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Topic: Electricity 101 (+/- 1) (Read 720 times) previous topic - next topic

Electricity 101 (+/- 1)

Disclaimer
For the most part, I think about (and understand) electricity on a pretty basic level:
 
    • I know that if I plug my appliance into an outlet it will probably work.

    • I know that if I do something to the appliance that lets the smoke out it will probably quit working.

    • I know if I am lucky (or far sighted enough) that when I roll up to the campground pedestal it will have a socket which is stamped with '125/250v' and a pair (that there are two is significant) of breakers labeled  '50amp' that matches the plug on my great big thick shore power cable.


Problem statement

A few days ago one or the participants on a boating forum I participate on wrote about his concern regarding what appeared to be overheating of is 50amp shorepower plug. He theorized that because both of his air conditioners were on the same 110 leg derived from the shore power connection that perhaps the neutral conductor was being overloaded and asked whether he should perhaps turn ON some devices on the other leg to balance the load.
(note: the smoke is beginning to be obvious!)

From his missive:
> As I understand it, on a 50 amp 125/250 cord, there are two 110-120
> VAC hots and one neutral return wire.
> Is it possible my return neutral is heating up
> because the two hots are 180 degrees out of phase,
> if I put a load on the non-air conditioner circuit do you
> think it may cause less heating on the neutral and the cord
> (assuming it is the neutral that is actually heating up)?


Most boats above a certain size look electrically just like our Foretravels.


The reply which I appreciated enough to burden you with is as follows

Date: Sat, 10 May 2014 20:21:25 -0400
From: Jim Healy via Trawlers-and-Trawlering
   <trawlers@lists.trawlering.com>
To: "trawlers@lists.trawlering.com
Cc: jim_gano@yahoo.com
Subject: Re: T&T: 50 amp 125/250 volt power cord

Jim,

In a 240VAC, 3-wire circuit, the two 120VAC legs ARE NOT out-of-phase with respect to each other.  In fact, that 3-wire circuit is a single phase, center-tapped 3-wire circuit.  It is very commonly thought - incorrectly - that the legs are 180 degrees out-of-phase.  They are NOT.  It only appears that way because the measurement is taken from the neutral, 1/2 way along the transformer winding.  That makes one end look positive and the other end look negative.  That is only a phenomena of the manner in which the measurement is taken.  Across the black to red wires, you have 240V.  From either the black or red wires to the white wire, you get 120V. 

Why do I emphasize the point?  Because understanding current flow in complex AC circuits depends on understanding what happens in each half-cycle of the AC current wave.  The laws that describe the flow of currents in a parallel circuit are called Kischoff's Laws.  If you have 120V loads on both legs of the 240V service, the returning currents divide at the neutral.  For two feeds protected by 50A breakers, the Neutral current will never exceed 50A, and in most operating situations will be well below the limit. 

Examples:

1.  If you have a pure 240V device like a water heater element, there is no neutral involved.  During one AC half-cycle, the black wire feeds and the red wire returns.  During the other half-cycle, the red wire feeds and the black wire returns.

2.  If you have a hybrid 240/120V device like a range/oven, it has both 240V loads (heating elements) and 120V loads (light bulb, hood fan, control circuits).  What happens then is that the heating element currents flow in the black and red wires (240V).  Only the 120V load current returns in the neutral.  Lets assume that the light bulb/fan/controller are hooked to the black wire.  Also assume the heating element draws 20A and the light/fan/controller draws 2A.  In that case, there will be 22A flowing in the back wire, 20A of the 22A total will return in the red wire, and 2A returning in the neutral.  The direction of flow reverses on the other half-cycle, but not the wires in which that current flows. 

the example most representative of us
3.  Now connect only pure 120V loads on both legs of the 240V circuit; that is, no 240V loads.  For example, suppose you have a 120V battery charger, 120V space lighting and 120V TV/DVR on the house side (Black wire in this example) drawing 17 amps, and the A/C compressors and raw water circulator on the other side (Red wire in this example) drawing 26 amps.  In this scenario, you will have 26 amps flowing in the red wire, 17 amps returning in and powering the loads of the back wire, and (26-17)=9 amps flowing in the neutral.  EVEN THOUGH THE DEVICES ARE NOT 240V devices, the base 17A will feed in the red wire and return in the black wire on one half cycle, and reverse direction in the other half-cycle, flowing through both loads, BUT NOT FLOWING IN THE NEUTRAL.  ONLY THE UNBALANCED CURRENT FLOWS IN THE NEUTRAL.

Above assumes all correct wiring throughout.

So the short answer to your question is, "No, the neutral is not overloaded."  The only time the neutral carries the entire load is when only one side of the circuit is turned on.  In example 3, above, if the house side were totally turned off, the A/C compressors/raw water pump would still draw 26 amps through the red wire, but with no return path available in the black wire, the neutral would, in that case, carry the full 26 amps.

If you have unusual heating occurring at the 240V connector, check the connections at both the male and female halves of the connection.  A high resistance in either place will heat the whole assembly.  If the connections and blades are all good, clean and tight, we'll need to investigate further.

Hope this is useful.

