can 2 Bucket Transformers share a neutral? House to house backup generator circuit.

[paulsonsmithfamily]

Good day brothers,

A widow next door cannot afford a generator, and the Lord has blessed us with one more than ample.

So on Wednesday we plan to:
Run a 600' 20A 240V circuit to her generator sub panel she had installed during a service change.

Using #10 THHN

I plan to not run a ground and keep them separate, but then what about the neutral? As we are on different transformers, could this cause an "oh dear"?

Would I need to add a 3 pole double throw disconnect feeding the sub panel?
If so, there is still a chance a ground/neutral off the sub panel makes it's way back to the 2nd transformer....


Please help us help someone else (safely)

 

[ActionDave]

.....I plan to not run a ground and keep them separate, but then what about the neutral? As we are on different transformers, could this cause an "oh dear"?

Would I need to add a 3 pole double throw disconnect feeding the sub panel?
If so, there is still a chance a ground/neutral off the sub panel makes it's way back to the 2nd transformer....


Please help us help someone else (safely)

You need four wires but you don't need to switch the neutral. Think about it this way....

Drive a ground rod, hook up a wire from the negative terminal of your car battery to the ground rod. Go to your neighbor's car and hook up a wire from the same ground rod to the negative terminal on her car battery. What happens? Nothing.

It's the same thing with the separate transformers. Your whole neighborhood is tied to the same power company grounded conductor, adding the neutral from your generator won't do anything, you need a complete circuit for any current to flow anywhere. As long as your transfer switch is properly wired everything is fine.

 

[JFletcher]

Given the overwhelmingly high labor to materials cost on this job, and the infrequency of which you or she will need to run the generator, I would up size your feeder*, and I believe under any circumstance you're going to need a 4 wire feed.

The neutral on her transformer will not be the neutral point on yours phase-wise, her transfer switch is not going to break her neutral, without her neutral you have no way of powering anything--but 240 amp loads, and 300.3 (B) makes it so that all conductors of the same circuit must be contained in the same Raceway or cable assembly.

You also need and EGC, otherwise there is no path to ground back to the ocpd to clear any faults.

Trenching in 600 foot of conduit at the required minimum depth of 18' more per table 300.5, column 3, is no small job. you can skip the conduit if you go with a direct burial cable, tho you have to go down another 6".

According to Southwire calculations, running 600 foot of copper conductor at 240 volts with a 20 amp load would require #4 wire to keep voltage drop below 3%.

*Actually, given that a 500 foot spool of 446 aluminum SE cable is almost $500, it would be better to buy her a portable generator for her house than to try to wire her house from yours.

 

[JFletcher]

I missed Dave's post... I agree with him in regards to her neutral, I was thinking more of an open Delta Bank with a center tapped neutral being on another phase other than AC... That would result in an instant Short Circuit. Regardless of if her neutral would work, 300.3 (B) still comes into play.

Code aside, a 7000 watt portable generator would cost less than a properly sized feeder, let alone trencher rental, conduit, additional switches, labor, etcetera.

I don't see a code compliant or feasable way to secure Poco power at two separate locations before the generator is started and loaded.

There's also the difficulties that will arise when the OP or his neighbor move out, dies, etc with a feeder running across property lines.

 

[ptonsparky]

Buy her a generator or take your spare over there as needed.

 

[JPinVA]

So on Wednesday we plan to:
Run a 600' 20A 240V circuit to her generator sub panel she had installed during a service change.

How is her generator sub-panel transfer switch configured? Neutral switched (in support of a separately derived system) or not switched (non separately derived)
How is the generator configured? Bonded neutral or floating neutral.

How is your gen transfer switch configured? Same questions as above

Are the switches designed so that the feeder between houses is ONLY energized by generator power?

 

[Besoeker]

What's a bucket transformer?
Does it transform buckets inyo pails?
:blink:

 

[paulsonsmithfamily]

How is her generator sub-panel transfer switch configured? Neutral switched (in support of a separately derived system) or not switched (non separately derived)
How is the generator configured? Bonded neutral or floating neutral.

Older bonded Neutral transfer.

How is your gen transfer switch configured? Same questions as above

Older bonded Neutral.

Are the switches designed so that the feeder between houses is ONLY energized by generator power?

