grounding

Hi,
First off I would like to wish everyone Happy Holidays and a Happy New Year. My question is as follows, I have just started wiring electrical panel and have a question regarding the appropriate way to ground.If you bring a source voltage to the panel from that source can you bond that ground directly to the panel? or would that create whats refered to as a "hot chassis" in which case you would have to run the source ground to a terminal block first and then move from there?
Any clarification would be very helpful, thanxs in advance.
Best regards
Lab

The box/panel is grounded, you run from there to a terminal strip for multiple connections to devices.

The purpose of a ground is to prevent items like metal panels from being "hot" if a short etc occurs.

Here`s some good info
http://www.patchn.com/forum1/viewtopic.php?t=108&highlight=grounding

Be sure and read Eric Nelson post i think
he explains what you want to know

That thread was about the output of a 480 to 120vac center tapped transformer and how it may or may not be connected to ground.

As is mentioned there is a difference between ground and a grounded conductor.

The "true" ground should be at the main panel. When you use a sub or control panel a ground wire should run from the main (with the power conductors) and be connected to the sub/control panel itself. You can have a multiple connection "at the ground connector in the panel" or run a wire to bonded terminals on a strip for ground connections of devices.

What you want is ONE path to EARTH GROUND for anything that is capable of carrying current. Any device, especially metal enclosures, should be connected to the ground from the source. All peripheral devices should (at least on that end) be connected to the ground in that panel.....and no other potential ground connection.

I pray I am stating this clearly.

I didnt have a picture handy so had to get one that isnt precise but will give you an idea, you DO NOT want any enclosure or device to have 2 ground connections. You DO want them to have A ground connection.
This pic shows how you get a ground loop.

Refering to the sketch above...

When connecting a device with shielded cable, the cable shield should only be grounded a one end, typically away from the device.

As stated earlier, control panels should be grounded once.

Remember, while nuetral and ground may go to the same point in the electric meter, and in the main disconnect or breaker panel, they are two seperate wires not tied together anywhere downstream, specially in a control panel.

Search threads for grounding control panels. Specially when your plc has AC, DC, and analog inputs. A step down transformer to 120 vac in a control panel, which also has a 24 vdc power supply may need to be handled differently.

Too late in the day and week to try to explain it now.

Try a search here, and check Ron's ste for more info.

regards.....casey

This whole thing on grounding can get a little confusing. Electricians including myself have made mistakes.

TO start off
The GROUND POINT is at the service entrance. This can be a grid or two ground stakes or ground stake and water pipe (depending on local jurisdiction). This is where the "ground potential" is set for the distribution system (be it a house factory etc).

In the US the ground conductor is bare or green. The ground conductor does not normally carry current and IS NOT normally a part of the circuit.
It is referred to as the GROUNDING CONDUCTOR.
Also it has been acceptable to rely on conduit for grounding. BAD business - I have seen one electrocution when the conduit was broken and a hot conductor contacted the conduit. I ALWAYS pull a ground in conduit.

Now here is where it gets confusing. The neutral is also a grounded conductor. In the US it is white or gray. It is a part of the circuit and does carry current.
It is referred to as the GROUNDED CONDUCTOR.

Here is where all the confusion starts from.
In a house panel the conductor (grounding electrode conductor) to the ground (grid stakes etc) is tied to the "neutral bus" OR if there are two buses (ground bus and neutral bus) they are bonded together -- so the ground (green) and the neutral "start from the same point" so to speak. This IS the ONLY time they are ever connected.

IN ALL equipment downstream including sub panels the neutral (white) is completely independent of the ground (green). In a sub panel the neutral goes to a neutral bus which is NOT connected to the panel frame (commonly referred to as floated). The ground goes to a ground bus which IS bonded to the panel frame. The reason for this is that if you tie the ground and the neutral togeter on a sub panel they are conductors in parallel and both will carry one half the current carried by the neutral. I learned this the hard way with a GFI.

In all control panels, junction boxes, receptacle boxes, switch boxes the ground (green) is connected (bonded) to the metal frame. The ground is also connected (bonded) to the ground connection on the receptacle and switch. The neutral is NEVER connected to the metal frame.