Jim
Peg and Jim Healy aboard Sanctuary, currently at Charlotte Harbor, Punta Gorda, FL Cruising Aboard Monk36 Trawler Sanctuary | Cruising Adventures in Another Life Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436:


For further reflection on my part

The Fortravel wiring plant doesn't know (nor really care) from a power distribution perspective whether we are connected to a 50amp or a 30amp connection. It gets 4 wires from the socket. It just blows the campground breaker quicker. I think that is a true statement.

I think I just got it... I'll tell you later ;D

Anyway, thankfully I only get carried away like this periodically. ::)
Elliott & Mary Bray
ex. 1996 36' U295 - Build 4879
ex. 2018 Coachmen Leprechaun 319MB
 

Re: Electricity 101 (+/- 1)

Reply #1 –
@Elliot Thanks for posting that is pretty interesting! I'm about like you. Just know that it's better when the smoke stays inside the wires. :))

see ya
ken
The selected media item is not currently available.ken & dori hathaway & Big Agnes
🍺1992 U300 GrandVilla WTBI #4150 FOT FBP 2011
✨6V-92TA DDEC Parlor Coach 350HP Series 92
🏁2011 Nissan XTerra Pro-4X

Re: Electricity 101 (+/- 1)

Reply #2 –
I agree with everything in Jim's well said reply with one exception:  his statement that the two legs are NOT 180 degrees out of phase. To me, the definition of "180 out of phase" is when  you measure 240 VAC from leg to leg, and 120 VAC on each leg to neutral.  His descriptions are spot on in my opinion and I'm sure he knows what he is talking about but I disagree on this one definition.
John Fitzgerald
1991 U300 (SAI) Side Aisle Island Bed 40'
Detroit 6V92 with Allison Retarder
Meridian (Boise), Idaho

Re: Electricity 101 (+/- 1)

Reply #3 –
I think Jim is correct.  It is a single phase system.  The two 120v legs are in phase.  In a three phase system common in industrial or agricultural sites each of the three legs are 120 degrees out of phase.  That is how they get 208 volts across two adjacent legs.  120x2xsin(120°)  120x2x0.87 = 208.

That is about all I can remember.  Avoid sparks.

Roger
Roger Engdahl and Susan Green
2001 U320 3610 #5879 (Home2) - 2014 Jeep Cherokee or 2018 F150
Hastings, MN

Re: Electricity 101 (+/- 1)

Reply #4 –
Jim's 120-volt discussion does not mention issues when 50-amp outlets are wired with both red and black being from the same wire, instead from two different sides of a transformer. Many of our generators are intentionally wired this way and some campgrounds are incorrectly wired this way. Measurements across the black & red will be zero volts instead of 240 volts. Since almost all RVs do not have any 240-volt loads, it is not possible to identify this situation without testing, as all 120-volt loads will work just fine.

But a problem occurs if the sum of the red and black wire amps is greater 50-amps, because the white neutral wire can usually safely carry only 50-amps. In this situation the white wire, connectors and transfer switch will be overloaded, damaged, burned, etc.

If the red & black generator or campground wires measure zero volts, it is important to not draw more than a total of 50 amps at one time. It is up to us to be aware of this situation.

Re: Electricity 101 (+/- 1)

Reply #5 –
Barry,
Don't we generate this exact situation when we use a 30-50amp adapter (or dogbone)?
If we take a look at the coach wiring immediately prior to the first transfer switch do we not have a zero potential difference between the black and red wires and 110v between the white and black/red 'hot' leads?

But wait, I think I see what you are saying..
The 30-50 adapter is connecting the pedestal's 30 amp circuit (protected by a 30 amp breaker) to the 50 amp plug by reusing the hot lead from the 30amp conductor.

If this same scheme 'advertised' itself as a 125/250 50 amp circuit  protected by a pair of 50amp breakers I understand your post
.
Elliott & Mary Bray
ex. 1996 36' U295 - Build 4879
ex. 2018 Coachmen Leprechaun 319MB
 

Re: Electricity 101 (+/- 1)

Reply #6 –
Elliott & Mary,

Think of the comment about being careful to not overload the white neutral wire this way:

The 50 amp circuit that is in our coach has a red-hot, black-hot & white-neutral, EACH designed to carry a maximum of 50 amps.

If plugged into a campground 20 or 30 amp outlet the pedestal only has one breaker and the white will only carry 20 amp or 30 amps, well within limits.

If plugged into a campground 50 amp outlet the pedestal has two breakers, so the maximum potential can be 100 amps.
If the red & black wires measure 240-volts as they should, the white neutral will never carry more than 50 amps, which is within limits.

If the red & black wires measure zero-volts, the white neutral could potentially be asked to carry 100 amps, double its rated capacity. But an RV with two roof air conditioners & microwave running at the same time, the total would be about 45 amps, within limit of the white wire. But if a toaster and hair dryer or a third air conditioner are added to the 45 amps, there would be overload, if a meter measures zero-volts across the red & black wires.

Since some generators are wired with zero volts across red & black, like our 8,000 watt, it just makes sense to be aware, so as to not overload the white neutral wire and transfer switch by powering more than 50 amps total.