Electrical Knowledge Repository - Automatic Standby Generator Ground Rod

Old EKR Archives: Subject - Automatic Standby Generator Ground Rod
jdcastle4ttu	My apologies for being long-winded, but… I will soon be installing, as part of a new construction, a Generac Guardian 15/13kW automatic standby generator, which includes an automatic transfer switch that does NOT switch the neutral. The transfer switch has a built-in 12-circuit sub-panel and the neutral is isolated from the ground lug, both in the switch and within the generator. Connecting the transfer panel to the generator are two 6-AWG red & black, one 8-AWG white, and a 10-AWG green. These four wires attach to the generator inside its control panel, with the green wire attaching to a terminal that is directly bonded to the generator’s entire frame. In addition, there is a ground lug attached to the bottom of the generator’s frame, and the manual says to drive a ground rod beside the generator and connect it to the lug on the frame. My questions is, since this is NOT a SDS, shouldn’t the generator’s only ground come from the 2 service panel ground rods. Wouldn’t driving a separate ground rod create 2 paths to ground and/or create a loop? Any input will be greatly appreciated.
blackrd	Q7. The instructions for the UL listed standby generators I install state: "drive a ground rod and bond it to the frame of the generator enclosure." This generator uses a solidly bonded neutral in the transfer switch, so this generator is not a separately derived system. Why do they say it must be grounded to an approved earth ground? A7. Probably because the engineer writing the specification for the manual and the engineer at UL who reviews the instructions think the earth serves some safety purpose. I have a standby generator with an unswitched neutral in the transfer switch and I didn't install a ground rod. The only time an earth ground is required by the NEC for a generator is when the generator meets the definition of separately derived system [250.30(A)(3)]. According to Article 100, the generator would only be a separately derived system where there's no direct electrical connection to the utility supply conductors. In other words, the neutral is also switched in the transfer switch. I hope this helps. From Mike Holts April NEC questions
wareagle	JD It sounds like you have it correct. When the neutral is not switched the generator is bonded to the existing electrical grounding. If you want to install a gnd rod it must be bonded to the frame of the generator. Since the ground rod at the main service and the generator gnd rod are bonded together, it meets the requirements of the NEC. The code give you the option to install the gnd rod. It is not required.
Ryan_J	The ground does absolutley zero nothing. I wouldn't even consider installing one.
jdcastle4ttu	This confirms my hunch that the ground rod was at best optional, and I won't install one as long as the AHJ is in agreement, but just for fun, would connecting an additional ground rod have any adverse effects? I ask this because this "third" ground rod would be bonded to the other two main rods by only a 10-AWG wire that connects the main panel to the transfer switch and then on to the generator. Thanks, Jonathan
Ryan_J	The ground rod won't do anything, good bad or indifferent.
stedder	Plus that's a lot of work... driving a ground rod and running that wire.
Tony Reid	I agree with Ryan & some others. I have installed these Generac Gen sets. That ground lug gives the Gen a ground path faster within the unit. However, the MFG insists on this ground & rod as not to void factory warranties.
Energreen	There's another possibility that I'm seeking responses to. Isn't it possible that the manufacturer and UL are considering the possibility that the generator housing, being a large piece of steel, could be hit by lightning? If it's hit, wouldn't it be better to have it grounded rather than have all that energy following the ground path directly into the building? (Very often, engineers look at even one occurance and then believe it can happen again. Which it can. So they design to avoid problems based on that one experience.) In general, follow the manufacturers instructions. They're written for a purpose. And sometimes that purpose is required to maintain the UL listing. EG
Ryan_J	I think if the generator gets hit by lightning, it's going to explode anyway! As far as the earth goes, if it is not a seperatly derived system, the equipment grounding conductor goes to the earth at the service anyway.
Energreen	I agree with both of those statements Ryan. If the gen is hit, it's toast. And the equipment gnd is tied to the service gnd anyway. Suppose the existing service has no ground rod? I just upgraded one that had no driven rod and only a cold water pipe gnd, which we know isn't the most reliable. Maybe they want to make sure there's a gnd rod in situations such as this, so the instructions require it? They know DIYers will be trying to install these things and they want to cover all bases...? (And yes, there will be those who never get an inspection. Especially propane fired units.) This might explain the manufacturers requirement for a gnd rod ?? Thanks, EG
Ryan_J	If you want my honest opinion.........I think Americans just have a thing for driving ground rods. Hoenstly. We have been putting them in for years and not asking why, so now it is just second nature.
macmikeman	I absolutely agree with you Ryan, one million per cent. Man for some real fun try even getting a 30" deep horizontal trench to lay one flat in when the ground you are standing on is solid volcanic rock. I think it was Mike Holt who went out into the field for some real life testing on true ground rod effectivness. The results of those tests proved beyond any doubt that we are only wasting metal.
apj	I install these generac generators also, and have often wondered about this issue myself. So, I assume that since the ground and neutral lugs in these generators are not bonded, a ground fault on the generator case would travel through the EGC through the transfer switch (where they are not bonded either), to the bond at the main service disconnect, then back on the neutral to the generator, through the windings to trip the generator breaker? Is this right? The manufacturer told me that the EGC and neutral are not bonded in the generator due to the fact that a generator has to have a floating neutral. Please explain this. Thank you.
jwhite	that is the way i see it apj. more important, if the neutral and ground were grounded at the generator and at the panel. then the ecg between the main panel and the generator would be sharing the neutal current with the neutral creating a saftey risk to anyone touching anything metal connected to that same ecg. electricity does not take the shortest path back to its source, it takes ever available path. even if one of those paths happens to be you.
apj	thank you jwhite. So a floating neutral is one that is not bonded? Is there anywhere in the code that says which side of the service main the transfer switch has to be? I assume article 702 applys to these residential units.
jwhite	Yes a floating neutral is one that is not bonded. I am not sure about the location of the transfer switch, that is to say if there is a requirement as to location. I know you can have a bond at the generator, if you switch the neutral at the transfer switch.
ssghill	Because the transfer has a solid neutral, you can't bond it in the generator. I agree the ground rod probably does little, but it does serve a function to the customer.... keeps their warranty!! If the generator in not installed correctly, Generac will not honor the warranty. They also have to be 3 ft. away from the house. Now let me as you this, don't you have to bond this electrode to the others in the system if available on the premises? How many DIYers would do that?
Ryan_J	You would not have to bond it to the other electrodes, because it is a supplementary electrode. 250.54.
ssghill	what is it supplimenting in the generator scenario?
Ryan_J	It is supplementary in that the NEC doesn't require it. Be careful...don't confuse supplementary electrodes (not required) with supplemental electrodes (required).
ssghill	If there is no electrode required, what will open a CB during a fault? the Neutral is isolated in this type of generator. Wouldn't you need some type of ground connection to the frame of the generator in the event a winding shorted to the frame?
kbsparky	The neutral is still connected to the ground in your main service panel. Just because it is not connected at the generator does not mean it is not connected at all. A short will still trip out the breaker in the panel.