120v brewing, can i run everything off any neutral?

[Rivenin]

Sorry if the subject was confusing...
But!

I'm rebuilding a control box and am curious, last time i kept the neutrals with whatever power source they came from... this was fine but made for a TON Of wiring.

I have 2x2000w elements, 2 ssr's 4 outlets, 5 switches, 5 120v LEDs...

basically this, but a slight bit modified

would i be able to take the neutral from either line? or would the neutral need follow the same HOT line that it came from?

 

[Minbari]

you want to keep them seperate, or tie them together. Not just use one. the return current for two 15 amp circuits on just one neutral would certainly not be a good thing. (Like starting fire not good)

 

[BadNewsBrewery]

Are you running them through GFCIs? That's the only place I could think you might have an issue.

 

[scottyb710]

If your using gfis you can't share.if not you can share neutrals but there supposed to be on a two pole breaker now so you turn off one breaker for both power circuits.

 

[jleiii]

If your using gfis you can't share.if not you can share neutrals but there supposed to be on a two pole breaker now so you turn off one breaker for both power circuits.

Taking this one step further with GFCI, you must isolate both input lines and neutral from each other or you will trip the GFCI's non-stop. They operate on minute differences in current flow between both wires. Even an LED indicator wired between circuits is enough to trip the GFCI.

 

[Rivenin]

oh duh... completely forgot about the GFCI and trip issues...
Non coffee fueled question i guess! thanks gents!

 

[The10mmKid]

you want to keep them seperate, or tie them together. Not just use one. the return current for two 15 amp circuits on just one neutral would certainly not be a good thing. (Like starting fire not good)

You're 50% correct Minbari.

It is a very common practice to feed two 120V circuits with '3-wire' and share the neutral.

The breakers are on opposite phases.

To the OP, if you still have your original panel intact, jumper the neutrals together in the box as a test to see if the GFCI stay happy. You can also measure voltage at the plug/terminal block hots. If the feeds are on opposite phases, you will measure ~240V.

As an electrician, I'd prefer you keep them separate. Even marked.

You do know you can 'daisy-chain' the wiring between all the devices!
(I would however 'home run' the power feeds to the SSR's)

Similar to this image. (Don't pay no mind to the labeling)

'da Kid

 

[Minbari]

You're 50% correct Minbari.

It is a very common practice to feed two 120V circuits with '3-wire' and share the neutral.

The breakers are on opposite phases.

'da Kid

yes, do you have to size the wire to do that? if you have two 15 amp branches (the black and the red) and a single white return, all on 14ga, then what happens when you pull 15 amps on both branches? you are returning 30 amps on the return.

weather it is common practice or even code, it seems like a bad idea to me.

 

[The10mmKid]

yes, do you have to size the wire to do that? if you have two 15 amp branches (the black and the red) and a single white return, all on 14ga, then what happens when you pull 15 amps on both branches? you are returning 30 amps on the return.

weather it is common practice or even code, it seems like a bad idea to me.

It's kinda hard to explain without the necessary understanding of how electricity behaves.

Maybe this link will clear it up a little or make it cloudy as hell.

http://www.electrical-contractor.net/forums/ubbthreads.php/topics/98066/3/Multiwire_circuit_neutrals.html

'da Kid

 

[Minbari]

I am a EE, I understand how it works

residential being on a center tapped transformer will run the phases 180° out of phase. so I see that it actually cancels out, interesting. now if the OP runs 2 breakers of the SAME phase. This will not work as the return current will be additive.

I usually work in high voltage single phase and DC, on those conditions, you have to really watch your return wires.

 

[The10mmKid]

yes, do you have to size the wire to do that? if you have two 15 amp branches (the black and the red) and a single white return, all on 14ga, then what happens when you pull 15 amps on both branches? you are returning 30 amps on the return.

weather it is common practice or even code, it seems like a bad idea to me.

See how the residential 60hz sinewave has the +120v side 'out of time' with the -120v side.
Also notice that the 220V line doesn't add up to the two 120V lines.

Almost have to get the test instruments out to believe it yourself.

'da Kid

 

[Minbari]

See how the residential 60hz sinewave has the +120v side 'out of time' with the -120v side.
Also notice that the 220V line doesn't add up to the two 120V lines.

Almost have to get the test instruments out to believe it yourself.

'da Kid

that is not how it works. you have 120 volts positive and negative. 120vac is the RMS value of AC. that means it is the ~.707 voltage of just one peak. you have a full sine wave on both the outputs of the center tapped transformer. The two halves of the transformer are out of phase with each other by 180°. The center tap become the Neutral.

"phase" is kind of a misnomer in household wireing of this type. There is one phase of 240VAC. the center tapped transformer simple changes the reference point from one side to the other giving a relative phase shift.

This is a better picture that shows it.

obviously to get 240 VAC you use the full transformer with no center tap.

again everything in AC is in RMS unless it says it is peak. (which in household wiring they dont) a "120vac" outlet in your home is really about 340VAC pk-pk.

