need to tie grounds together in a outlet box

Bill D · May 5, 2015

In a outlet box with separate cables, each cable with it's own ground wire, for 240 volt and 120 volt circuits supplied by 20 and 30 amp breakers. Do I need to tie all the grounds together then tee off to each outlet or is it good practice to just run each ground separately to the single outlet it supplies. Does it make a difference if the box is metal or plastic.
I guess they have to run separate grounds to each outlet if they are isolated ground outlets.
Bill D.

technocrat · May 5, 2015

tie them together - this is the basic concept of a Main Earth Neutral system.

SAF · May 5, 2015

Bill, Tie ALL the cable grounds together. Provide a pigtail for each device (outlet, switch)and one for grounding the metal box if you have one. If you needed an isolated ground for a piece of equipment, it should be separate and insulated green and not connected to the common ground anywhere except the source panel and the load. Providing a dedicated and isolated ground doesn't relive the requirement for the common ground for the rest of the devices and metal box.

SAF Ω

Winterfalke · May 5, 2015

You can buy solid copper wire with a loop at the end around a green grounding screw just for installing in boxes and tying all the grounds together. If it is a metal box, it should be tied into the ground and there should be an appropriate sized hole in the back just for that screw. If it is plastic, you just nut them together. I'm pretty sure metal boxes tied together with metal conduit don't need to be grounded individually, but your local laws may be different. Some places do screwy things, it's best to check your local codes.

adama · May 5, 2015

Over here that would not meet code, not because of the grounding, but because of the risk of a fault on say one of the higher voltage wires over voltage - over current if it shorted to one of the lower power circuits.

steve-l · May 5, 2015

technocrat said:
tie them together - this is the basic concept of a Main Earth Neutral system.

NEVER at the service point and only once in a system, usually at the power entrance point. Commonly called the equalization bus. Ground is not ground the world around. Violation of this rule creates ground loops. Electric distribution is not instantaneous. Propagation takes approximately 2 Nano seconds per foot of path. Paths are rarely equal.

sable · May 5, 2015

adama said:
Over here that would not meet code, not because of the grounding, but because of the risk of a fault on say one of the higher voltage wires over voltage - over current if it shorted to one of the lower power circuits.

I think we are talking about two lines of 120V to make the 240V rather than lines of different voltage in the same box.

crossthread · May 5, 2015

Ground loops are often misunderstood. They can make your life very interesting to say the least. I think most of the time the misunderstanding is from the same folks that say "it's got a short in it" when something doesn't work. A lot of the time the problem is that it has a "long" in it (the circuit isn't complete). I have seen industrial wire troughs where over 200 volts AC is riding on a 24 volt DC line due to lack of shielding and ground loops. Makes for a fun day. I guess the best rule of thumb is never connect a shield at both ends and make sure your grounds are in order. And NO, ground is not ground the world around. My first job was troubleshooting variable speed drives on tobacco converting machinery. The first one I had to put a scope on to trace drive pulses taught me an interesting lesson. Ground was at 380 volts DC and the scope probe was of course at ground. Needless to say it pretty much blew the end off the probe. I never used a scope again without cutting off the ground lug on the power cable. This was in the early 70's and there are devices now that lift the ground but back them you just took a pair of pliers and cut off the ground lug. I am pretty sure that here in VA you have to not only twist the ground wires together but put a wire nut on them as well. You might even have to tape up the connection on top of the wire nut like you do with hot and neutral.

WizardOfBoz · May 5, 2015

sable said:
I think we are talking about two lines of 120V to make the 240V rather than lines of different voltage in the same box.

Adama, Sable's right. Home wiring here normally comes in as a [added]split single phase [deleted partial "wye"]: two hot legs (240 V difference) and a neutral (each hot leg to neutral is 120V). The hot legs come in through two breakers, physically connected, as a service disconnect. Each hot leg is connected to a buss, one on the left and one on the right of the box. A breaker is attached to either one bus and neutral (120V), or across both busses (240V). One connects about the same number of 120V circuits to each buss to balance things a bit. So you kind of have to have both voltages in the same box. Or three separate boxes (240, 120 Phase A and 120 Phase B).

To the OP's point, there usually is a long square bar with wire holes and set (grub) screws to clamp ground and neutral wires. One or more buss bar for each.

Pic below. The colors (black for both hot and neutral) is confusing, but it gives you the idea:

gbent · May 5, 2015

bosleyjr said:
Home wiring here normally comes in as a partial "wye": two hot legs (240 V difference) and a neutral (each hot leg to neutral is 120V).

Isn't it a partial center tapped delta? Partial wye would be two legs at 208 leg to leg, giving 120v to ground.

Mark Rand · May 5, 2015

Actually, it's centre-tapped single phase. There's no phase difference between hot1-neutral and neutral-hot2.

When you get a three phase supply it's a bastard cross, with a neutral that's not in the middle.

Apologies for any offence caused if folks have started to get symmetrical supplies these days.

WizardOfBoz · May 5, 2015

Ok, I know a lot of theory, and I've added and deleted services to my house, but I realized that I wasn't sure about wye or delta. I accept that my "wye" surjection is wrong. I had to look this up to learn something. Turns out it's called "Split Phase" wiring. It derives from the center-tapped single phase Mark noted. The idea is that you only have to run two conductor single phase to a distribution transformer (saving a conductor over three phase) and then you use a center-tapped transformer to deliver either high voltage (across the whole transformer output) or low voltage (the center tap to either of the other conductors). gbent, your question bugged me: I recognized that if there were phases involved the 240 to 120 ratio was problematic. Mark is correct: with the center-tapped split phase, the 2:1 ratio is a natural outcome.

