
Originally Posted by JPraski View Post
That's what I thought you meant. No, you're backfeeding the neutral, it doesn't really have 120V on it. It has the potential of the other bus, or phase, of the panel being read through the wire, basically, although you can't really use that potential. Kind of a 'ghost voltage' if you want to call it that.

Well.. unless you actually do have 220V from black to white. Then, yeah, I could see ballasts fail a lot. Make sure your 3 way switches are only wired from one side. (I mean, don't feed power to both switches. Someone I know did that, happened to get it on the 2 opposite phases, and turned 220v on his stairway lights when he threw both switches, but 110v hen he only threw one.)
Hey JP - the problem above was in the outbuilding, no 3 way switch involved. Im a bit embarrassed to tell all how I figured this out - but basically, I flipped the switch for the light circuit off - not the breaker. I then went to replaced the failed ballasts (plural) and got zapped by the neutral! Yeah, I know, prob with the VOM first. The long and short of it is that at that point, I tore it all out, had the new panel put in, and all is now well. I dont think I would make a good lab rat, this isnt the first time I have zapped myself assuming all was well without probing with the VOM first! Especially considering all the other "odd" things I have found around here.

Thanks so much for the response - I think I am going to like it here!

-lower32
lower32 is offline   Reply With Quote
Old 12-28-2010, 10:38 PM   #10
Member
 
Join Date: Jan 2009
Location: South of Boston, MA
Posts: 17,248
Rewards Points: 2,000
Default

NEC, 6 Ga THWN, and an outbuilding


A light switch can just be a hot loop
With full power going to the light/ballast
So shutting the switch off turns the light off
But may not kill power in the enclosure
Scuba_Dave is offline   Reply With Quote
Old 12-28-2010, 10:41 PM   #11
Mad Scientist
 
Join Date: Dec 2009
Location: Minnesota
Posts: 775
Rewards Points: 518
Default

NEC, 6 Ga THWN, and an outbuilding


A word of advice about those cluster- houses with layer upon layer of screwy electrical work: Get a decent non-contact voltage detector, and learn to wave it at everything before you touch it. I keep mine handy in my shirt pocket or even clipped to the end of my sleeve. Wiring at my buddy's house is so screwed up I wouldn't even flush the toilet without waving a voltage detector at it first...
McSteve is offline   Reply With Quote
Old 12-28-2010, 10:48 PM   #12
Member
 
Join Date: Dec 2010
Posts: 65
Rewards Points: 75
Default

NEC, 6 Ga THWN, and an outbuilding


Yeah, like Dave said, I never even considered a switched neutral. And I like what McSteve said- wonder if he has the same electrician who wired his buddy's house as did my buddy's house?

but it can still be a backfed neutral. Bascially, if you have absolutely balanced load, the neutral carries no current. None at all, at least at the panel. (Of course it does on a branch circuit.) When it's unbalanced, it carries current. This unbalance will read as hot against ground, and both phases. It's definitely enough to zap you, though. It can do it with the breaker off, even. It's usually not too bad unless there's a big imbalance, though. Which there generally starts to happen when you start flipping breakers, so be careful.
JPraski is offline   Reply With Quote
Old 12-28-2010, 10:50 PM   #13
Newbie
 
Join Date: Dec 2010
Posts: 8
Rewards Points: 10
Default

NEC, 6 Ga THWN, and an outbuilding


Quote:
Originally Posted by McSteve View Post
A word of advice about those cluster- houses with layer upon layer of screwy electrical work: Get a decent non-contact voltage detector, and learn to wave it at everything before you touch it. I keep mine handy in my shirt pocket or even clipped to the end of my sleeve. Wiring at my buddy's house is so screwed up I wouldn't even flush the toilet without waving a voltage detector at it first...
Thanks McSteve - advice duly noted and I have invested in one! Like I sad above, when I started to figure out what I was dealing with, I first stepped back and "mapped" all the circuits to the respective breaker. The sig other still pops a breaker on occasion as previous owner (dweeb) has some circuits *highly* loaded and others with near insignificant demand. I have been moving things around to disperse the loads better. GF still occasionally pops a breaker when she runs the heated ceiling fan, space heater, and vacuum all at the same time, but at least I think I have it all safe now.
lower32 is offline   Reply With Quote
Old 12-28-2010, 10:53 PM   #14
Member
 
Join Date: Jan 2009
Location: South of Boston, MA
Posts: 17,248
Rewards Points: 2,000
Default

NEC, 6 Ga THWN, and an outbuilding


I had the same problem when I moved in here
2 circuits that were overloaded
....then a 15a circuit that had.....just the front doorbell transformer
Scuba_Dave is offline   Reply With Quote
Old 12-28-2010, 11:04 PM   #15
Newbie
 
Join Date: Dec 2010
Posts: 8
Rewards Points: 10
Default

NEC, 6 Ga THWN, and an outbuilding


Quote:
Originally Posted by Scuba_Dave View Post
I had the same problem when I moved in here
2 circuits that were overloaded
....then a 15a circuit that had.....just the front doorbell transformer
I am going to "one up ya" on that!!

I have a 20 amp breaker in the panel, with of course 12/2 on it. It was the sole remaining breaker I couldnt figure out where it went. I traced it from the garage, through the basement floor, under and *through* the kitchen cabinets, then lost it as it went up towards the attic. I then just switched it off to see what wasnt working.

One year later, I still dont know where it goes or what it is hooked up to, if anything!

Egawds, the panel is 200 amp service, all breaker spots are full, and it is only a 1500 sq foot house.

The popular "theory" is that it was run for a trash compactor or something of the like, but then why does it run up into the attic? rhetorical question. (I havent been able to find it in the attic yet, I dont like digging through that insulation on my belly)

-lower32

Advertisement

lower32 is offline   Reply With Quote
Reply


Thread Tools
Display Modes

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Ampacity of #6 THWN feeder McSteve Electrical 9 05-23-2010 11:57 AM
THNN Vs. THWN Wire indy Electrical 15 11-13-2009 11:12 PM
Easy THWN question Piedmont Electrical 5 10-09-2009 05:27 PM
Transition from NM to THWN Miranda7 Electrical 17 05-26-2009 06:43 AM
Outbuilding wiring and underground wiring questions tpagel Electrical 7 07-08-2008 09:38 AM




Top of Page | View New Posts