There is the RMS, and both peaks:

 

[ajdelange]

"phase" is kind of a misnomer in household wireing of this type. There is one phase of 240VAC. the center tapped transformer simple changes the reference point from one side to the other giving a relative phase shift.

I don't know about that. There are two non grounded conductors which happen to be 180 ° out of phase (and thus antipodal) in what is normally (and I suppose properly) termed a 'biphase system'. But in an office or apartment building (and I have even been in some homes - OK not in the US) the two phases are 120 ° apart (all referenced to the neutral and an arbitrary sinusoid at the line frequency). That's the voltages. Unless the power factors, even in a biphase system, are exactly 1.0 on each leg in a 120/240 (or 120/208) system the currents are not antipodal and thus a common neutral does carry current even when the magnitudes of the loads are the same. In a 120/208 system if each leg has a 1 amp purely resistive load a common neutral would carry 1 amp.


The interesting case of split loads with a common neutral is a 120/208 system where each load draws 100% of the breaker's rated current but the load on the leading phase is reactive such that the power factor is 0.8 (not a terribly low power factor). Now the common neutral current is 142% of the breaker's rating (if I did the math right) but the breaker will not trip. This makes me question as to whether code would permit the common neutral setup as suggested in #7 for apartments and offices with 120/208. With biphase this should not be a problem unless power factors got really low.

 

[Minbari]

If it is 120° apart, then it has 2 of 3 legs of a three phase in it

 

[bu_gee]

Only the "hot" wires are breakered. The neutrals all come back to a single bus which is directly wired to system neutral. Where I live we get a two-phase, two wire feed from the power company and supply our own ground and neutral. My phases are 120° apart.

Minbari: I have learned the hard way that being an EE has nothing to do with home wiring. I would say it is much more similar to plumbing that can kill.

 

[ajdelange]

If it is 120° apart, then it has 2 of 3 legs of a three phase in it

Correct. 120/208 panels are derived from two phases and the neutral of a Wye connected 3 phase feed. Which was my whole point. In such homes the 'phases' are indeed phases and use of the term, phase, is, thus, not a misnomer.

 

[ajdelange]

Where I live we get a two-phase, two wire feed from the power company and supply our own ground and neutral. My phases are 120° apart.

Where do you live and where do you get the neutral from? In most of the US and Canada the power company distributes 3-phase Wye to the neighborhood and usually a single phase (14.4 kV) to the individual groups of houses (with, of course, the neutral). A 60:1 center tapped transformer is hung between that phase and the neutral. I have seen systems where the 'neutral' is provided by the earth itself (e.g. sea water ground systems) but that depends on unusually good ground conductivity.

I have learned the hard way that being an EE has nothing to do with home wiring. I would say it is much more similar to plumbing that can kill.

In 50 years as a double E I have been called many things but never a lethal plumber.

 

[bu_gee]

Where do you live and where do you get the neutral from? In most of the US and Canada the power company distributes 3-phase Wye to the neighborhood and usually a single phase (14.4 kV) to the individual groups of houses (with, of course, the neutral). A 60:1 center tapped transformer is hung between that phase and the neutral. I have seen systems where the 'neutral' is provided by the earth itself (e.g. sea water ground systems) but that depends on unusually good ground conductivity.

Pepco territory just outside of Washington, DC. I don't recall exactly, but I think they call it "floating neutral." My neutral is wired to my ground stake. I get stupid problems like one of the phases stops working so half of the house wired on the right side of the panel suffers a blackout.

It is one of the many reasons it takes 10 days to restore everybody's electricity when we get a few rough thunderstorms.

 

[ajdelange]

So if I went into your neighborhood and looked at the pole that feeds your house (assuming the distribution isn't buried) I would see two (or 3) transformers with one secondary wire from each of two going to your house and the other wire from each to a ground stake next to the pole? If I measured 120 V at an arbitrary outlet I would measure 208 at your clothes dryer? If I looked in your panel I would see only two wires entering from the pole and a third wire going to a ground stake just outside your house and that wire would be connected to the neutral bus in the panel?

 

[bu_gee]

So if I went into your neighborhood and looked at the pole that feeds your house (assuming the distribution isn't buried) I would see two (or 3) transformers with one secondary wire from each of two going to your house and the other wire from each to a ground stake next to the pole? If I measured 120 V at an arbitrary outlet I would measure 208 at your clothes dryer? If I looked in your panel I would see only two wires entering from the pole and a third wire going to a ground stake just outside your house and that wire would be connected to the neutral bus in the panel?

Both (maybe all 3) phases are in one transformer can and there is no ground stake at the pole. Aside from that, you are correct.

