3-wire connection bonded?

[lonesouth]

I just moved to a new house and have not had time to run a new dedicated circuit for the generator. What I do have access to is a 3 wire dryer circuit that was abandoned. When using a 3 wire circuit, which of the below is the best solution? I will install an interlock on the circuit.

1. connect the bonding strap in the generator and use the single neutral to carry the ground
2. leave the bonding strap disconnected and drive a ground rod to use as the ground

This is won't be permanent, just looking for something to get me by until I get the remodel done or through hurricane season.

 

[Scoobyshep]

I would advise against using it. You would be better off installing a quick backfeed directly to the panel.

But if you are going to have to do it for a temporary emergency feed, then leave the bonding strap installed.

leaving the strap disconnected without a 4 wire feed leaves you open to the risk of a floating neutral (this is why separately derived systems have a bond in the first place) essentially it establishes the neutral and ground to be the same potential and gives the neutral a place to dissipate the unbalanced load. in systems where this bond is not established the higher load will have a lower voltage and the lower load will increase voltage proportionally .

watch your current dryer feeds are typically 10 awg and limited to 30 amps.

 

[lonesouth]

I would advise against using it. You would be better off installing a quick backfeed directly to the panel.

But if you are going to have to do it for a temporary emergency feed, then leave the bonding strap installed.

leaving the strap disconnected without a 4 wire feed leaves you open to the risk of a floating neutral (this is why separately derived systems have a bond in the first place) essentially it establishes the neutral and ground to be the same potential and gives the neutral a place to dissipate the unbalanced load. in systems where this bond is not established the higher load will have a lower voltage and the lower load will increase voltage proportionally .

watch your current dryer feeds are typically 10 awg and limited to 30 amps.

Thanks. It has a 50amp breaker, but I'll verify wire gauge before moving on.

 

[Scoobyshep]

Thanks. It has a 50amp breaker, but I'll verify wire gauge before moving on.

Youre welcome. if its 50 then is likely a range outlet Should be #6

 

[nextalcupfan]

Personally I would do everything possible to run a 4th wire. Either by changing the 3 wire to 4 or just running a 4th wire separately.
Neither should take much time or effort.

 

[Bmxenbrett]

breaker and wire gauge dosnt matter if your trying to power your house with a 3 wire 240v plug. You have no neutral! This means you can not provide 120v to the pannel. By doing this you may be putting 120v out to both prongs on a 120v outlet, this is very bad. If you use the 3rd wire for a neutral you will have no ground, again very bad.

if you really need to hook it up and are almost in the eye of a hurricane go to a big box store and buy 4 conductor wire and use the current breaker for your dryer. Two wires to each side of the breaker, one to the neutral bus bar and one to ground.

 

[Scoobyshep]

breaker and wire gauge dosnt matter if your trying to power your house with a 3 wire 240v plug. You have no neutral! This means you can not provide 120v to the pannel. By doing this you may be putting 120v out to both prongs on a 120v outlet, this is very bad. If you use the 3rd wire for a neutral you will have no ground, again very bad.

if you really need to hook it up and are almost in the eye of a hurricane go to a big box store and buy 4 conductor wire and use the current breaker for your dryer. Two wires to each side of the breaker, one to the neutral bus bar and one to ground.

Not quite.

In the older 3 wire range and dryer outlets they used to share the same conductor for neutral and ground. Definitely not ideal, nor the best practice, but as indicated above it was going to be replaced with a proper 4wire cable. It will work, but shouldnt be used unless absolutely necessary.

 

[gatorbob]

In the older 3 wire range and dryer outlets they used to share the same conductor for neutral and ground. Definitely not ideal, nor the best practice, but as indicated above it was going to be replaced with a proper 4wire cable. It will work, but shouldnt be used unless absolutely necessary.

Exactly.

What is the cable type?

A lot of the old range/dryer circuits used SEU cable with two insulated conductors and one bare conductor. I'm not sure if the bare conductor was always the same gauge as the two insulated conductors. It could be smaller but I honestly forget.

