Revisiting the PTO-generator question

Trev · Dec 19, 2002

Thanks, Spence! Great info and links... I'll keep them for when I meet with our electrician.

I only have a 7500 watt rated, 10000 watt surge portable generator so a 100 watt or even 60 watt switch would have been enough.

But, doesn't the switch also have to be rated to handle the full load from the power company as well? You can't just get one rated only for the generator, can you? I have a 200-amp service coming in, and my switch would have to handle at least that, plus some safety headroom, correct? Or am I, as usual, missing something?

Thanks again,
Bob

djradz · Dec 19, 2002

Even though the generator may only serve a portion of the load, the switch must be rated the same as the service. When the generator is not in use and the normal supply is from the utility, the switch could see up to the maximum of the service panel in load. Also, I am not sure the neutral needs to be switched. Someone should address the tying of all three neutrals together permanantly. This may be acceptable.

Spencer · Dec 19, 2002

Ah yes, you are correct old wise one. I knew there was another reason I wanted at least a 200 amp switch. Thank you for catching my error before someone else caught it and ripped me a new one.

Spencer · Dec 19, 2002

djradz,
In the event that power lines have been downed, their neutral wire could come in contact with other wires couldn't it? Thanks for pointing out my switch rating error without making me sound like an idiot.

Anonymous Poster · Dec 19, 2002

Neutrals-
There are a couple reasons for isolating the Neutral when switching to generator; One of witch you already figured out, the neutral from the power company could easily be in contact with a hot wire someplace down the road. The neutral is always the top wire on the pole in secondary circuits on the pole, so it's easy for a neutral to short when a limb falls on it. I don't know why the Neutral was so positioned back in the early days of utility standards, I ain't that old, but I do know anyplace I go in America the Neutral will be on top.
The second reason for isolating the neutral is because there is often voltage above ground on the neutral. This occurrs for a lot of reasons, including flourescent ballasts, and inductive loads, and needs to be dealt with.
Even MORE Important in generator installations is GROUNDING the genset. The first connection made when hooking up the generator should be the mechanicle ground. Did you ever notice the GROUND prong on an electrical plug is longer than the others, so ground connects first.

Trev · Dec 19, 2002

Even MORE Important in generator installations is GROUNDING the genset. The first connection made when hooking up the generator should be the mechanicle ground.

In the case of my PTO generator, I would assume this would entail actually having a ground strap bolted to the machine, and a copper rod that is pounded into the ground before the machine is used?

They should come with those, if that's the case.. wouldn't you think? Is there any specific specs that need to be followed, e.g., how heavy the wire is, how deep you pound the rod, etc.?

Thanks, yet again..
Bob

gerard · Dec 19, 2002

I used a standard 6 ft ground rod that I bought separately, drove it into the ground near where I run my generator, and ran some copper wire to it that I keep rolled up unless the generator is being used, then I just connect it.

Anonymous Poster · Dec 19, 2002

The grounding conductor should be of equal size to the current carrying conductors.
May I humbly suggest bonding the genset ground to the distribution of the electricity, be that breaker box or whatever.
You'd be surprized at the resistance that can occurr between a ground rod and another one 10 feet from it. You may not see it on your VOM, but I assure you it shows up on my Null Balance megger. Ground problems can get both expensive and dangerous real quick.

John_Mc · Dec 19, 2002

Even MORE Important in generator installations is GROUNDING the genset.

Any reason you couldn't just clamp on to the same ground rod the main power into the house uses? The generator receptacle on my house is right near one of these rods. I've been trying to think of a situation where using the same rod would be a problem, and so far have not come up with one.

Anonymous Poster · Dec 19, 2002

Clamping onto the existing ground rod would be the second best way to do it. Best would be connecting to the ground bus in the breaker box, where the wire from your ground rod connects.

djradz · Dec 20, 2002

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 · Dec 20, 2002

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 · Dec 20, 2002

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 · Dec 20, 2002

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 · Dec 20, 2002

Unless you're living next to an ocean, don't get concerned about storing fuel oil. 

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 · Dec 20, 2002

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

Trev · Dec 20, 2002

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

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 · Dec 21, 2002

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 · Dec 21, 2002

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:


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.



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.


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 · Dec 21, 2002

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.