A hot from one voltage panel and a neutral from a different voltage panel?

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KyleFowler

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Location
Maryville, TN
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Electrician
So this is purely a thought experiment that I can't believe just struck my brain. I tried Googling this and tried searching for it in this foum but came up empty handed. I'm sure I'm not the first person to think of this.

What would your multimeter read if you placed one multimeter lead on a 120v leg of power and the other multimeter lead on a 480v panel's neutral bar?

Even better what would lets say a 120v incandescent bulb do if hooked up this way?

What would your multimeter read if you placed one multimeter lead on a 277v leg of power and the other multimeter lead on a 120v panel's neutral bar? And what would a 277v incandescent bulb do if hooked up this way? Anybody ever seen something wild like this?

Or even crazier has anyone ever stuck one multimeter leg on a 120v leg and the other multimeter lead to a 277v leg and seen what voltage pops up?

I can't believe I've never thought to try this stuff before and I am planning on testing this stuff out with my multimeter when the infrequent opportunity arises. And while tempting I wont actually try hooking up a load, seems unsafe lol. The closest I've ever done something like this is measuring ungrounded Delta phases to earth ground voltages before but that was years ago. If memory serves correctly I believe I got wild readings that may have been higher than phase to phase.
 

infinity

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Ultimately both neutrals are connected together so you should read near the phase to ground voltage that is at the panel.
 

KyleFowler

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Location
Maryville, TN
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Electrician
Ya I guess you are right, in my head I was thinking about the feeder to a transformer containing no neutral but there is still an earth ground on the load of the transformer bonding to the neutral to handle ground faults. Whomp whomp. Im still curious to test this out in the field and also what my multimeter would read between a 120v leg to a 277v leg
 

ActionDave

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... Im still curious to test this out in the field and also what my multimeter would read between a 120v leg to a 277v leg

A 120V hot to any available ground/neutral will read 120V, a 277V hot to any available ground/neutral will read 277V.

Extend your thought experiment and ask what happens if tie a 277V hot to a 120V neutral and then open the neutral. What does your meter read from neutral to ground now?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
181202-1703 EST

KyleFowler:

There is nothing wrong with what you want to do. Actually it can be very useful in troubleshooting. From a safety perspective nothing more than what you should normally do.

To troubleshoot certain problems I would suggest you place two three prong receptacles at the main panel, one for each phase. Use at least one extension cord to go from the main panel to where you want to make measurements. Depending on what you want to measure it may be useful to simultaneously have two cords. Also you may want to make measurements at the main panel. The extension cords are just long test leads for you. You also should have a test load. A 1500 watt portable heater is useful.

At the main panel the voltage difference between the EGC bus and the Neutral bus shoukd be in the low millivolt range.

At the destination of a circuit under consideration with no load on the circuit the voltage difference between the main panel points and the destination points (EGC to EGC, neutral to neutral, and hot to hot) should be near zero. This comparison will identify cross wiring or possibly opens.

Use the EGC reference from the main to measure to the destination neutral or hot with with the heater load on and off. Both changes in voltages should be about the same. If not, then the difference may imply a poor connection some place. You can estimate the expected change in voltage resulting from the heater load by assuming the heater is 10 A at 120 V, knowing the expected resistance per foot for your wire, estimating wire length, and calculating expected voltage change. The destination measurement should be at a duplex socket with the voltages measured from 1/2 of the receptacle and the heater load on the other 1/2.

Go play and see what results. Note that your hot goes thru a voltage drop across its breaker in the main panel. A 10 A change produces about 0.35 V change across a 20 A QO breaker.

Voltage measurements between different panels will have errors that result from voltage drops between the neutrals of the different panels.

.
 

GoldDigger

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Thought experiments and tests aside, note that it is a code violation for a branch circuit hot and the corresponding branch circuit neutral to originate in different panels, even if they come from the same voltage service.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
181202-2331 EST

GoldDigger:

That was my original thought of what he wanted to do. But later it seemed like he was talking about measurements. So we really need to know what his thinking was.

.
 

