Yet Another Sub Panel Question

[fotoflojoe]

Guys,

Fairly soon, I'll be installing a sub panel. After much research and reading, I've gotten everything figured out - almost.

- Main service in house is 200 amps
- Sub panel will be 100 amp
- The garage is 30 feet from the house
- Pulling 3 #3 THWN and 1 #CU Ground through 1.5" schedule 40 conduit
(Conduit is already in place)
- Burying 2 8 foot ground rods within six feet of sub panel

For me, the devil is in the details:
Where cable enters the house, does it need to pass through a junction box, or can the cable come through the LB fitting/joist and then be directly fastened along a joist?

Does the cable need to enter the garage above the sub panel? Or, can it come in below and be routed up into the panel?

Haven't been able to find any information on these points. They might seem like silly questions to some, but they're the kind that drive me nuts.

 

[Steve in Mi]

Guys,

Fairly soon, I'll be installing a sub panel. After much research and reading, I've gotten everything figured out - almost.

- Main service in house is 200 amps
- Sub panel will be 100 amp
- The garage is 30 feet from the house
- Pulling 3 #3 THWN and 1 #CU Ground through 1.5" schedule 40 conduit
(Conduit is already in place)
- Burying 2 8 foot ground rods within six feet of sub panel

For me, the devil is in the details:
Where cable enters the house, does it need to pass through a junction box, or can the cable come through the LB fitting/joist and then be directly fastened along a joist?

Does the cable need to enter the garage above the sub panel? Or, can it come in below and be routed up into the panel?

Haven't been able to find any information on these points. They might seem like silly questions to some, but they're the kind that drive me nuts.

Wire meant to be run in conduit SHALL run in conduit. Another LB in the wall cavity and conduit into the panel from there.

Above or below entry - either is fine, take the easy route.

 

[jamm]

Wire meant to be run in conduit SHALL run in conduit. Another LB in the wall cavity and conduit into the panel from there.

Above or below entry - either is fine, take the easy route.

X2

 

[fotoflojoe]

Awesome. Thanks guys!

 

[dipper]

keep the wire in conduit all the way to the panel.

and don't forget to keep the neutrals and grounds seperate when you tie in the new circuits at the subpanel.

 

[fotoflojoe]

Thanks again.

Hey Dipper, I just spent the last hour perusing your garage build. That's some great stuff there! Congratulations!

 

[tdkkart]

and don't forget to keep the neutrals and grounds seperate when you tie in the new circuits at the subpanel.

Just a general question:
Why would the neutrals and grounds be required to be seperated in the subpanel if they are not in the main house panel??

What's good for the goose isn't for the gander??

 

[eddiethekub]

Just a general question:
Why would the neutrals and grounds be required to be seperated in the subpanel if they are not in the main house panel??

What's good for the goose isn't for the gander??

NO!. The only place where the neutral & ground meet is at the main panel.

 

[ddawg16]

NO!. The only place where the neutral & ground meet is at the main panel.

Correct per the NEC code. The best reason I can figure out is for proper ground reference.

Basically, when the neutral comes to your house, for the most part, it is floating. At the house you are going to tie it to earth ground. That sets up the reference so that each 120 leg is actually 120 Volts to ground.

If you tie the neutral to earth ground at the garage....if there is any differance in ground potential between the garage and the house, you are now going to have a ground loop between the house and garage.....in other words, the neutrals could have a slight difference in potential between the house and the garage....I could see how under certain conditions that this could cause enough difference in potential to be a hazard....this is more commonly called a ground loop.

By keeping the ground seperate from the neutral at the garage, you maintain proper potential between the 120's and neutral. You are still safe in your garage because ALL of your outlets are GFI'd....so if there is a ground fault, the GFI trips and you are ok.

 

[tdkkart]

By keeping the ground seperate from the neutral at the garage, you maintain proper potential between the 120's and neutral. You are still safe in your garage because ALL of your outlets are GFI'd....so if there is a ground fault, the GFI trips and you are ok.

Heh, this is where it gets better, see my double tapped breaker/home inspector thread.
My house has a 200amp panel, installed who knows when, most likely when the house was built in 1976.
The garage subpanel is fed by a 60amp breaker from the house. This panel was likely installed sometime after the house was built, but before I bought it in 1995. There is no ground at the subpanel, it is only tied back to the house panel via the neutral. Grounds and neutrals tied together in the subpanel. No seperate ground lug in the panel.