Her end has a typical interlock on the top two breakers. I will have a 20A breaker in our basement to turn on during an outage.

*** My main concern ***
When the grid powers back on and the pole "bucket" transformers energize, with the neutrals/grounds being bonded is there a chance of imbalance?

What if one of use lost a neutral at the POA or elsewhere? Both our systems are newer.


Thanks for the input brothers.

 

[ptonsparky]

Your EG and Neutral if not disconnected will parallel the primary neutral. IDK how much that will affect your small conductors.

We have somewhat similar install t a feedlot where standby power to waterwells are from one generator while normal power is from pole top transformers local to the well. 480v. No neutral loads.

 

[mivey]

You need four wires but you don't need to switch the neutral. Think about it this way....

Drive a ground rod, hook up a wire from the negative terminal of your car battery to the ground rod. Go to your neighbor's car and hook up a wire from the same ground rod to the negative terminal on her car battery. What happens? Nothing.

It's the same thing with the separate transformers. Your whole neighborhood is tied to the same power company grounded conductor, adding the neutral from your generator won't do anything, you need a complete circuit for any current to flow anywhere. As long as your transfer switch is properly wired everything is fine.

At the generator house (#1), a fault travels along the safety ground back to the panel then transfer switch then generator.

At the no-gen house (#2), a fault travels to the sub-panel back to house #1. It also takes a parallel path through the #2 service, along the POCO neutral (as well as phone, CATV, etc.), down the #1 service to the single-point bond, to the transfer switch, then to generator.

Separate premises should have separate sources.

 

[mivey]

Your EG and Neutral if not disconnected will parallel the primary neutral. IDK how much that will affect your small conductors.

We have somewhat similar install t a feedlot where standby power to waterwells are from one generator while normal power is from pole top transformers local to the well. 480v. No neutral loads.

Yes. The single-point ground on the no-gen structure creates the parallel path, even with the neutral switched at the generator.

Even switching both neutrals leaves a path along the POCO neutral during a fault. A separate source at each premise avoids that.

 

[JFletcher]

In your original post, you asked for help on how to do this safely. Your proposed method of using her service neutral to eliminate it in the feeder from your generator is not up to code.

Any imbalance from 120 volt loads, which would be pretty much everything with only a 20 amp breaker, the hot is going to come from one leg of your newly installed feed from your generator, neutral will be her SE cable back to the poco's grounding system, and circuitously back to the generator through the neutral to ground bond in your main panel. This is not allowed under 300.3(B) nor any of its exceptions.

You have a similar problem with the EGC, or lack thereof. It is a violation of 250.32 (B) to not run an EGC in that feeder.

Operationally, about the only things she's going to be able to run from a 20 amp 2p circuit would be her refrigerator, some lights, modem, some small battery chargers, a box fan, and one space heater or window shaker on the opposite leg of the refrigerator. all of this would have to be thought out in advance and the appropriate breakers flipped in her generator sub panel, otherwise you stand a good chance of overloading your 20 amp breaker.

The voltage drop on a number 10 run 600-foot at 15 amps is about 8%, so I retract my earlier statement regarding wire size. everything else stands, including the main thing which is this project, unless you are getting donated labor and materials, and can borrow or own the trencher, is simply going to cost more money than buying her a generator.

Edited to add... Since you are planning on using number 10 THHN, which I assume is dual rated as thwn for this outdoor application, I will also make the assumption that you are using a Raceway and not a cable type. In that case you can get a neutral and ground in the same conduit without having to upsize it.

 

[ActionDave]

At the generator house (#1), a fault travels along the safety ground back to the panel then transfer switch then generator.

At the no-gen house (#2), a fault travels to the sub-panel back to house #1. It also takes a parallel path through the #2 service, along the POCO neutral (as well as phone, CATV, etc.), down the #1 service to the single-point bond, to the transfer switch, then to generator.

Separate premises should have separate sources.

I will accept your assertion about the separate premises, but don't we have just one giant parallel grounded conductor, along with everything attached to it (water pipes, ground rods,..) already? especially on the poco side of the system?

 

[LarryFine]

Yes, but to wire that way is akin to pulling a line conductor from one circuit and a neutral conductor from another.

