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-   -   NEC, 6 Ga THWN, and an outbuilding (http://www.diychatroom.com/f18/nec-6-ga-thwn-outbuilding-90679/)

lower32 12-28-2010 08:38 PM

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

McSteve 12-28-2010 08:45 PM

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.

McSteve 12-28-2010 08:51 PM

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.

lower32 12-28-2010 08:53 PM

Quote:

Originally Posted by McSteve (Post 558354)
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

Scuba_Dave 12-28-2010 08:54 PM

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

JPraski 12-28-2010 08:57 PM

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?

lower32 12-28-2010 09:03 PM

Quote:

Originally Posted by JPraski (Post 558370)
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

JPraski 12-28-2010 09:17 PM

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.)

lower32 12-28-2010 09:34 PM

Quote:

Originally Posted by JPraski (Post 558394)
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

Scuba_Dave 12-28-2010 09:38 PM

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

McSteve 12-28-2010 09:41 PM

A word of advice about those cluster-:censored: 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...

JPraski 12-28-2010 09:48 PM

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. ;)

lower32 12-28-2010 09:50 PM

Quote:

Originally Posted by McSteve (Post 558419)
A word of advice about those cluster-:censored: 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.

Scuba_Dave 12-28-2010 09:53 PM

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

lower32 12-28-2010 10:04 PM

Quote:

Originally Posted by Scuba_Dave (Post 558433)
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


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