Nice link, David. Multiwire branches is probably a better term that "Edison" wiring but I think they are the same.
Nice link, David. Multiwire branches is probably a better term that "Edison" wiring but I think they are the same.
My understanding is with a 220 line split into two 20 amp circuits with non-inductive loads the neutral carries something between zero to 20 amps:
L1=20A, L2=0 A, Neutral=20 A
L1=20A, L2=10 A, Neutral=10 A
L1=20A, L2=20 A, Neutral=0 A
This would be easy to test. With loads distributed on both sides of the line (lights, etc) put an amp meter on both legs at the service entrance, write down each reading, and compare it to the reading on the neutral line.
This makes sense since a straight 240v 20A load the electrons are traveling down both L1 and L2 and there is no neutral at all.
Because a shared neutral carries less amperage a residential service entrance wiring is often sized with the neutral smaller than the hot, for example 4/6/8 for 100 amp, 1/2/6 for 150 amps (H-H/N/G) This was code when I built my shop about 3 years ago.
I found this on an electrical forum:
Please note...the shared neutral line does not carry 40 amps if both 110 circuits are fully loaded. The shared neutral line actually carries 0 amps when both 110 circuits are fully loaded. Why? Because the current from one 110 volt circuit is 180 degrees out of phase with the other 110 volt line, therefore cancelling out in the neutral line. Trust me, the shared neutral line never carries more than 20 amps. If it did, the neutral line in a 12-3 cable would have to be larger than the black or red lines, but it isn't. Just saying...
This picture gets slightly more complicated with inductive loads in which case the currents between phase A and phase B may not necessarily be exactly 180 degrees out of phase, but the currents will still cancel each other out (in the neutral wire) to a degree. In any case, the current in the neutral wire will not exceed 20 amps.
I do agree that the DPDT switch is not needed and perhaps not even wise. An minimum it complicates things and costs more.
JKJ
I've always liked the water analogy too, since electrons flow in many ways like water. Without a source, the ground wire doesn't really do much most of the time.
Instead of a pipe, imagine the ground as being like the LA river (the one they use for all those movie car chases and races) - sure there might be a trickle of stray water present at times, but most of the capacity of the riverbed is unused except in a emergency, and then you're really glad you have it sitting there waiting to absorb the extra flow instead of flooding everyone's houses.
~Garth
Here are some diagrams that may help:
Improper Multiwire diagram.JPGMultiwire diagram.JPG
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
I stand humbled by the combined intellectual might arrayed against me!
My only defense is ignorance (and forgetting that whole parallel vs series nuisance). I learned something, so its been a GOOD day.
I mean this absolutely respectfully, but you are repeatedly failing to understand basic concepts, here. If you want to learn this for fun, that's fantastic, but you are not competent to do this work, yourself - for your own safety, please hire it done. (That's not an insult - I am, for instance, not competent to do brain surgery...).
The neutral is NEVER used to ground the equipment. It is not a "sometimes/depends" issue. There is ALWAYS a separate ground wire, and it NEVER carries current in normal operation. Yes, a 120V circuit will have a neutral wire at approximately the same potential as the ground, but you can't use it as a ground - it serves a separate purpose, and differentiating those purposes is essential for safety.
David,
That link does explain it well - and confirms what I was thinking. Doing it this way (12/3 to a properly-rated switch into one 240V load *or* two equal 120V loads) is safe, even if it is complicated. Thank you!
Chris,
I am pretty sure that I understand the similarities and differences. Electrically, so long as there is a wire that grounds the chassis, we are good. (Don't break out the pitch forks yet!) Realistically, it is important to use the correctly-labeled/colored wire for other people's safety (as they would not expect to have current potential on a ground/green wire and the neutral/white wire be used for earth grounding) - which is why we have the NEC - standardization.
Now now - let's not confuse DC with AC <laughs> That is a different beast in many ways. I get that we effectively have 120V of potential in two phases (which still strikes me as strange that we don't call it two-phase, but whatever - that is a different conversation), each of which is 120V different from the neutral, but one is +120V and one is -120V due to phase.
This and some of what David's first link said clarified the why of which wire (neutral with potential to carry, ground without potential to carry in normal operation). No worries.
Dan,
I appreciate your candor, but I am not failing to grasp them. What I am doing is failing to adequately express my questions and/or discussing a slightly different concept while confusing overloaded terms like neutral, ground, earth ground, potential, etc.
No insult taken, but let's be honest - even if I never understand the basics that you are trying to impress upon me, connecting power equipment (at this potential) is nothing more than an exercise in attaching the correct wire to the correct lug. Monkeys could do it <laughs> and I certainly can. Clearing up the misunderstanding of what wire - by convention - goes where does not contravene that; on the contrary, it *reinforces* that! Just like the hot wire (clad in black) goes to the gold lug, the neutral wire (clad in white) goes to the silver lug, and the ground wire (unclad/clad in green) goes to the green lug - that much can be managed, **safely**, without understanding that electrons flow like water, or that AC is passed in a sine wave or or or ... I am interested in learning more of the theory - and have - but even before that started, I was capable of matching colors/following instructions <grins>
<in all seriousness> Understood. And thank you, again, for your help!
