Generator/Electrical question

[SPYDERLK]

The reason for the 4 wire is because since about 1992 the NEC (National Electrical Code) has required separate ground and neutral wires whenever the box is considered as being a subpanel. For your purposes, the generator is considered to be the main panel and your house is the subpanel.

At the main panel you only need two hot wires and a neutral, which is also connected to an earth ground within about 10' of the panel, but at the subpanel you are supposed to have 4 wires. The subpanel is supposed to be wired with all the bare ground wires to one bus bar that also is grounded to the metal of the box, and then grounded back to the main panel via its own wire. Then all the 110V white neutrals are landed on another bus bar that is NOT grounded to the metal of the subpanel box. Instead, it is wired all the way back to the main panel where it is connected to the same ground bar that the other ground wire is connected to, and then to the earth ground.

As I recall, it has to do with the fact that if a ground rod is driven at the main panel and another ground rod is driven at the subpanel, they will almost never be at the same potential due to differences in ground moisture, etc. By wiring the ground back to the main panel a ground rod isn't needed at the subpanel, but can be used if wanted (it doesn't really do anything, so why bother). The problem with having two ground points at two different locations w/o having a ground wire tieing the two together is that they won't be at the same potential, and electricity can do some very strange things if something gets shorted.

It will work just fine the way you have it, but I wouldn't put another ground rod at the generator. With only 25' of wire I wouldn't even worry about it, and there is no good reason to tie the two grounds together at the generator. Just use it like before and consider your house as still being the main panel instead of the subpanel.

The EEs are mistaken on this point and this code quote supports that fact altho the reasons for it are not perfectly clear in the code. The key point is that there can be different potentials (volts) to ground if correct connection protocols are not observed. On any current carrying lead some voltage is dropped across that lead due to its resistance. This means that the two ends of the lead are at different potentials. Thus if you connect one end of your neutral (which is a current carrying lead when the load is not equal in both legs of the 240 circuit) to the case (ground) of your generator, the other end of that current carrying neutral will be at a different potential than your gen case. Since that other end is tied to a good ground at the service box, that end will be at ground potential. The end back at your generator will have a potential, a voltage, to ground. Since this end is also connected to the case of your gen, that means your gen case also has a potential to ground - the one you are standing on. In favor of the EEs, this potential is low - - not enuf to feel the shock when you touch the gen - - if something has not gone wrong to somehow cause a high resistance in your neutral circuit. Even if everything is right this type of low potential will keep horses from drinking from an electrically heated water trough. Dont connect a neutral to any case except the service box. Thats why the gen has 4 wire output on 240, to alleviate the temptation presented on 3 wire 240 output gens, to combine neutral and ground at the gen.

 

[_RaT_]

Larry, I'm confused. The main panel on the house that also has a main disconnect will still be the main panel right? In it, you have isolated neutral and ground buss's. The neutral will go to the generator neutral but not combine with the generator equipment ground. The generator ground will tie to the ground buss only on the main panel which is grounded via a ground rod. Here we have to bond neutral and ground in sub panels but not in the main service panel. Another question I have is that I know that generators are sold as either construction or as back power generators. They ground differently as I recall. I have enjoyed trying to follow your posts, but two glasses of wine have me thinking to hard. Rat...

 

[SPYDERLK]

RaT, yeah, I feel your pain. This is a hard enuf thing to get clear in ones own mind let alone try to tell another in an understandable fashion.

Yes, I consider the main panel to be the house panel - differing from kmdigitals interpretation. I dont think this distinction is important in this case tho. What I think is important is that a ground wire should not normally carry current - instead it should do so only to conduct an electrical fault to earth. My favorite idea for this hookup would be:
1) Run the neutral and 2 hots to the house panel.
2) Connect the neutral gen wire to neutral/ground at the house panel.
3) Run a 4th wire from generator case ground to neutral/ground at the house panel. This wire would carry virtually no current, like maybe acouple milliAmps, unless something goes wrong with the gen. With no or very low current in this wire the gen ground and house ground are the same potential.

An alternative would be to drive a ground rod at the gen instead of running the 4th wire. Good either way, but the 4th wire allows some more freedom to move the gen around.

Any combination of ground and neutral at the generator is an unoptimum situation lending toward potential differences and currents where they cant do any good. In this setup, even driving a ground rod at the gen doesnt solve everything. The neutral wire grounded to earth at gen and house just uses the earth as a parallel neutral path - an extra neutral wire, ensuring constant ground currents and causing the 2 groundpoints to be at different potentials.
Im not familiar with the differences in construction and backup generators.
Did I understand you to say that CA doesnt bond neutral and ground at the main panel. How does that work? Do all houses have subpanels then?
Larry

 

[Tom_Veatch]

...
My favorite idea for this hookup would be:
1) Run the neutral and 2 hots to the house panel.
2) Connect the neutral gen wire to neutral/ground at the house panel.
3) Run a 4th wire from generator case ground to neutral/ground at the house panel. This wire would carry virtually no current, like maybe acouple milliAmps, unless something goes wrong with the gen. With no or very low current in this wire the gen ground and house ground are the same potential.

An alternative would be to drive a ground rod at the gen instead of running the 4th wire. Good either way, but the 4th wire allows some more freedom to move the gen around.
...