IF a transformer is hooked up this is a "new system". One side of the secondary should be grounded. - I am purposefully avoiding 3 phase by the way - . There are exceptions -- door bell transformers come to mind. One side of the secondary is grounded to the grounding conductor (green).

FINAL THOUGHT
In general everything is grounded.
Like anything else there are exceptions to the rule.
Not all installations are covered by NEC -- utilities, and marine come to mind.
The US Navy submarine I served on electrical system was not grounded at all - Navy electricians were slightly confused by this neutral stuff when we became civilians.
Not all equipment is required to be grounded by NEC either.

Dan

there is also the GROUNDING ELECTRODE

NFPA 79 ("The Electrical Code') also refers to a conductor
called the GROUNDING ELECTRODE.

It refers to services, I believe, and should not be
confused with the GROUNDING CONDUCTOR or
a more generic term GROUNDED CONDUCTOR, as Dan has pointed
out already.

One of the more pervasive myths out there is the size of the
grounding conductor. The old electrician thumb rule is
'one size smaller' than the circuit conductors. This is
totally wrong, and, almost always ends up with an oversized
(i.e. overexpensive) ground wire. The size of the ground
wire is related to the size of the branch-circuit/short-circuit
protection ahead of it.

Ground

In Canada, we Ground at the panel, min 2 ten foot rods, or the copper water line, before the first valve. The goal is to get less than 10 ohms on the ground path. Ive had to drive 4 ten foot rods in more than one instance. The inspector has checked on more than one occasion. Everything metal from the panel is bonded to this ground. The question here sounds kind of like a subpanel. This is not considered a new "system" but the definition varies by country and even by province. Check with the local inspector, they are there to help you make the installation safe. The code is MINIMUM standards for saftey. You pay them to protect you. Ask questions. Do it right. Bruce

Great explanation, Dan!...

Much clearer than my attempt at Ron's...

---

jdbrandt said:

NFPA 79 ("The Electrical Code') also refers to a conductor
called the GROUNDING ELECTRODE.

It refers to services, I believe...

Click to expand...

Yes. The grounding electrode conductor is the wire that runs from the main service panel to the ground rod.

jdbrandt said:

...and should not be
confused with the GROUNDING CONDUCTOR or
a more generic term GROUNDED CONDUCTOR, as Dan has pointed
out already.

Click to expand...

This scares me, Jeff. From what you wrote, it sounds like you propose that the terms GROUNDING CONDUCTOR and GROUNDED CONDUCTOR are interchangeable. Let's hope that's NOT what you meant.

jdbrandt said:

One of the more pervasive myths out there is the size of the
grounding conductor. The old electrician thumb rule is
'one size smaller' than the circuit conductors. This is
totally wrong, and, almost always ends up with an oversized
(i.e. overexpensive) ground wire. The size of the ground
wire is related to the size of the branch-circuit/short-circuit
protection ahead of it.

Click to expand...

I've never heard that 'rule of thumb' if you're referring to the size of the grounding electrode conductor. There's a simple table in the NEC to determine the size. You might be referring to sizing of the grounded (or neutral) conductor (NOT the grounding conductor!). I have heard the 'one size smaller' thing for neutrals on a 3-wire service (2 phase conductors and 1 neutral). If you don't want to take the time to calculate the neutral load, then yes, 'one size smaller' is often larger than necessary, but it can't hurt to err on the high side! Wire ain't all THAT expensive...

beerchug

-Eric

P.S. How many here have experienced almost finishing driving a ground rod, only to see the tip start popping out a few feet away. Damned rocks!...

Neutral Sizing...

I just found THIS article about neutral conductor sizing...

beerchug

-Eric

ALL sizing of conductors must be done in accordance with NEC (in Canada what is the equivalent??).

In the tables there are instances where the sizes of the GROUNDED conductor (white or neutral) and the GROUNDING conductor (greeen) are smaller than those of the UNGROUNED conductors (AKA hot, phase, black or red or whatever). These exceptions I believe are in the service entrance area where the neut can be smaller than phase (hot) expecially on residential (Edison 3 wire ie 240 / 120 single phase). The GROUND ELECTRODE CONDUCTOR is also smaller ie # 4 copper on a 200 amp service (using 2/0 copper for the ungrounded conductors) but I believe you will find this nearly restricted to residential.