 

[Subdivisions]

So if I went into your neighborhood and looked at the pole that feeds your house (assuming the distribution isn't buried) I would see two (or 3) transformers with one secondary wire from each of two going to your house and the other wire from each to a ground stake next to the pole? If I measured 120 V at an arbitrary outlet I would measure 208 at your clothes dryer? If I looked in your panel I would see only two wires entering from the pole and a third wire going to a ground stake just outside your house and that wire would be connected to the neutral bus in the panel?

There's no ground stake at the pole. In the US it's arbitrary as to the outlet voltage. Some places its 110, others its 120. It's double across the hots since they're 180 out of phase. 208 sounds more like a 3 phase voltage. The neutral is run in from the transformer and the ground is staked at the residence (usually). This is generally considered single phase 220

 

[Subdivisions]

double post

 

[Minbari]

Correct. 120/208 panels are derived from two phases and the neutral of a Wye connected 3 phase feed. Which was my whole point. In such homes the 'phases' are indeed phases and use of the term, phase, is, thus, not a misnomer.

My point was for the united states, where you have power from a single phase 240v center tapped transformer. Its not two phase.

And are you saying that house electrical is beyond my comprehension because i am a EE? Lol

 

[ajdelange]

Both (maybe all 3) phases are in one transformer can and there is no ground stake at the pole. Aside from that, you are correct.

Something just doesn't add up here. The absence of a neutral wire from the pole means that all neutral current has to flow through the earth and your ground rod. In a home where the panel phases are 120 ° apart the neutral current is half the sum of the currents in the low voltage circuits in your house. For example if one 'side' of your panel's loads were 30 amps and the other 50 you would have 40 amps of neutral current. If your ground rod impedance is 2 ohms that means an 80 volt drop across it. The absence of a ground rod at the pole means that neutral current has to flow to the nearest grounding point in the system which may be up to 600 m away if the utility follows the usual practices wrt multi grounded neutrals (as I understand them, anyway).

Forgive me for being so persistent but I have never heard of anything like this. There has to be neutral on the pole (well there doesn't have to be but its absence would be stranger still). Why wouldn't they bring it into the house? I suspect failure to do so violates code.

So if you don't mind let me ask a couple more questions. For what you describe there would have to be at least two wires on fairly hefty insulators running to the pole with the transformer but definitely more than one. Is that what you have? The transformer can would have to have at least two high voltage bushings (look like long insulators) connected two the two high voltage wires on the pole cross-arm. Do you have that? The transformer would have to have at least 3 and perhaps four low voltage bushings two of which we know goes to your house. Where do the other(s) connect?

How does the appearance of your transformer differ from this picture?

 

[ajdelange]

208 sounds more like a 3 phase voltage.

That's what he says he has. Two 120° phases.

 

[ajdelange]

My point was for the united states, where you have power from a single phase 240v center tapped transformer. Its not two phase.

In fact it is. Both EE's and electricians refer to the red and black wires as 'phases' and the white as neutral and it is appropriate to do so because the voltages on them are at different phase angles with respect to an arbitrary sinusoid of the same frequency. If those angles are different by 90° I don't think you would have a problem calling them 'phases' nor would I expect that you would if they are 120° apart. But you do if they are 180° apart. How about 179.999999? If you had an electronic circuit with a biphase clock and the two clocks were complementary would you call that a single phase clock?

As an EE you should be familiar with phasors. Two or more phasors represent a polyphase system. Does the fact that two phasors are pi apart mean that that the system is no longer polyphase? Yes, it represents a special case of a polyphase system, one in which the voltages of the phases are antipodal but it does not change the fact that it is a polyphase system.

Now lets consider the currents in a typical home biphase system. They are not antipodal if, for example, one phase is connected to a resistive load and the other to a reactive one or, indeed, if the power factors of both loads are not identical. The phasors do not line up, they are not 180 ° apart, it's not a single phase system.

And are you saying that house electrical is beyond my comprehension because i am a EE? Lol

No, just that you don't grasp this concept. Double E's usually do. I most often have this argument with electricians.

 

[ajdelange]

There's no ground stake at the pole.

Actually there should be.

NESC Section 096.C: Multi-Grounded Systems:
The neutral, which shall be of sufficient size and
Ampacity for the duty involved, shall be connected to a
made or existing electrode at each transformer location and
at a sufficient number of additional points with made or
existing electrodes to total not less than four grounds in
each 1.6 km (1 mile) of the entire line, not including grounds
at individual services.

What I don't know is whether Pepco follows NESC. I'd guess they would. My dog walk this morning made it clear that Quebec Hydro does.

 

[scottyb710]

You guys have some weird ways of getting power in Kansas your neutral comes from the transformer. It's fun to see the different ways things are done.

 

[ajdelange]

I'd forgotten this thread but since it popped up again...

I get stupid problems like one of the phases stops working so half of the house wired on the right side of the panel suffers a blackout.

Thought I had it figured out (corner grounded delta) but that wouldn't work so I've scrubbed the post.