KyleFowler

Member
Location
Maryville, TN
Occupation
Electrician
181202-2331 EST

GoldDigger:

That was my original thought of what he wanted to do. But later it seemed like he was talking about measurements. So we really need to know what his thinking was.

.

Ya I understand you cannot just grab a neutral from anywhere, I was just more curious about voltage readings for science sake lol. I have yet to get a chance to gather any readings. I'm still really curious what the multimeter voltage reading would be from a 277 volt hot to a 120v hot
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
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Electrical Contractor
Ya I understand you cannot just grab a neutral from anywhere, I was just more curious about voltage readings for science sake lol. I have yet to get a chance to gather any readings. I'm still really curious what the multimeter voltage reading would be from a 277 volt hot to a 120v hot
Again, it can be anywhere from 277 + 120 to 277 - 120.
 

jeremysterling

Senior Member
Location
Austin, TX
If you perform commercial service work you will occasionally (usually incidentally) find loads wired like your theoretical incandescent lamp. I find them during inspections, troubleshooting, or panel schedule surveys. Rooftops are an example of a place where an incompetent person may choose to be creative when "developing" a circuit.

Sometimes, I'll ampclamp panel feeder wires and discover an imbalance that can only be explained by a "bootleg" neutral somewhere in the branch circuitry.

The challenge can be explaining to customers why you must correct the miswire, even though the customer claims, "Its been working fine since the last 'electrician' wired it up."

The NEC installation rules are your guidance. Safety concerns and best practices are additional reasons to correct the wiring.
 

GoldDigger

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Ya I guess you are right, in my head I was thinking about the feeder to a transformer containing no neutral but there is still an earth ground on the load of the transformer bonding to the neutral to handle ground faults. Whomp whomp. Im still curious to test this out in the field and also what my multimeter would read between a 120v leg to a 277v leg

If, as is most likely in the absence of exempting features, both neutrals are required to be grounded, the voltage between the two will be at most the sum of the neutral voltage offsets resulting from the resistive voltage drop in the neutrals carrying unbalanced current.
If both neutrals are ungrounded, and there are no other grounds in the two voltage systems (e.g. corner grounds) then all that you could possibly be reading would be a phantom voltage seen on a high impedance meter.
In that case, you can probably assume that the neutral "voltages" will be the result of unbalanced stray capacitance to ground. In that case the result could vary anywhere between zero and 397 volts.
(I am ignoring the possibility of an offset voltage greater than the nominal voltage as the result of restriking arc fault to ground, since that would be a decidedly abnormal situation.)
 

kwired

Electron manager
Location
NE Nebraska
Ya I understand you cannot just grab a neutral from anywhere, I was just more curious about voltage readings for science sake lol. I have yet to get a chance to gather any readings. I'm still really curious what the multimeter voltage reading would be from a 277 volt hot to a 120v hot
POCO's do this a lot with their MGN- it is one conductor carrying both primary neutral and secondary neutral currents.

We could do that with premises wiring and it would work, have to be careful on determining the load and make sure you ran a conductor that can handle the load. Opening a conductor with load on it has some negative consequences and I'm sure is a big reason we are pretty much limited to just multiwire branch circuits being what can share a common neutral conductor.
 

JFletcher

Senior Member
Location
Williamsburg, VA
Ultimately both neutrals are connected together so you should read near the phase to ground voltage that is at the panel.

How so? Say a building has a 480 volt Delta service... Panel number one is fed from a 480 y / 277 volt transformer. Panel number 2 is fed from a 480/208 volt transformer. How are the neutral points of those two transformers connected?

Along the original posters question, I'm curious as to what would happen if one were to take a high leg Delta service and center tap either the AB or BC legs in addition to the normal center tap on AC, and measure voltage between the two 'neutral' points.

As to the original posters question, and Larry's answer, I do not think one would see 397 volts with three-phase systems; the two waveforms at best would be 150 degrees or 210 degrees out of phase, not 180. I admit that such an experiment is a bit beyond my mathematical ability to prove, know anyone that has the math that can prove it, I'm all eyes.
 

kwired

Electron manager
Location
NE Nebraska
How so? Say a building has a 480 volt Delta service... Panel number one is fed from a 480 y / 277 volt transformer. Panel number 2 is fed from a 480/208 volt transformer. How are the neutral points of those two transformers connected?