If you have a split phase 240/120 volt system, and add another leg that is 120° in phase different from both original hot legs, you have a "hot leg" or "red leg" delta. This gives you 3 and single phase 240, and also 120V. Very cool.

I beg Mark's pardon as I've translated English English to whatever it is we speak here in the colonies. (Centre to center).

Thanks, gbent and Mark.

gbent · May 5, 2015

Mark Rand said:
Actually, it's centre-tapped single phase. There's no phase difference between hot1-neutral and neutral-hot2.

Doesn't there have to be a phase difference between hot1 and hot2? If there was no phase difference, the voltages wouldn't be additive would they?

Mark Rand · May 5, 2015

gbent said:
Doesn't there have to be a phase difference between hot1 and hot2? If there was no phase difference, the voltages wouldn't be additive would they?

I meant that hot1-neutral is identical in phase to neutral-hot2. Note the order of the wires in that sentence

. Also note that "hot1-neutral" would be interpreted as the "hot1 voltage waveform minus the neutral voltage waveform". So they add up like boseyjr says. That's for the normal domestic/retail 120+120=240 system.

If it's 208/120, then that's a true symmetrical three phase setup.

I just found a lovely set of diagrams here that explain all the variations you chaps get to foreigners like me.

Bill D · May 5, 2015

OP here: This is single phase but supplied by different breakers. I do know that neutral is tied to ground only once at the main service entrance. I understand that ground should only be tied to a a grounding rod at the main service entrance. You should not add another grounding rod for a detached shop. (But I have read that some huge machines recommend a dedicated ground rod next to the machine)I guess it helps to tie grounds together so if you are touching a grounded metal machine and a power tool at the same time they are at exactly the same potential. That would include any static being generated by a moving machine.
That is why I asked about tying grounds together once past the main entrance panel. Here in the USA there are only three, or four, wire colors for single phase power. green or bare for ground, white for neutral, and any other color is a hot leg.
Bill D.

I used to know why you do not tie neutral and ground together after the main entrance but I have forgotten why. Now I just remember it should never be done for safety reasons.

sfriedberg · May 5, 2015

Bill D said:
I used to know why you do not tie neutral and ground together after the main entrance but I have forgotten why. Now I just remember it should never be done for safety reasons.

The neutral is a current-carrying conductor, and can be at a significant potential above ground due to line resistance and/or imbalanced split-phase or multi-phase loads. In fact, you should assume it's never at ground potential, and is therefore not safe to touch.

The equipment grounding conductor (safety ground) does not carry current under normal circumstances. Through the process of bonding, all exposed metal surfaces of equipment and raceways (conduit) are held at ground potential and are therefore safe to touch.

There are page after page on this topic in the NEC. So that's obviously just a nutshell summary.

kb1cjp · May 5, 2015

I assume the real question the OP is asking is; he has one dual gang box with a 20amp 115v outlet and a 30amp 240v outlet. The two circuits are feed from two separate breakers and two seperate feed wires. As a result both feed cables have their own ground conductor, both going to the same ground bus in the load center.

As a result he wants to know if he should connect the two incoming grounds lines together.

I don't know the answer to the question but I assume the OP is conserned about making a ground loop by tieing the two ground cables together.

Brian

Bill D · May 5, 2015

kb1cjp said:
I assume the real question the OP is asking is; he has one dual gang box with a 20amp 115v outlet and a 30amp 240v outlet. The two circuits are feed from two separate breakers and two seperate feed wires. As a result both feed cables have their own ground conductor, both going to the same ground bus in the load center.

As a result he wants to know if he should connect the two incoming grounds lines together.

I don't know the answer to the question but I assume the OP is conserned about making a ground loop by tieing the two ground cables together.

Brian

my question exactly.
bill d

sfriedberg · May 5, 2015

Bill D said:
my question exactly.

Tie all the grounds (green) together, and bond the grounds to the outlet box if it's metallic. Do not tie the neutrals (white) together in an outlet box.

kingmt01 · May 5, 2015

Bill D said:
OP here: This is single phase but supplied by different breakers. I do know that neutral is tied to ground only once at the main service entrance. I understand that ground should only be tied to a a grounding rod at the main service entrance. You should not add another grounding rod for a detached shop. (But I have read that some huge machines recommend a dedicated ground rod next to the machine)I guess it helps to tie grounds together so if you are touching a grounded metal machine and a power tool at the same time they are at exactly the same potential. That would include any static being generated by a moving machine.
That is why I asked about tying grounds together once past the main entrance panel. Here in the USA there are only three, or four, wire colors for single phase power. green or bare for ground, white for neutral, and any other color is a hot leg.
Bill D.

I used to know why you do not tie neutral and ground together after the main entrance but I have forgotten why. Now I just remember it should never be done for safety reasons.

Firstly forget colors unless you were the one that just hooked them up. power doesn't know what color the wire is. It is very comon to run 240 across two conductors(White & Black) Equipment grond hasn't been in the code all that long so I wouldn't expect it working on something that has been there over 20 years.

Now to the OP. With a new circuit If you have one feed wire going into a junction box feeding othere circuits then you would tie all of those grounds togather but in your case they are two bifferent circuits so they don't get tied togather. Pick only one to ground your box if it needs it. Ground loops are any path that make a complet circle & are a very bad thing.

need to tie grounds together in a outlet box

Diamond

Stainless

Stainless

Stainless

Diamond

Titanium

Titanium

Titanium

Diamond

Diamond

Diamond

Diamond

Diamond

Diamond

Diamond

Diamond

Aluminum

Diamond

Diamond

Plastic

Similar threads