You can never go wrong making the GROUNDED conductor (white or neutral) and the GROUNDING conductor (greeen)the same size as that of the UNGROUNED conductors (AKA hot, phase, black or red or whatever).

EXCEPT on three phase.
Things get a little flaky here. The two common are delta and wye. Delta is generally used for balanced loads ie motors etc.
WYE is generally used to supply both 3 phase loads (motors) and single phase. Most common are 480 / 277 and 208 / 120. The 277 is a lighting circuit often broken out into single phase circuits for smaller rooms.
120 is used for lighting and receptacles.

DELTA often does not have a neutral. IF it did (most common was on 240 3 phase) then you got 3 legs of 240 (as usual), a centertap on one phase gave you 120 volt from centertap to each phase conductor supplying that phase. From the centertap to the third phase was what was called a wild leg - been so long I forget this voltage -- anyway this wire was supposed to be orange. The centertap was grounded, was colored white and was the neutral.

Not too long ago on wye 3 phase it was common to make the neutral one size smaller than the UNGROUNED conductors (AKA hot, phase, black or red or whatever). The thinking was that even with unbalanced loads ie 277 volt lighting, or 120 volt receptacle circuits (120 / 208) that the neutral current would never exceed the phase current. If you go thru the trig and calculate the neut current with one leg open it will equal phase current at certain points of the sine wave. That worked great until the computer and variable speed drives with switching power supplies came along. Neutrals (common to all 3 phases - not a neut on a single light circuit or recept circuit) started burning out. Now we have to deal with harmonics. The harmonic induced currents from each of the phases add in the neutral where it is common to all 3 phases ie in the "main distribution ie sub panel feeds etc. This is why NEC now requires neutral sized at 140% of phase conductors on three phase.

CAVEAT This is my understanding. I have no hands on with harmonics problems, and am taking a class this winter on PLCs and VFDs to overcome my ignorance and get caught up with the times. I want to hear where I am wrong and need the education.

Dan

NEC (in Canada what is the equivalent??).

Canadian electric code or CEC

Dan,
the Neutral being rated at %140 or even higher is correct. Our company does alot of large data centers and it has become rule of thumb to enlarge the neutral.
The new trick that Liebert has been pushing for the really large computer room loads is to use 277/480 PDUs on the data ctr floors and distribute it as 20,30 and 60 amp tails to the computer systems.

Just finished a data ctr that had dual 13Kv service with 13Kv generators. This fed into 4 indoor substations that brought it down to 277/480 then onto the 4 10,000A online UPS systems. Finally it was steped down at the PDUs to 3 phase 120/240. The PDUs were set up so no computer rack was farther than 30' from it. All neutrals rated at %150-%175

DAN:

Sometimes I will use 2/0 for my hot leads and a piece of 3/0 for the nuetralon 200 amp underground single phase residential services, and a couple of inspectors go crazy. That keep saying you can't upsize the "N".

Heck, if I wanted to fight with it, there is nothing in the code that says I couldn't use 535mcm as long as the conduit was sized right.

To be safe, since I usually have have a couple of different brands of 2/0 or 3/0 laying around, I will use something different for the "N", since I can never find my white tape (I have probably 6 rolls of it). I haven't done anything in Com Ed territory for a couple years, so I still have some 3/0 laying around. The biigie and all the local municipal electrics only want 2/0 (cheaper lugs for them) on 200 amp, and #4 for 100 amp.

Occasionally, I'll put a 320 amp into a new house, and really mess with their minds.

hopefully, back to flying a desk and laptop soon!!!

regards.....casey

Why should they care about "N" size, they regularly pass houses with no ground rods, no fan rated ceiling boxes, missing gfci's, and on...

Guys, lets all get something straight, before something gets fried..... In reading the posts, some refer the neutral as being a grounded conductor,

QUOTE]You can never go wrong making the GROUNDED conductor (white or neutral) and the GROUNDING conductor (greeen)the same size as that of the UNGROUNED conductors (AKA hot, phase, black or red or whatever).[/QUOTE]

The NEUTRAL, in any codebook that I use, is Referred to as the "IDENTIFIED CONDUCTOR". This conductor must be WHITE OR GRAY in colour. This should not even be correlated in speech as the Ground....ever.