Along the original posters question, I'm curious as to what would happen if one were to take a high leg Delta service and center tap either the AB or BC legs in addition to the normal center tap on AC, and measure voltage between the two 'neutral' points.

As to the original posters question, and Larry's answer, I do not think one would see 397 volts with three-phase systems; the two waveforms at best would be 150 degrees or 210 degrees out of phase, not 180. I admit that such an experiment is a bit beyond my mathematical ability to prove, know anyone that has the math that can prove it, I'm all eyes.
If installed in accordance with NEC the neutral points of each SDS will all be bonded to the premises grounding electrode system - making them effectively all the same potential. You may have slight voltage differences because of voltage drop on neutral conductors depending on exactly where you are measuring and how much neutral load is present.

Center taps on other phases - I didn't do the math but draw a triangle with 60 degree angle and two 15 degree angles, make the two long sides 120 or 240 (whichever voltage you want to represent) and calculate the length of the short side - that will be the voltage you would see. You will see same voltage to the opposite corner as you ordinarily do on the high leg on all three sides. You can only ground one point in the system though or else you create low impedance path and have high current flow between bonded points.

And yes phase displacement does impact what you will read between 480/277 conductor and separately derived 208/120 conductors, and you seldom would see 397 volts between systems, but is not out of question to see over 300. I think you still can find this with somewhat simple trigonometry if you don't understand more complex math, just lay each wye over one another with both neutral points at same position and at proper displacement. Draw lines between points to be measured and use trig to calculate length based on whatever triangles you have created.
 

david luchini

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As to the original posters question, and Larry's answer, I do not think one would see 397 volts with three-phase systems; the two waveforms at best would be 150 degrees or 210 degrees out of phase, not 180. I admit that such an experiment is a bit beyond my mathematical ability to prove, know anyone that has the math that can prove it, I'm all eyes.

For a standard 30deg shift delta wye transformer, the range would be about 183v to 386v.

For a standard 0deg shift wye-wye transformer, the range would be about 157v to 353v.

For a 180deg shift wye-wye transformer, the range would be 240v to 397v.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
181205-1157 EST

JFletcher:

With a wild leg delta that has one secondary, AC, center tapped, and grounded, that single center tap is called a neutral. I don't know when and why this was called a neutral. Possibly dates to Edison and that it was a mid point.

I will avoid the name neutral here.

Your question is what are the various measured voltages between center taps and end points of a delta secondary? This is a simple analysis of phasors (vectors).

For secondary AC's mid point (center tap) to either end point the voltage is 1/2 of AC. Same for each other secondary.

The phasor sum of the three secondaries is 0 volts. This comes from the three phasors forming an equilateral triangle with 60 degree corners. Draw a line between two center taps and you form another equilateral triangle. Thus, for 240 V secondaries the answer is 120 V.

Note: the 207.84 volts from B to the AC center tap comes from a right triangle of 30, 60, 90. 240 times the sin of 60 or cos of 30 is the length of the one long leg. These, sin or cos, are from 1/2 the sq-root of 3. or 0.866 .

.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
With a wild leg delta that has one secondary, AC, center tapped, and grounded, that single center tap is called a neutral. I don't know when and why this was called a neutral. Possibly dates to Edison and that it was a mid point.
If you recognize that the high-leg delta originated as a modification to existing single-phase installations, it's easy to understand why the neutral conductor kept the designation.
 

kwired

Electron manager
Location
NE Nebraska
If you recognize that the high-leg delta originated as a modification to existing single-phase installations, it's easy to understand why the neutral conductor kept the designation.
some call any "grounded conductor" the neutral, but that isn't always correct. The center tap on high leg delta is neutral to the one side of the delta though. It just isn't neutral to the entire secondary. More recent years NEC decided to consider it as a neutral to the entire system though.
 
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