Generator wiring question

wrote:


The neutral carries the difference between the hot leg currents. If the load current is 100A on one leg and 0A on the other the neutral will carry 100A. Increasing the load on the other leg decreases the current in the neutral, so with 100A through both hot legs the neutral carries 0A, not 200A, and it can be the same size cable as the two hots.
Conceptually you get the right answer if you treat both loops of the circuit as separate and then add the currents, If the neutral has 100A flowing in from one hot leg and simultaneously 100A out to the other they will sum to 0
The relevant circuit analysis principle is that the currents into and out of a wire junction have to sum to zero, since you can't create or destroy electrons. A capacitor at the junction doesn't negate this rule, it turns the solution into a problem in differential calculus whose answer is an equation of voltage versus time.
The Romans used the same net-sum principle to run an empire-wide checking system. When a merchant wrote a check for a cargo of wheat in Egypt the amount was simply deducted from tax payments sent back to Rome, and when he returned home the merchant had to reimburse the treasury (or become lion poop). Thus only the heavily guarded tax shipments were at risk from storms or pirates. -jsw
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On 5/26/2018 2:12 PM, Jim Wilkins wrote:

in a properly balanced load center neutral current will vary , but the idea is to minimize it .
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wrote:

The ship designers can predict and apportion the loads, which may not be practical for residential, commercial or industrial installations subject to unexpected, unengineered and perhaps undocumented changes. Plus warships are necessarily relatively symmetrical and redundant and lack the enormous motor startup surge capacity of the grid. The analysis I gave works for all situations, not just your well-designed and controlled ones.
I've traced and numbered my home outlets so I know which side, odd or even, they are all on. But I have to plug the portable air compressor and MIG, microwave, window air conditioners and heat treating oven into whichever hot leg's outlet is nearby. The 120V heating and cooling loads will never be balanced because they cycle randomly. I have a 200A electric heat service so balancing <20A loads doesn't matter, and the big compressor, TIG and plasma cutter are 240V anyway.
What can you tell of the South Dakota incident? https://www.tapatalk.com/groups/warships1discussionboards/south-dakota-s-electrical-problems-at-guadalcanal-t24931.html
http://www.kbismarck.org/forum/viewtopic.php?t 34
http://www.dcfp.navy.mil/mc/museum/War_Damage/57.pdf "That won't help you very much. The report was whitewashed to cover up what had happened. You may look in vain for any reference to the sabotage."
-jsw
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On 5/26/2018 3:43 PM, Jim Wilkins wrote:

and I'm trying to apportion loads to minimize neutral leg current . Just seems like the right way to do it - though I can see why a pro wiring a new house probably just sticks 'em wherever with no regard to balancing loads .
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wrote:

I think balancing the loads is a good idea too, but I wouldn't let it override other standards. https://www.thespruce.com/common-electrical-codes-by-room-1152276
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On 26/05/18 22:56, Jim Wilkins wrote:

In the UK and Europe the neutral takes the same current as the live in
pulling current from one side of the centre tapped transformer.
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wrote:

Our hots and neutrals are the same gauge and as I pointed out the neutral current can't be more than the greater of the hot currents; the other 180-out 'phase' can only reduce it. Any imbalance doesn't feed back beyond the pole or distribution transformer which has a single 'phase' primary, so I don't see a significant problem as long as the currents stay within the wire, breaker and transformer ratings.
Three phase imbalance does feed back into the grid.
I tried to roughly balance them in the industrial equipment I designed because I had no idea what else might be on the circuits, then or later. However the control circuit was all on the same breaker pole and usually there wasn't anything comparable to put on the other one(s). -jsw
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On 5/26/2018 4:56 PM, Jim Wilkins wrote:

#12 and 20A breakers . 2 circuits per room (plus designated , like for A/C) except the kitchen , it gets 4 plus the refr/dishwasher . 2 for the island alone (on different legs of course) because it's nickname is "appliance central" .
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On 5/26/2018 4:56 PM, Terry Coombs wrote:

I'd say that "right" implies that otherwise is wrong and it really doesn't make ANY difference, except to the power company. Because the only neutral current that you're minimizing is that in the "drop" to your panel. You have no control over the neutral currents in your panel and circuits.
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On 5/27/2018 3:32 PM, Bob Engelhardt wrote:




your statement that I have "no control" over neutral currents . I may not have a perfect balance , but I have a pretty good idea of what loads will be on most circuits and that influences my decision on which leg to place that load . And what difference does this make ultimately ? Probably not a helluva lot . But if I can somewhat balance the load that my supply transformer sees that can only be good . Or should I draw the whole 200 amps (or as much as is 110V loads) my panel can handle from one side ? If for no other reason than voltage sag I can't call that a good idea . Symmetry !
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On 5/27/2018 5:10 PM, Terry Coombs wrote:


Careful, now... I said "in your panel and circuits". Your balancing does not affect any neutral current in your branch circuits. Only in the aggregated current in the neutral wire coming to your panel.
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On Sun, 27 May 2018 17:36:39 -0400, Bob Engelhardt


Unless you are using "edison circuits" - or sub-panels
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Actually, if the load on the two 120V legs of the 240/120 service is reasonably well balanced, the neutral cable can be substantially *smaller* than the two hots. For example, I just helped my son install a new service panel in his house, upgrading from 100A to 200A service, and he just passed his final electrical inspection last week -- with two 3/0 copper cables for the hots, and 1/0 copper for the neutral. It actually would have passed with the neutral as small as AWG 4, but I happened to have enough 1/0 on hand already, left over from a long-ago project, and neither one of us had any 4.

*subtract the currents

Correct.
Further example, in case it's unclear to anyone: 75A on one leg and 90A on the other leaves 15A on the neutral.
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Adding works if the direction is considered. I would have written 'add algebraically' to an audience of engineers. https://www.conservapedia.com/Algebraic_addition
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And I would have understood what you meant. Best to assume, though, IMHO, that most of the people reading this discussion aren't engineers, and would be confused by the terminology.
Since one current is arriving, and the other leaving, at the same point, they perforce have opposite signs -- but electricians and laymen don't think that way. They see 70A current on one leg, minus 30A current on the other, equals 40A in the neutral. and don't really care about the fine points of Kirchoff's Current Law.
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Agreed, as long as they stick to single phase. I didn't want to plant an incorrect idea in case they move on to three phase.
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wrote:


Greetings Terry, Maybe you can educate me a little. After reading posts in reply to my post I got out the amp clamp and measured the current on both wires of the 125 volt receptacles in my shop. Plugging in a motor and turning it on the meter shows the same current draw on both the neutral and hot wires.In this case approximately 2.8 amps. I wired my shop with wire ways so it is easy to make measurements as the wire way covers come off easily and the wires just lay in the wire way. All the 125 volt receptacles on one wall are fed from the same breaker, on another wall another breaker, and so on. I did balance the load in the breaker panel so that two walls are fed from one leg of the 250 volt supply and two walls from the the other leg. I don't understand how the neutral can be balanced and show less current than the hot except at the breaker panel where the power comes in. What am I missing? What don't I understand? I did wire the shop myself but I was helped by a licensed electrician, the electrical code book, and the wiring was inspected and bought off by a particularly picky inspector. Thanks, Eric
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On 5/26/2018 8:01 PM, snipped-for-privacy@whidbey.com wrote:


each on one leg of the 220 power supply and to neutral . Putting your meter on the neutral should show zero or very close to it . Look at it like you've put 2 110 volt loads in series across a 220 supply with the connection between motors hooked to neutral . If the two loads are unequal the difference is carried by the neutral . If the two loads are the same neutral current will be zero .
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wrote:

The neutral current will be zero only on the grid side of the breaker box neutral terminal block. The current going out to each grinder through the black wire will return through the white wire, then pass to the other grinder's white wire through the terminal block.
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On 5/27/2018 6:01 AM, Jim Wilkins wrote:

implies .
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