 

[mivey]

I will accept your assertion about the separate premises, but don't we have just one giant parallel grounded conductor, along with everything attached to it (water pipes, ground rods,..) already? especially on the poco side of the system?

Yes. The POCO system (a MGN system anyway) uses a multi-point grounded system. Even without a MGN, the system gets capacitively coupled to earth. On the NEC system, we use a single-point grounded system.

With only one bonding point, the SPG keeps load current off of safety grounds and other things like telecom shields, building steel, pipe, etc., at least except for the short hop from main to transformer. However, if the service neutral breaks we can see load current running all over the place as it finds a combined path back to the transformer secondary (the premise source) as it tries to replace the neutral.

Not lifting the neutral at house #2 puts load current everywhere as it tries to get back to the source at house #1. However, a ground fault follows that long parallel path(s) between houses even with the neutral lifted at #2. A separate gen for #2 is better. Or even better yet just directly plugging in #2's appliances is safer (equivalent to lifting the line, neutral, and ground at house #2).



On the POCO primary side, load current runs on the multi-grounded neutral conductor as well as on other ground paths. Some things come to mind that help mitigate the ground currents.

For one, we try to limit the neutral/ground current (balance feeders, use shorter single-phase feeders if loads are high, use two-bushing transformers as needed, etc.).

As for currents on everything attached, a high NEV is caused by high neutral current. Limiting the NEV helps limit excess ground current away from the main line (where the NEV is carried from the main line by conductor bonds). Some NEV is expected and is not classed a stray current source because it is considered normal.

Maintenance to check for broken/poor main line neutrals helps mitigate localized abnormal high ground currents.

Also, any ground currents tend to stay near the power line. It is a physical phenomenon and can be modeled during system analysis.

 

[mbrooke]

Yes. The POCO system (a MGN system anyway) uses a multi-point grounded system. Even without a MGN, the system gets capacitively coupled to earth. On the NEC system, we use a single-point grounded system.

With only one bonding point, the SPG keeps load current off of safety grounds and other things like telecom shields, building steel, pipe, etc., at least except for the short hop from main to transformer. However, if the service neutral breaks we can see load current running all over the place as it finds a combined path back to the transformer secondary (the premise source) as it tries to replace the neutral.

Not lifting the neutral at house #2 puts load current everywhere as it tries to get back to the source at house #1. However, a ground fault follows that long parallel path(s) between houses even with the neutral lifted at #2. A separate gen for #2 is better. Or even better yet just directly plugging in #2's appliances is safer (equivalent to lifting the line, neutral, and ground at house #2).



On the POCO primary side, load current runs on the multi-grounded neutral conductor as well as on other ground paths. Some things come to mind that help mitigate the ground currents.

For one, we try to limit the neutral/ground current (balance feeders, use shorter single-phase feeders if loads are high, use two-bushing transformers as needed, etc.).

As for currents on everything attached, a high NEV is caused by high neutral current. Limiting the NEV helps limit excess ground current away from the main line (where the NEV is carried from the main line by conductor bonds). Some NEV is expected and is not classed a stray current source because it is considered normal.

Maintenance to check for broken/poor main line neutrals helps mitigate localized abnormal high ground currents.

Also, any ground currents tend to stay near the power line. It is a physical phenomenon and can be modeled during system analysis.

Question- would it really cost POCOs that much to connect everything phase to phase? I think that would solve a lot of problems and potential mitigation.

 

[mivey]

Question- would it really cost POCOs that much to connect everything phase to phase? I think that would solve a lot of problems and potential mitigation.

For a single phase line you would have to have three conductors instead of two, plus added hardware for the extra phase conductor so no small cost.

For two-phase and three-phase lines you can just balance the loads. This can be more complex with L-L rather than L-G..

 

[mbrooke]

For a single phase line you would have to have three conductors instead of two, plus added hardware for the extra phase conductor so no small cost.

For two-phase and three-phase lines you can just balance the loads. This can be more complex with L-L rather than L-G..

But to be fair you ditch the neutral which saves some $$?

 

[mivey]

But to be fair you ditch the neutral which saves some $$?

Not with a MGN system.

 

[mbrooke]

Not with a MGN system.

On single phase circuits your neutral simply becomes phase two, on 3 phase circuits you end up with 3 wires instead of 4. At least how I imagine it. But if you say so I'll take your word for it.