As someone else already said, you will have no dedicated grounded conductor for the generator with this setup. I think the risk would be the same as an old 3-wire range/dryer when a cable/receptacle/lug fails but it will nonetheless work.

Assuming the issue is you don't have time to feed new cable through buildings/sheetrock to fix this, there may be something you can do entirely outside on a temporary basis. Connect your 3rd conductor to the generator's neutral lug. Run a new single conductor cable of the correct size directly from the ground lug on the generator to your house's existing ground rod. Temporarily secure the new wire to the ground using landscaping fabric garden staples or some other method. The bonding strap would be removed inside the MEP.

 

[Scoobyshep]

Usually 1 gauge smaller, and unless hes really unbalanced it will be fine



And its good to see someone else who understands grounding and bonding.

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[gatorbob]

Usually 1 gauge smaller, and unless hes really unbalanced it will be fine

And its good to see someone else who understands grounding and bonding.

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It's confusing stuff and I'm still not sure having the grounded conductor not land at the service entrance would be a problem that matters in the real world. I imagine it's fine.

Might make sense to at least change out the female range receptacle with a male generator inlet you can buy at Home Depot. 30 minute job to swap that and make up a new cable. This is important in case someone trips over the cord or something pulls on it from outside due to the wind. The dog or whoever won't get electrocuted by having current on the range cord's male prongs. I believe that's why they call it a suicide cord. The new cord you make yourself (or buy premade) will have a female end for safety. Hard wire the other end directly to the generator. This is the bigger problem overall compared to the 3-wire issue, IMHO, and none of this expense goes to waste.

I would do this before ever worrying about an interlock on a temporary setup. Colored duct tape could be used as a cheaper and easier to install interlock for a weekend storm assuming no one else touches the breaker panel. Plus, the odds of this 50A range breaker being in the right position to even install an interlock are slim if you believe in Murphy's law. If multiple breakers have to to move, what are the odds the conductors are long enough to re-arrange everything in 100F heat with the power off. I don't have that kind of good luck.

 

[Scoobyshep]

Thats exactly why they are called suicide cords.

As for the return to bond. Code calls for bonding at the first means of disconnect so its either at the meter or in the panel. If its old enough to have a 3 wire outlet then odds are the bond is in the panel. If it isnt then theres 4 conductors going to the panel, at that point the ground will be carrying the unbalanced load to the bond and it will go to the neutral buss back on the neutral. Here the biggest risk is current on the ground (if everything is bonded properly youd never see it) if the cable lands in the panel with the bond ground and neutral are the same thing.


Isnt electricity fun?

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[LEOK]

The problem is that most ground rods and clamps don't provide a good ground. Which is supposed to be under 20 ohms. While places like data centers and communication closets are spec'ed and tested to be below 5 ohms. However the power company grounds every pole and every stepdown transformer will have driven grounds usually 20' or more with welded connections. Mostly for lightening protection. So the danger your service entrance neutral gets taken out by a tree limb. Now your 20 ohm plus ground can be a problem. If you somehow can provide a better ground. If you regularly fry things like DSL moderns redoing your house ground will fix it.

 

[Scoobyshep]

Actually the codes for grounding are 25 ohms or less OR 2 rods. So you can have really dry sandy earth and test a very poor rod and as long as you have 2 meet code. So your ground rods dont actually do as much as you think they do.

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[LEOK]

Your right poor ground don't do anything, the scenario is you loose your neutral service line to your meter now you ground becomes hot from bonding. So a metal water line could be a better ground and you could complete that circuit standing in the rain working on your gen. So that is why you should put in quality tested ground. Plus I spent 20 years doing data circuits and poor building grounds fry a lot of equipment in Florida.

 

[Scoobyshep]

In my years, ive done grounding from houses up to industrial and a few cell towers too. Lot of cad welding and the weirdest overkill bonding. Worst one i had to do was a 110 foot rod. That one was in a delta field so 3 rods 110 feet apart 110 feet down. When I redid my home service the inspector was shocked to see the rods welded

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