Generator/Electrical question

[frankc]

The setting: Cabin 8 miles from any public power source. So, there is little chance in my life that public electric will ever be available. SO, when I wired the cabin, I set up the electric service box just as I would if it was being served from a power company, and bonded the Neutral and Ground bars together, then run to an external ground rod. I supply power to the box via a 240V 50Amp breaker wired from a 6-gauge 2wire plug. (2 hots, 1 ground std welders plug).
This has worked just fine no problems.

THE QUESTION: I just got a new generator that has a 4 wire plug on it, (2 hots, seperate neutral, ground). I know the right answer is to spend the money and change my setup from the current 2/1wire to a 3/1 wire setup, but given the expense of that heavy wire and that I need 25ft of it, I'm wondering since the neurtral and ground are bonded in the box(thus they both go to ground), why can't I just buy the appropriate 4-prong plug to plug into the generator but jump the nuetral to the ground in the plug there, thus I could still use my current wiring.

Basically I'm just wondering why I can't pass the neutral to the ground in the plug at the generator since that's whats basically happening in the main electric box????

 

[SPYDERLK]

Frank, there is probably something operating on 120V in that welder - like a fan. The electrical draw of such fan will complete its circuit thru the neutral line, causing a small voltage drop in that line. This means that out at the end, where your welder is, the neutral will have a voltage above true ground. Now you connect that neutral to your welder ground and the small voltage on the neutral is present on the welder case. This is the rationale for the rule that the ground should not normally carry current. You would probably never feel the small voltage unless something went wrong with the fan causing it to draw a bunch more current, or worse, if something went wrong with the neutral connection at the service box. In this latter, one leg of the Hot would appear on your welder case. That would be 120 and yould feel it! - - A couple of alternatives; 1) run a single solid wire conductor from your box ground to welder plug ground, or 2) drive a ground rod at the welder and ground welder to it. I think the 1st would be totally satisfactory to attach the ground wire to your existing line w/o buying 3/1 cable Just label it well.
Larry

 

[frankc]

Thanks for the prompt response Spyder!
But, I must have not articulated the whole scenerio correctly, so let me try to clarify.

No Welder is being used, the old plug is just sometimes refered to as a 50AMP Welder plug!

My NEW Generator is a PTO generator and has a 4-prong 50AMP plug on it (2 hots, 1 neutral, 1 ground),
My OLD generator had a 3-prong 50AMP plug on it (2 hots, 1 ground).
I have 6-gauge 2/1 (2 hot, 1ground) wire that is wired into the utility panel just like the power company would wire in from the street, then on the other side it plugs into the generator.

So, since the NEW generator has a seperate neutral I am wondering if I could just jump that neutral to the Ground wire in the plug out at the generator so that I wouldn't have to run all new heavy wire from the utility panel out to the generator. I should add that I was planning to put in a seperate physical ground rod outside next to the generator and ground the case of the generator to this rod. Hope I did a better job explaining, do you think my plan is OK???

 

[SPYDERLK]

OOps, sorry - read that wrong, but same idea applies. Dont tie the generator case to a current carrying line (neutral) because this will cause your generator case will have some degree of electrical heat when the two sides of 120 are not perfectly balanced.
Larry

 

[SPYDERLK]

Frank, your original explanation was fine - - I just saw welder and grounding of neutral and didnt really read it thru. The ground rod is good. Just tie the gen ground pin to the rod and float the neutral at the generator end. That way your case will not go hot and flow some of the current thru the earth everytime the neutral carries current. You shouldnt need the hardwire ground to the box - - the earth will take care of that unless it is really really dry.

 

[gizmo]

EE here. It will work just fine to tie the two together.

 

[SPYDERLK]

EE here. It will work just fine to tie the two together.

It may work fine because of the low level of AC that the case of his generator will have on it. However, the case of the gen is supposed to be at ground, and it is counterproductive and could be dangerous to tie the case to a current carrying lead since that assures that the case is not at ground potential unless the case has a rock solid connection to ground. If you float the neutral the case remains at ground potential without such a critical need for a high quality ground.
Larry

 

[schmism]

EE here. It will work just fine to tie the two together.

what he said.... id go a step further and were ever you plan on parking your PTO generator often (i figure you have some spot you generally park it) put a second ground rod there and ground the generator to the ground....

 

[AndyMA]

Second EE here. Just tie the two together. They are bonded at the main box for a reason. There should be no potentional between neutral and ground.

Andy

 

[kmdigital]

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.