Pa's Wiring Conundrum (Longest Ever)

[al hildenbrand]

Here,

Assume all the connections are tight and solid.

Think of the source voltage at the secondary terminals of the PoCo transformer. There's 240V (or some higher voltage) from line (L1) to line (L2) and 120V line to center tap.

Put the motor out on the end of 371' of conductor (one way).

As the 70A start current flows around line L1 and the grounded conductor connected to the center tap, the 120V at the xfmr will decrease by the voltage drop of the line conductor resistance times the current.

At the same time, the 70A start current returning to the xfmr center tap will cause a voltage rise toward the line L1 further decreasing the load voltage at the saw motor.

The brightening of lights on the L2 side comes from the voltage on L2 not changing while the grounded conductor and (after the service disco) the neutral voltage rise moves away from L2 towards L1.

 

[George Stolz]

I don't get it.

 

[al hildenbrand]

Hi George,

The first diagram shows what the PoCo service lateral between the PoCo transformer and your Pa's service disco looks like, IMO, along with a path including the conductive water mains of the municipal water system.

The voltages in Black are a voltage measured from point to point on the circuit, the points shown by the arrows.

The voltages in Red are voltages measured to a theoretical equipotential ground plane, that is, the ground symbol.

To simplify things a little bit, let's pretend that all the neighbor's discos are open. Also, I am assuming the two lights illustrated are 13 watt compact fluorescents, so their current is very small, by comparison to Pa's saw start current, and is negligable, especially as it affects the voltage on L2.

The lower diagram is an equivalent diagram of the first.

The 70 Amp start inrush on the 120 Volt saw motor will cause voltage fluctuations at loads connected to each and/or both lines L1 and L2. The voltage fluctuation for the light at the house is entirely a function of the resistance in the PoCo service conductors all the way to the transformer secondary terminals.

The less resistance in the PoCo service conductors, the smaller the voltage fluctuation at the house light.

The barn voltage fluctuations are a function of the PoCo service conductors and the house to barn feeder conductors.

Even though the feeder size is increased, and all the connections (PoCo's and Pa's) are good and tight, the PoCo service conductors between the transformer and Pa's service disco will cause voltage fluctuations. The question is "how much?" is coming from the long PoCo conductors.

 

[George Stolz]

Al, first off, thanks for devoting so much time to thinking about my situation and then drawing out your diagrams to help me understand. I truly appreciate it.

Now, I once again don the dunce cap. I just don't get it.

I guess, at the outset, my problem is that I see the parallel neutrals and ground paths as decreasing the resistance (since resistors in parallel are less than the smallest resistor in the circuit). So that primary rule is a roadblock in my getting to understand what you're trying to tell me.

I appreciate your patience, and I hope you will give it another try.

 

[ramsy]

Al, ..I appreciate your patience, and I hope you will give it another try.

Al may only be addressing how high-Z PoCo lines to transformer can create v-drop unevenly between phases, and perhaps the contribution some municipal water mains (earth) can provide to a high-Z service neutral, regardless of feeder size or parallels.

The question is "how much?" is coming from the long PoCo conductors.

Shouldn't the PoCo service conductors be lifted to check their voltages without any earth assistance / plumbing bonds.

 

[al hildenbrand]

So that primary rule is a roadblock in my getting to understand what you're trying to tell me.

I appreciate your patience, and I hope you will give it another try.

The resistance between the house and the transformer secondary terminal is not as low as your primary rule leads you to think. . .the evidence is in the fluctuation of the light at the house on the other leg than the leg that is supplying the saw.

The fluctuation that you called out earlier in the thread

So, you think the upswing on the opposite phase inside the house could be from the undersized feeder going out to the barn?

is caused by a high grounded service conductor resistance.

The high neutral resistance contributes to the voltage sag at the load, and the voltage rise measured from the neutral to the other leg.

Or. . . .The voltage upswing at the house on the opposite phase is the contradiction to what you think your primary rule dictates.

 

[al hildenbrand]

Shouldn't the PoCo service conductors be lifted to check their voltages without any earth assistance / plumbing bonds.

Do you suggest temporarily disconnecting the neighborhood service main bonding jumpers from their respective grounding electrode systems, for the purposes of the test?

 

[ramsy]

Do you suggest temporarily disconnecting the neighborhood (bondings) for the purposes of the test?

No

The idea is to avoid lifting the service neutral or perhaps main bondings providing unique service-earth combo performance at a dweling disconnect while loads are running. Since the remaining phases or line-to-line voltages, with poorly grounded/neutral performance could damage shared neutral loads L-N on multi-wire branch circuits.

The PoCo's outside lineman probably know best how to safely test the service, for an accurate v-drop reading at dwelling disconnects.

The problem:
Opening a load side disconnect / switch only shows service line-line voltage drop without a load. Since neutral bonds are not usually switched, controlling any earth assitance (GEC) to the service L-N performance test requires lifting the main bondings.

It seems to me, when a service neutral is suspect, the only guarantee of avoiding damage to inside wireing is don't lift main bondings (GEC) then risk a load test without earth assistance.

Maybe load test the service conductors L-N with a portable load, maybe the saw on a suicide cord to service conductors, if its protected or fused of course.

 

[George Stolz]

Update: With the new feeder (4/3 AL), the voltage still drops to 99V on the saw's phase, and the opposite phase raises to 127V as before. So, I guess a call to the POCO is in order.

At least the romex is out of the ground...

 

[al hildenbrand]

Hey George,

Any feedback from the PoCo yet?

 

[George Stolz]

No, Mom's holding off until my nephews are out of town. "Too much commotion to think," she said.