It is this exact mentality that gets people hurt or killed every day. There is a reason that the NEC is almost two inches thick and nearly every page is written in the blood or ash of people that thought that "even monkeys can do it". The fact is that right now you don't even know how to ask the questions you need to be asking to do the work safely. The irony is that you are worried about your child when you might well be getting ready to burn his house down.
For the record; I'm a state recognized journeyman lineman and I have 25 years experience as a high voltage substation and metering tech.
John,
I appreciate your input, but the NEC is that thick because it covers FAR more than just 120V and 240V lines. I have wired plenty of 120V lines, both 15A and 20A; wiring a 240V line is only marginally more difficult than that. As you are a journeyman lineman, please go and tell me what percentage of the NEC deals with 240V lines compared to the percentage that does not - and no, you don't get to count the pages that talk about how to mount NM, or BX, or conduit - just the actual wiring (connection) portion.
And no, I am not about to 'burn his house down'. This is not the 1800s where I am stringing knob and tube through walls, it is not in a home built circa 1970 where I have to worry about possibly connecting aluminum wire to copper and dealing with thermal expansion. Let me lay it out straight for you and the others who do not quite grasp what I am saying:
Electrically - not by code, not by convention, not according to any authority, a neutral wire and a ground wire are the same. Read that again - e l e c t r i c a l l y. They are both conductors connected to earth. Nothing more, nothing less. By convention - and this is part of what I gathered today - electricians and those educated in the NEC expect the white-clad wire to be possibly carrying current when the circuit is active. Also by convention these same people expect the ground to not be carrying current unless there is an issue.
None of the 120V outlets I have wired in the past have ever been mis-wired. None of the 120V circuits I have wired have been under-wired/over-breakered (e.g., 20A breaker on 14/2 wire). None of the circuits I have wired have had too many receptacles on them (per MD: 10 on 14ga. and 12 on 12ga.) None of the GFCIs I have wired have been backwards w/r Line and Load. Never have I pierced the insulation of the NM I am running. When using metal boxes, I have never forgotten to ground them. Is there something else that I can tell you that I have done safely to allay your fears?
The second thing that I learned is that 240V motors do not require a neutral wire. I was mistaken because of some of the other posts that I have read (I am not certain if it was here or on another forum) where the connection was listed as HHN instead of HHG. That is my mistake, I will own it, and if it will appease you, I am happy to edit the original post to reflect that (though it will then make all following replies much less intelligible/useful in the future).
Believe it or not, I still appreciate your input. Be safe - you work with far more dangerous potential than I ever hope to see.
For reference, knob-and-tube wiring is actually incredibly safe - the huge distance between conductors, combined with the wires generally being ran in empty wall cavities (no insulation), made it nearly impossible to either short or overload. Even K+T wiring that was glowing hot would pose little risk, since there are ceramic insulators between it and anything flammable.
I'm with John, though.
To say that neutral and ground are "electrically" the same (whatever that means...) is a meaningless equivalence. What you really mean is that the neutral and ground are at NEARLY the same potential. The 12V battery in my car and the 12.8V battery in my laptop are also at NEARLY the same potential, but I guarantee there would be a fire if I tried connecting them in parallel.
The bare conductor wiring I saw in Moctezuma Mexico scared me. Several places had bare wires stapled to concrete block walls a few inches apart. Not much chance of fire, but in one store these were stapled where someone (a child?) could touch or lean against them. No inspectors, no codes, anyone could wire anything according to their own understanding. Someone is electrocuted?, oh well.
It is against code and safety to consider the neutral and ground the same and to even fasten (bond) them together in some cases. (There is a green bonding screw in electrical panels that must be removed if used as a sub panel.) Although the ground wire is in fact connected to the neutral in the MAIN panel, you do not bond the neutral to ground on a sub panel. I understand connecting them there is a significant hazard if a problem develops with the neutral line. It may not make logical sense that this could be a problem but when people have been shocked by touching grounded things when the neutral developed a fault.
A little knowledge is a good thing. If the electrical code is too hard to read, maybe search with google for "why does a subpanel need separate ground and neutral" or something similar. It will be a valuable education for the OP and anyone else who does their own wiring.
If you want to read only one thing, try this, one of the best explanations I've read:
http://www.woodcentral.com/woodworki...subpanel-long/
A teaser from that text: "...OK, so now you're thinking So what. ... It's all at ground potential so what's the problem?..."
For the extended story and a rare exception read the entire thread. Or just don't connect neutral and ground anywhere in the shop within a sub panel.
JKJ
Last edited by Chris Padilla; 09-01-2016 at 11:08 AM.