3 phase Grounding Question
Now that I have more than one piece of 3 phase equipment, I'm getting ready to install a load center to the output of my phase converter to easily distribute power. My question involves grounding. I know on single phase, the ground can be a size or two smaller than the power carrying conductors and it can also be shared between a couple circuits. Is there any difference on 3 phase? I would like to run two 30A circuits together in EMT sharing a 12awg ground. The reason being that I'm cheap and I can buy a roll of black 10awg for all the hots but I already have a roll of green 12awg for ground. Technically the EMT should handle any grounding but I'd like to make it as NEC compliant as possible which means running a separate ground. Any comments would be welcome.
Ground is ground! Your fine with the smaller green wire! But remember Ground is NOT Neutral! It should not be used to carry current! Therefore you can not use it to connect 120VAC. You must have a separate line if you intend to use Neutral.
Would someone with much more electrical knowledge than I please explain bonding,might be relevant here.
Science in and of itself when it comes to things like satellite Earth-stations and FPTS sites, but in a nutshell, 'bonding' is largely going around the plant or structure and applying conductive straps or 'bonds' to all the metal that might pick-up induced electrical potentials from any source, be they near-miss lightning strikes or heavy machinery / gen-set power switching transients.
Originally Posted by racer55
Ultimately, once 'bonded' together, it all gets grounded, but is only peripherally related to the power distribution, proper.
Items in want of 'bonding' can includes metal building panels, roll-up doors, metal framing, roofing, handrails, catwalks, metal doors and frames, and the machines themselves. Object of the exercise is to protect staff and the more delicate parts of any electronics gear in the area.
Bonding is covered by codes, more so yet by Telco SOP, and literature can be found online.
I think racer55 is specifically asking about the bonding of the neutral wire used in North American single phase to earth ground. I'll give it a try:
Originally Posted by racer55
Bonding ensures that the neutral is at ground potential, and that limits the voltage potential from any live single phase conductor to ground to 120VAC.
Bonding of the neutral to ground is limited by the NEC to one location only (typically at the panel at the service entrance that also has the main breaker) because the system ground is designed to not carry any current, which allows the ground conductor to be smaller wire, and in some instances the conduit pipe itself. If the neutral were to be bonded to ground at several locations, and the neutral wire between those locations was broken, that would turn the ground conductor into a current carrying conductor, a duty for which it is not properly sized. Not good.
The exception to the rule on multiple bonds between neutral and ground is that a separately derived system has its own bond. For instance, a control transformer wired between two legs of 240VAC 3Ph. to produce 120 VAC control voltage on a machine tool is a separately derived system, and one side of the transformer secondary will be bonded to ground. That ensures that both legs of the control circuit don't float to some indeterminate voltage, where one leg is at greater than 120VAC potential to the grounded machine frame.
I think a simpler explanation is that all conductive materials to which electrical power is associate with be attached to ground so that it is always at Zero potential and can at no time be used to carry current. Neutral will then be bonded or connected to ground at the source of power such as a transformer, power line or anything else so as to maintain it's zero potential with respect to ground.
That indeed simplifies both my response and the one Dennis made.
Originally Posted by Froneck
But Dennis did the better job w/r the NEC, and that is probably what the OP needs 'soonest' to stay healthy AND out of trouble with the 'civils'.
Forgot to mention w/r economizing on the wire by using all one-colour. I'd go the extra expense to avoid that, because it is extra work to track wot's what, not easy to check in later years, hence all too happy to get you in trouble.
ISTR (Dennis?) that the code permits it, but wants adding coloured tape or such at each termination to ID it.
That can go awry Real Soon Now when gotten wrong or lost-off as tape ages or wires are trimmed back for a new connection.
I'm not familiar enough with all the niceties of the NEC to comment on where colored tape can be used... for that we need a real electrician, not a maintenance hack such as myself. However, I do recall that a bare conductor has the same meaning as green under the NEC, so when faced with a lack of green wire, I just use what I have and skin the insulation off. Hard to mistake that for anything else.
"I'm not familiar enough with all the niceties of the NEC to comment on where colored tape can be used... for that we need a real electrician, not a maintenance hack such as myself. However, I do recall that a bare conductor has the same meaning as green under the NEC, so when faced with a lack of green wire, I just use what I have and skin the insulation off. Hard to mistake that for anything else."
