Revisiting the PTO-generator question

[djradz]

Old wise one #2, I feel I need to further reinforce your "bonding" suggestion in that one should NEVER drive a second ground rod just to "better" ground a specific piece of equipment without also connecting it to the same ground system for the house, business, whatever.

Has anyone checked the NEC on the need for any additional gound at the generator. Whenever we used a generator, we have always assumed it is actually grounded through the ground conductor in the cable, house wiring, and back to the ground connecition in the main panel.

Lastly, not sure I complete agree with the need to break the neutral from the utility. The utility ties that neutral cable to ground at every customer transformer along their system. In addition, the cable is tied to ground by the homeowner's ground rod. I'm not sure on this one, ... just some thoughts.

 

[gerard]

My genset connection is a four prong twistlock, two hots, a neutral, and a ground which grounds back to the service panel. I then separately have a ground wire from the genset directly to a ground rod that grounds the genset. Are you saying that the separate ground wire I'm referring too has to be the same size as the service cables that feed from the genset? That would be pretty big and I wouldn't be able to leave it hooked up the way I do now. (Right now it's just single strand 12 guage, bare copper) I thought this was just kind of a back up ground which would shunt at least most of the current to the ground if the genset frame itself became live for some reason? The grounding screw on the genset which is specifically there for this purpose is not that large/heavy duty and doesn't look designed to have a large 4 gauge type wire to it.

 

[Anonymous Poster]

2 answers here-
Breaking the neutral from the utility is a better degree of seperation than not breaking it, and especially in emergency situations, where you have no lnowledge of what voltage may be on that wire due to the emergency. The posibility always exists that a primary may be resting on the neutral down the road. The utility ground would NOT necessarily carry that voltage to ground and render -0- volts neutral to ground at your location.
This is another reason the "Gentran" type of switching system in intrinsicly unsafe. I double checked a Square D manual transfer switch, 100 amp size, and it does switch neutral, not just the 2 hot legs.

Second subject, genset ground wire- The ground wire to the genset should be of the same size as the current carrying leads from the genset to the load. If the set is providing 30 amps, the load wires should be #10, and the ground should be # 10. A larger wire sure won't hurt anything, as you never know what fault curent could occurr in these situations.

 

[djradz]

I am no expert on this subject and I believe someone should really be referring to the NEC or NEC handbook on this, because I think we may be providing some misinformation. Whether any ground connection is required at the generator should be further researched. My guess is the NEC would show there should be an equipment grounding conductor along with the neutral run back to the main panel, and nothing more. However, I know may and maybe most folks do not even have that but just mearly use the neutral for both purposes.

If a second ground rod is driven, I do not think the same size conductor is necessary at the generator. This is just like grounding your main panel. Your service entrance cable may be 1/0 or 3/0 but your grounding conductor is much less. These conductors are sized not to carry normal current, but to maintain zero potential and to carry instantaneous fault current until a breaker trips. In actuality, this conductor for this case may only need to be #18, but I think there may be a minimum of #8 in the NEC. Again, the Code should be consulted.

 

[Trev]

<font color=blue>Unless you're living next to an ocean, don't get concerned about storing fuel oil. </font color=blue>

Okay, good to know. Thanks.

BTW, what do you think of the attached wiring setup? Would this meet code would you say?

Happy Holidays to all,
Bob /w3tcompact/icons/smile.gif

 

[Anonymous Poster]

How'd you manage to get hold of pics of Raytheon's new install down at McMurdo, do you know Steve too?

 

[Trev]

<font color=blue>How'd you manage to get hold of pics of Raytheon's new install down at McMurdo, do you know Steve too? </font color=blue>

No, it just seemed the obvious way to wire it up. Good to know that others also came up with a similar solution.. means I must be on the right track! /w3tcompact/icons/grin.gif

 

[bgott]

http://www.electrical-contractor.net/

Go here and use the search function to look for "generator" in all open forums. There are quite a few discussions on transfer switches and code requirements. I use the search function rather than posting most of the time, they are heavy on " consult a licensed electrician" over there if they think you're a DIY'er.

 

[EdDekker]

I don't see a problem not disconnecting the neutral in the transfer switch.

The neutral originates at the center tap of the distribution transformer (on the utility pole or in ground power company box) and runs to the main panel it must be bound to the earth ground at the service entry or main panel of each house. I believe but have not found a reference that it is also bound to a ground at the utility transformer (via the ground wire running down the utility pole from the transformer to a ground rod).

If there was a non-zero voltage on the utility side on the neutral line it would be a shorted to ground at EVERY house and possibly at the distribution transformer.


In fact the NEC specifically allows the neutral to not be switched by a generator transfer switch. (It also allows the neutral to be switched with an additional generator ground requirement).



The NEC itself is not online but I found the following online pages which quote or paraphrase the section of the NEC:

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from http://www.egsa.org/standards/_docs/word/std109c.doc

250-5(d). Separately Derived System. This section requires grounding a separately derived system meeting the requirements of 250-5(b). A generator may or may not be a separately derived system. A fine print note explains that an on-site generator is not separately derived if the neutral is solidly interconnected to a service-system supplied neutral. Such generators should not be separately grounded. Other generators are separately derived and must be grounded if they can be grounded at 150 volts or less to ground or supply 480/277 volts.
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from http://www.lni.wa.gov/scs/electrical/currents/elc98_11.pdf

NEC 250-5 (FPN No. 1) states: An alternate alternating-current power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral." The most common example of this is an on-site generator installed as a backup source of power. This is the type that would employ a transfer switch to transfer utility and generator power. The transfer switch does not break the connection between the generator neutral and the utility grounded circuit conductor (neutral). The generator is required to have an equipment grounding conductor to ground any exposed noncurrent-carrying metal parts. This equipment ground must be a type specified in NEC 250-91(b) and run from the normal system ground to the generator frame. It should be noted that a supplementary grounding electrode at the generator location is allowed under NEC 250-91(c). A grounding electrode is not required by the NEC, but could be recommended by the generator manufacturer.
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The section is also quoted is also in: http://www.eren.doe.gov/der/microturbines/pdfs/mtstandards.pdf page 5.5


The transfer switches intended to switch a 220 volt service between utility service and a generator I have found are two pole switches (switching the two hot lines) with a non-switched neutral. This includes both the manual switches and the automatic switches (see the Kohler web site http://www.kohlerpowersystems.com/onsite_ts_auto_all.html for example).




(I currently have the 12 circuit gentran transfer switch I had installed when I built the house, this was a mistake. I had the panel rewired once to change the 12 circuits and still don't have the 'right' 12. I have been looking at both an LP system with automatic start or a manual 200 amp transfer switch. )

 

[Trev]

Thanks, bgott. I'll go look at it. Fortunately, I have a "licensed electrician" coming out at 10am or so today. Now the only painful part left will be in the wallet.. but I figure a properly wired transfer box will mean once less thing to worry about, and may increase the property value a bit.