The home inspector wrote up the panel as improperly wired, says the neutrals and the grounds have to be seperated, which of course requires that I go get a buss bar, a hunk of ground wire and a ground rod. No big deal if it'll keep them off my back.

All of this, and he didn't say anything about there being not one GFI outlet in the garage or kitchen, nor the 1/2 dozen outside outlets.......

 

[sajohnson]

Heh, this is where it gets better, see my double tapped breaker/home inspector thread.
My house has a 200amp panel, installed who knows when, most likely when the house was built in 1976.
The garage subpanel is fed by a 60amp breaker from the house. This panel was likely installed sometime after the house was built, but before I bought it in 1995. There is no ground at the subpanel, it is only tied back to the house panel via the neutral. Grounds and neutrals tied together in the subpanel. No seperate ground lug in the panel.

The home inspector wrote up the panel as improperly wired, says the neutrals and the grounds have to be seperated, which of course requires that I go get a buss bar, a hunk of ground wire and a ground rod. No big deal if it'll keep them off my back.

All of this, and he didn't say anything about there being not one GFI outlet in the garage or kitchen, nor the 1/2 dozen outside outlets.......

The grounds and neutrals in the sub panel definitely are supposed to be on separate buses (ground bus connected to the panel, neutral bus isolated from the panel).

You do not need a ground rod(s) for the sub panel as long as you have a proper gauge grounding conductor connecting the ground bus in the sub panel to the ground bus in the main panel.

That said -- more grounds never hurt, as long as they're tied together.

At my house I have at least seven (7) different grounds -- all connected per the NEC with #6 copper. The best is the well casing -- 60 feet of 6" steel pipe! I actually saw a spark when I connected it to the rest of the ground system! The main house has three ground rods -- one where the coax cables enter, and two (one old, one I installed) for the main panel. The antenna tower has one, and so does our little "guest house". I also connected the grounding system to the cold water pipe.

The sub panel in the garage is fed from a 100 amp breaker and I used tri-plex with along with a fourth grounding conductor (they didn't have quad-plex or whatever it's called). I may install one or two ground rods at the garage, I just never got around to it. I've actually got a few ground rods in the garage along with the clamps and wire.

Something to keep in mind -- because soils vary from one location to another and some are very dry, the typical 8' ground rod often isn't really a very good ground. The resistance to Earth ground is often varies quite a bit. That's why the NEC specifies that all grounds must be tied together -- because otherwise with a lightning strike one ground might look 'better' (lower resistance) than another. Lightning traveling along coax might look at the ground on the other side of the house (if that's where the main panel is) and decide that it looks better than the ground rod right there at the coax entrance. **** -- there goes lots of expensive equipment (or at least a surge suppressor). I've thought about buying a ground resistance tester but the good ones are very expensive.

Regarding keeping grounds and neutrals separate in sub panels, the reason is that grounding conductors are meant to only carry fault current. If the grounds and neutrals are connected at the sub panel, current flowing thru the neutral(s) (grounded conductor) would have two paths back to the main panel -- the neutral and the grounding conductor. Under normal circumstances, a ground wire should not carry any current. At least that's how I understand it.

There are some cases where a dedicated ground isn't required. For example, wiring for ranges. For homes built prior to 1993 (I think), the pig tail can have just three (3) conductors -- two hot, one neutral (no dedicated ground). The neutral is used for the ground. The reasoning is that if the load is 240 v only, the neutral will not carry any current under normal conditions. Even with both 240 v and 120 v loads, if the 120 v loads are balanced (each phase has the same load), the neutral will not carry any current. I'm not going to attempt to explain why -- I think it's magic! I really don't quite understand, but the current flowing to the two loads must cancel out.

 

[Aceman]

Heh, this is where it gets better, see my double tapped breaker/home inspector thread.
My house has a 200amp panel, installed who knows when, most likely when the house was built in 1976.
The garage subpanel is fed by a 60amp breaker from the house. This panel was likely installed sometime after the house was built, but before I bought it in 1995. There is no ground at the subpanel, it is only tied back to the house panel via the neutral. Grounds and neutrals tied together in the subpanel. No seperate ground lug in the panel.