In the past, the service conductor NEUTRAL, which was also "bonded" at the service entrance panelboard, thereby becoming BOTH a NEUTRAL (groundED) and a protective (groundING) conductor, could be bare.
No longer. The service NEUTRAL must now be insulated, just like the ungrounded conductors.
You are USUALLY permitted to change the function of a conductor by "identifying" it, namely by applying to all visible ends a PVC (or equal) tape of the relevant color.
I ALWAYS wire my services with BOTH black (L1 and L2) and white (N) conductors. White is white is white is NEUTRAL.
After the first panelboard, which, necessarily, is the service entrance panelboard, TWO ground-level conductors must be provided: a groundING conductor (green, or "identified" as green) for the safety ground, and a groundED conductor (white, or "identified" as white) for the neutral.
Therefore, all EXCEPT the service entrance panelboard MUST have two grounding bars, one which is "bonded" (green) and one which is not "bonded" (white).
The NEC covers all these cases.
The AHJ (authority having jurisdiction) "may" change these, to suit local conditions.
Thanks for the info guys. I'm definitely aware of bonding the neutral and ground at the main box and nowhere else. And I will label the ends of the black conductors, just didn't mention it.
During My last visit to the NEC code book I noticed they have removed color coding. At one time specific colors were assigned to voltages, ground and neutral. Now it's up to the facility do determine color coding. In my small shop I keep the old color code. Red, Black and Blue were 240VAC White Neutral. Yellow, Brown and Orange 480VAC Grey Neutral and Green ground. I also keep the same colors on same phases. When I was the engineer the building was 100,000 sq/ft. I required the same so as to avoid any confusion.
Yes complying with the local inspector is wise! His bite was bigger than mine! That goes with NEC too but reading that code book is like reading a book in code! I bought the Black NEC Book, Then the Blue colored book to explain it, Then a Black and Yellow book to explain the other 2. Now I'm retired and gave them to my son!
"Now it's up to the facility do determine color coding. In my small shop I keep the old color code."
My take on it is this: the conductor manufacturers don't want to produce any more colors than they have to. Black and white have well-known UV and other resistant characteristics and Ideal, and others, are producing PVC tapes, for purposes of "identifying" conductors, in a wide range of colors, so why bother?
However, with the code revision which gives the installer almost infinite discretion, I would be a little upset if I discovered that s/he had wired my structure with all-green wire, and then had gone back and "identified" the L1 with black PVC tape, the L2 with red PVC tape and the N with white PVC tape.
I guess I could reluctantly accept all-white, but I would not be pleased.
All-black with PVC tape, as required, makes the most sense to me, if customary colors (red and white, etcetera) are not available.
At my local electrical supply wholesaler/retailer, black and white are always stocked in sizes which are appropriate for services (#2 THHN for 125 amps, etcetera), so there is really no reason not to install a white neutral. You still have to "identify" L2 and L3, though.
Actually if it's your facility you can determine the color code and tell the contractor. Where I was the engineer I did make it mandatory that green always be used as ground, white low voltage Neutral and Grey (Taped Black) for high (277VAC applications) black every thing else. After that getting contractors to keep the phasing straight was impossible. So I had our men go over the wiring to color code to phase. All same colors didn't have a voltage difference between them except maybe a few volts.
Other than 240 corner-grounded and 480 corner-grounded, both of which are delta and are deprecated by the recent codes (and most AHJs), but are grandfathered, the most common three-phase systems are:
1) 120/240 delta, which also provides 120/240 single-phase with generally higher capacity and also 240 delta three-phase with generally lower capacity; two single-phase lines,
2) 120/208 wye, which is a good choice as all three lines can be used for single-phase; requires a transformer for 120/240 or 240, and
3) 277/480 wye, which is a good choice for a multi-use premises with lots of fluorescent lamps (which generally operate at 277); requires a transformer for 120 or 120/240 or 240.
Should you have multiple sub-panelboards of the (1) kind, I suppose those which are dedicated to three-phase, only, could do away with the neutral (groundED) conductor, but, of course, not the safety ground (groundING) conductor.