Let's see if I'm understanding this correctly, the OP has 3 wires from the 'on house' generator connector which feeds his 'in-house' service panel, 2 hot wires and 1 "non-hot" wire he called 'ground'.

If I'm reading it correctly, your alternative to running a 4th wire is to connect the already present "non-hot" wire to the neutral position in the 'on-house' generator plug/connector and to the neutral bus in the 'in-house' panel. Then drive a ground rod and connect it to the ground position of the 'on-house' generator plug/connector.

Thus, when the generator is plugged into the 'on-house' connector, the two hot wires from the generator connect with the two existing hot wires in the house, the generator neutral wire connects with the 'non-hot' house wire which is connected to the panel's neutral bus. The generator ground wire connects only to the driven ground rod and makes no direct connection with any of the three existing house wires.

I hope I'm reading it correctly, 'cause that's exactly what I did under much the same circumstance.

 

[SPYDERLK]

Let's see if I'm understanding this correctly, the OP has 3 wires from the 'on house' generator connector which feeds his 'in-house' service panel, 2 hot wires and 1 "non-hot" wire he called 'ground'.

If I'm reading it correctly, your alternative to running a 4th wire is to connect the already present "non-hot" wire to the neutral position in the 'on-house' generator plug/connector and to the neutral bus in the 'in-house' panel. Then drive a ground rod and connect it to the ground position of the 'on-house' generator plug/connector.

Thus, when the generator is plugged into the 'on-house' connector, the two hot wires from the generator connect with the two existing hot wires in the house, the generator neutral wire connects with the 'non-hot' house wire which is connected to the panel's neutral bus. The generator ground wire connects only to the driven ground rod and makes no direct connection with any of the three existing house wires.

I hope I'm reading it correctly, 'cause that's exactly what I did under much the same circumstance.

Tom. Yes, thats it. That ground is there when you need it, but not doing any work until you need it.
Larry

 

[gizmo]

blah, blah, blah...
Again, tie them together and it will work just fine. If you are concerned about potentials on the generator case, run some 120vac load in the cabin and check the case voltage to ground.

 

[Tom_Veatch]

Michael, I'm not an EE but I do have a fairly good grounding in basic circuit theory. I don't believe anyone has any doubt that tying neutral and ground together at the generator will work, electrically. But the NEC is pretty explicit and consistent in it's requirement that the safety ground not carry current in normal operations.

I've gotten crosswise with code enforcement folks more than once. Everytime it's happened, they've prevailed, and it's cost me extra time, effort, and money to tear out and rebuild. So, even if I don't apply for a permit, unless there is some real, specific, strong, overriding, and defensible reason to do otherwise, then my attitude is build it such that it satisfies whatever building codes apply. That's just my opinion and is non-binding on anyone else.

 

[hayden]

Get the 4 wire cable and do it right. The whole point of a separate ground and neutral is to provide protection in case a fault develops elsewhere in the system. With 50A at 240V outside on a tractor, perhaps in the rain, I'd want a safety ground.

In your previous setup are you sure the 3rd wire was ground and not neutral? It makes a big difference. Neutral would be a center tap on the generator windings such that you have 120V between it and each of the line wires, and of course 240V from line to line. If it's a ground, then it would not be tied into the generator windings at all and just be bonded to the generator frame. If it's really ground and not neutral, it's a miracle you didn't burn up any appliances since the voltage on the 120V circuits in your house would vary anywhere between 0 and 240V depending on the loads. The netral center tap is what forces the two halfs of a 240V curcuit to be 120V, and not some random combination of voltages adding up to 240V.

 

[frankc]

WOW, I thank everyone for responding. I think that just to set all questions aside, I'll tell you that the old (2hot 1ground) plug was a:
NEMA 6-50
The NEW plug (2hot, 1 neutral, 1 ground) is a:
NEMA 15-50

The Generator is a Tiger Power PTO generator, it has a Ground bolt on the case that is intended to go to a PHYSICAL ground rod.
The generator is actually only 4 feet away from the ground rod that is used to ground the MAIN indoor panel. I could use that ground rod or put in a seperate one that only the generator case is attached too, Let me know which is preferrable.

Based on my understanding of the conversations, I should:
1) Ground the case of the generator to physical ground.
2) Run the 2 hots as I would normally.
3) Run the neutral from the generator thru the 3rd wire of my cord to the neutral bus bar in the Main Panel.
4) Run a seperate insulated wire into the NEMA 15-50 plug that plugs into the generator connecting this to the ground Pole in the plug then run the other end to the ground rod that I installed in step 1.

Is that correct??
And yes, I do want it to be safe. The generator is outside (actually under the deck which is roofed, so it does stay dry unless the wind is really blowing.
And as I said, if there is a safe solution without having to replace that heavy wire cord, I'd like to use it. Just go price 6-gauge 4wire cable and you'll be shocked at what it costs.
Thanks for everyone's help.

 

[SPYDERLK]

Frankc, you are well set up. That close, I would ground to the house ground rod if you can get a good clean coupling. Right there is your 4th wire - - hardwired from gen ground to ground rod to house box. I would consider not doing #4 if you are confident of your connections at the ground rod. It would be redundant.
Larry