The home inspector wrote up the panel as improperly wired, says the neutrals and the grounds have to be seperated, which of course requires that I go get a buss bar, a hunk of ground wire and a ground rod. No big deal if it'll keep them off my back.

The home inspector is mistaken. The code is not retroactive, up until the 08 code took effect 3-wire feeds to outbuildings were allowed. Your installation was code compliant when it was installed.

For the record, you cannot create a "ground" by just driving a ground rod and seperating the grounds and neutrals in the subpanel.

 

[tdkkart]

The home inspector is mistaken.

Wouldn't be the first time either, get this.

My air conditioning unit has a data plate on the side of it which says 20amp.
Doesn't say if that's running or starting, just 20 amp.

The inspector wrote it up because it's connected to a 30 amp breaker.
30 amp circuit with 10 gauge romax running in the open in the basement.
Says the breaker should be 20 amp??

No problem, I pulled the 30 amp breaker and put in a 20, to bad they likely won't have AC for the first couple hot days next spring cuz I can guarantee that the unit pulls more than 20 amps on startup. Oh well, not my problem.

I can see not wanting it on a 50amp circuit, but a bit extra insurance isn't gonna hurt anything.
Does this mean that our washer that draws 10 amps has to be on a 10 amp breaker?? What breaker does the electric dryer that draws 28amps need??How about the OL's vacuum that draws 5 or 6 amps??

Goober............


BTW, he didn't write up the 2 double tapped breakers in the garage panel.........

 

[kapena]

There are some cases where a dedicated ground isn't required. For example, wiring for ranges. For homes built prior to 1993 (I think), the pig tail can have just three (3) conductors -- two hot, one neutral (no dedicated ground). The neutral is used for the ground. The reasoning is that if the load is 240 v only, the neutral will not carry any current under normal conditions. Even with both 240 v and 120 v loads, if the 120 v loads are balanced (each phase has the same load), the neutral will not carry any current. I'm not going to attempt to explain why -- I think it's magic! I really don't quite understand, but the current flowing to the two loads must cancel out.

I'm not an electrician, but my understanding of the 240 v current flow is that each of the two 120 v feed lines has a different phase. Since they are different phases, they can use each other's feed line as a return line to complete the circuit. Being in different phases allows both currents to go opposite directions on the same line without interfering with each other.

 

[sajohnson]

I'm not an electrician, but my understanding of the 240 v current flow is that each of the two 120 v feed lines has a different phase. Since they are different phases, they can use each other's feed line as a return line to complete the circuit. Being in different phases allows both currents to go opposite directions on the same line without interfering with each other.

That's correct. The transformer on the utility pole is center tapped, with the center being the neutral and ground and the hots at each end, 180 degrees out of phase (when one is at +120 vrms the other is at -120 vrms with respect to the center tap).

What I'm not clear on is how the current flow on the neutral wire from any 120 volt loads cancels out (if the loads are matched, say 1Kw on each phase).

For example, lets say there are two 1,200 w loads, one on each phase, each pulling 10 amps, each using the same neutral (as with the typical 3 wire pigtail). Instead of the neutral carrying 20 amps, it ends up being zero (0) amps!

 

[BigChevy80]

That's correct. The transformer on the utility pole is center tapped, with the center being the neutral and ground and the hots at each end, 180 degrees out of phase (when one is at +120 vrms the other is at -120 vrms with respect to the center tap).

What I'm not clear on is how the current flow on the neutral wire from any 120 volt loads cancels out (if the loads are matched, say 1Kw on each phase).

For example, lets say there are two 1,200 w loads, one on each phase, each pulling 10 amps, each using the same neutral (as with the typical 3 wire pigtail). Instead of the neutral carrying 20 amps, it ends up being zero (0) amps!

Your service transformer has a single 240v winding in it. The neutral wire is attached to the exact center of the winding, which splits it into two 120v windings. So you have exactly half the resistance from either side to neutral.

If both legs of the transformer have the same load on them, it is basically the same as them being wired together in series across the 240v winding. Since they both are drawing the same amount of power and have the same resistance, the current flows from one leg, through the loads and directly out the other. If one side has a lower resistance than the other, the difference in current will flow through the neutral because there is nowhere else for it to go.

