Yet another sub panel question

[NHBandit]

Getting ready to make my connection from the house to the garage sub panel. I have already learned that I should not have the neutral and the ground tied together in the sub panel. Got a seperate grounding bar and installed it in the box and threw the green screw that came with the box into the woods as far as I could throw it. So now I'm looking at the man panel in the house and they are tied together. White wires & bare ground wires all going to the same bar.. WTF.. I have a 100 amp breaker I bought for the main panel. I am using 4 wire MHF to feed the garage. When I attatch the wires in the main panel I assume I simply attatch the 2 black wires to that breaker. I assume I will need to go buy a couple of lugs for the ground and the neutral wires ? When I attatch those 2 wires to the bar in the main panel aren't I effectively tying the ground & the neutral together ? Do I need to worry about getting zapped when I attatch the neutral even though I will have the main breaker turned off ? Please no long drawn out questions about wire size, etc. from the guys who just love to get all technical & ****. I have already insured that the wire is correct, the sub panel is wired correctly, etc. I know it's alot to ask but a simple answer is all I need at the moment. And yes... I already tried doing a search.... Thanks.

 

[sparky36000]

The neutral and grounds are all bonded together at your house service panel, that is correct. Keep them separate at your sub panel. Yes, you can still get shocked if your main is off, lugs going into the main breaker are still live, be careful with your bare ground wire, but your neutral bus won't shock you. Two blacks to your 100a breaker, white and bare to neutral bus at house panel, separate buses at you sub.Depending on you panel brand, some neutral buses have holes for up to #2, some only to #6. You may need to find adapter lugs if yours doesn't fit. Hope this helps.

 

[NHBandit]

The neutral and grounds are all bonded together at your house service panel, that is correct. Keep them separate at your sub panel. Yes, you can still get shocked if your main is off, lugs going into the main breaker are still live, be careful with your bare ground wire. Two blacks to your 100a breaker, white and bare to neutral bus at house panel, separate buses at you sub.Depending on you panel brand, some neutral buses have holes for up to #2, some only to #6. You may need to find adapter lugs if yours doesn't fit. Hope this helps.

Thanks sparky. Just what I needed. The confusing part is the ground & the neutral being tied together in the main panel. I'm still not sure why it matters that they are seperate in the sub panel since they are tied together on the other end of the same wires. I am using #2 wire so I'll have to go look again to see if there are holes that big in the bar.

 

[sparky36000]

Thanks sparky. Just what I needed. The confusing part is the ground & the neutral being tied together in the main panel. I'm still not sure why it matters that they are seperate in the sub panel since they are tied together on the other end of the same wires. I am using #2 wire so I'll have to go look again to see if there are holes that big in the bar.

You wanted the simple answer, so you'll just have to trust me that it's the right way to do it without going into the technical stuff. good luck and be careful

 

[Gary S]

The reason for the ground and neutral being tied together in the main and separate in the sub is "ground loop".
http://en.wikipedia.org/wiki/Ground_loop_(electricity)

It isn't something you need to understand or worry about. Just know that the NEC was written by someone who understands it and knows how to make things work the best.

 

[NHBandit]

You wanted the simple answer, so you'll just have to trust me that it's the right way to do it without going into the technical stuff. good luck and be careful

touche' LoL.. I looked and the holes in the bar aren't big enough for the #2 wires. Guess it's back to Lowes yet again. It's getting to the point where if I miss going there for a couple days they call to make sure I'm ok... Next up is figuring out how to make a neat hole in the concrete block foundation of my house to run the wire through and how to seal it up.

 

[hh76]

Thanks sparky. Just what I needed. The confusing part is the ground & the neutral being tied together in the main panel. I'm still not sure why it matters that they are seperate in the sub panel since they are tied together on the other end of the same wires. I am using #2 wire so I'll have to go look again to see if there are holes that big in the bar.

If they were tied together at both ends, they would paralleled. That means that any current that normally travels on the neutral wire would be shared with the ground wire.

Think about the wires as two different paths to the same point. Electrons starting at the neutral in the sub panel are forced to take the neutral path, and if they start at the ground buss in the sub panel, they are forced to take the ground path. If you connect the neutral and ground at the sub panel, you essentially connect the starting points for the paths. You give the electrons a choice of paths, and they will split up between the two. You don't want current on the ground path.

 

[pattenp]

By NEC, if you are using #2 aluminum MHF the feed breaker can only be a max of 90A, not 100A. Also the MHF needs to be in conduit inside the structure.

 

[NHBandit]

By NEC, if you are using #2 aluminum MHF the feed breaker can only be a max of 90A, not 100A. Also the MHF needs to be in conduit inside the structure.

Thanks. I guess when I go to Lowes later I'll swap the breaker for a 90A as well. It's cool. The girl at the returns desk knows me pretty well too..

 

[pattenp]

Thanks. I guess when I go to Lowes later I'll swap the breaker for a 90A as well. It's cool. The girl at the returns desk knows me pretty well too..

You'll be very lucky if Lowes has a 90A breaker. What they have usually jumps from 60A then to 100A.

 

[larry_g]

One other small point is to make sure that the ground bar is bonded to the box/frame of the panel, so you may have to go find that green ground screw. Also check that the neutral bar is isolated from the box/frame of the panel before any wires are attached.

lg
no neat sig line

 

[nwav8tor]

If your garage is detached from the house, have you installed grounding rods for the sub-panel?

 

[Alchymist]

One other small point is to make sure that the ground bar is bonded to the box/frame of the panel, so you may have to go find that green ground screw. Also check that the neutral bar is isolated from the box/frame of the panel before any wires are attached.

lg
no neat sig line

The ground bars are typically bonded by their mounting screws. The green screw is for grounding the neutral when needed.

 

[theoldwizard1]

If they were tied together at both ends, they would paralleled. That means that any current that normally travels on the neutral wire would be shared with the ground wire.

True, but this is not the real problem.

No 2 pieces of wire are exactly a like and neither is their resistance. We may only be talking fractions of ohms, but those parallel wires are different. The current will not split up evenly and ultimately you will have a "difference in potential" (that is the scientific way of saying "voltage"). Connecting them together completes the circuit and you have current flowing (a "ground loop").

Ground loops can be from milivolts to many volts and cause interference with electronic equipment.