Sounds complicated, but it's actually pretty simple... I think.

 

[Torque1st]

For safety use a Neutral AND a Ground back to the main panel. If the Neutral should become disconnected between the two panels with only a ground rod at the subpanel for ground very high voltages and fluctuating voltages can appear on the Neutral circuit of the subpanel. This can cause equipment failure and expose the homeowner to lethal voltages on hot chassis equipment with those nice polarized plugs or equipment with a short to a poor "ground".

Lightning induced transients either direct or power line coupled are also a major problem.

 

[sajohnson]

For safety use a Neutral AND a Ground back to the main panel. If the Neutral should become disconnected between the two panels with only a ground rod at the subpanel for ground very high voltages and fluctuating voltages can appear on the Neutral circuit of the subpanel. This can cause equipment failure and expose the homeowner to lethal voltages on hot chassis equipment with those nice polarized plugs or equipment with a short to a poor "ground".

Lightning induced transients either direct or power line coupled are also a major problem.

I'm not sure I follow.

If the neutral wire between the main and the sub panel were to become disconnected, there would be no return path and any 120 volt loads would simply not work. In that situation, it wouldn't matter if there was a ground wire between the two panels and/or a ground rod for the sub panel -- since the neutrals and grounds are isolated from each other.

The main reason I used a dedicated grounding conductor and a neutral between the panels is that the grounding system at the house is a far better ground than even two or three ground rods at the garage would be (of course, adding more ground rods for the sub panel wouldn't hurt -- they would become part of the grounding system).

I'm not sure how disconnecting the neutral feeding a sub panel would cause a hazardous situation. Even with a ground fault (hot wire shorted to a metal case) where the resistance to ground is so high that the fault current will not trip the breaker, having the neutral connected would not help, since it is completely isolated from the ground wires.

One scenario where it would matter would be if the chassis were both grounded _and_ connected to neutral. In that case, with a poor (or no) ground, the neutral feeding the sub panel would provide a path to ground back at the main panel.

I think it is important to emphasize that the resistance to ground with the typical 8' ground rod can be very high -- especially with dry sandy soil. Also, even if it is initially low, it may very well increase over time due to corrosion. Depending on soil conditions, even having a rod at the garage and a ground wire connecting it to the main panel ground may not be sufficient. For anyone so inclined, there are ways to lower the resistance:

http://www.ees-group.co.uk/downloads/Better Grounding.pdf
http://www.arema.org/eseries/script...library/2000_Conference_Proceedings/00057.pdf
http://www.iaei.org/subscriber/magazine/01_d/johnston.htm
http://www.iaei.org/subscriber/magazine/05_e/johnston.htm
http://www.mikeholt.com/onlinetraining/page_images/1113918256_2.jpg

The above are just a couple examples -- there is undoubtedly even better info out there. The illustrations at the last link are good.

 

[BigChevy80]

If the neutral wire between the main and the sub panel were to become disconnected, there would be no return path and any 120 volt loads would simply not work. In that situation, it wouldn't matter if there was a ground wire between the two panels and/or a ground rod for the sub panel -- since the neutrals and grounds are isolated from each other.

When the subpanel loses it's neutral connection, any circuits with loads on them will feed current back through the neutral side of other circuits. Since both legs feed back through the same neutral, you have the potential to have up to 240v feeding back through a neutral wire. You also have the possibility of feeding 240v through the 120v circuits, most likely destroying anything connected to them.

Since the neutral connection is broken, the current will try to return through any means possible. Through the neutrals of other circuits, through the loads on both busses of the panel and out to the other side of the transformer winding.

 

[sajohnson]

When the subpanel loses it's neutral connection, any circuits with loads on them will feed current back through the neutral side of other circuits. Since both legs feed back through the same neutral, you have the potential to have up to 240v feeding back through a neutral wire. You also have the possibility of feeding 240v through the 120v circuits, most likely destroying anything connected to them.

Since the neutral connection is broken, the current will try to return through any means possible. Through the neutrals of other circuits, through the loads on both busses of the panel and out to the other side of the transformer winding.

A couple thoughts:

1) This is may be one of those cases where a picture is worth a thousand words.

2) Having a fourth (grounding) conductor would not help, since it should be isolated from the neutral(s).