Burnt Neutral

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Burnt Neutral

July 29, 2015, 01:40:25 pm

We arrived early at the Wisconsin FMCA rally and had to do without shore power for a few days. We were using the generator to power both A/Cs when they turned off. I noted on my EMS display that the power had been interrupted. Wha....

The EMS display was dark (no errors) so it was likely the problem occurred before the EMS. I have mine hard wired after the automated transfer switch so that's where I looked. Definitely the problem. A short wire connecting the generator neutral to the shore power neutral had burned up and the out neutral wire was badly discolored (not melted).

So what's the deal? Here's what I think. A few days ago, I believe I overloaded that neutral wire. Trying to recall but I think we had both A/Cs going, microwave and a toaster. Probably other stuff too. I was checking the EMS display and saw one leg loaded much more than the other but neither was near 50 amps. As I understand it, that neutral is only rated for 50 amps even though we have a 100 amp system with the two legs.

I replaced the burned wire temporarily, started the genny and all worked with no error code on the EMS. Shut it down, started again and no go. I ended up screwing with this thing for hours (never say die) and finally giving up. The following day when power became available, I plugged in and all is normal.

Check out the photo. The neutral doesn't look quite that bad but bad enough. I've ordered a new ATS and from now on I'm going to monitor amps and make sure we stay below 50 amps total.

Can't post the photo for unknown reasons. I'll add it next time I stop in.
jor

Re: Burnt Neutral

Reply #1 – July 29, 2015, 01:49:56 pm

Yes, IF (repeat IF) this was on shore power an improperly wired outlet could cause the melted neutral.

Properly wired, the neutral carries only the difference between amps on L1 and L2. So, with both A/C's running, max difference would be dependent on what other appliances were running-- but very likely under 20amps.

But, if the park is improperly wired and L1 and L2 are on the same side of the CG box, the neutral will carry the SUM of L1 and L2.

Easy to tell with a voltmeter if the CG is mis-wired. L1 to L2 should read 240 VAC. If mis-wired, it will read ) VAC.

And, the damage could have been several CG's ago, with the overheated wire resulting in resistance, leading to more heat, leading to more resistance...........

Re: Burnt Neutral

Reply #2 – July 29, 2015, 01:56:58 pm

This may help answer the neutral wire overload question:
Two Generator Questions

Re: Burnt Neutral

Reply #3 – July 29, 2015, 02:36:30 pm

Yes,

Brett is correct. If wired correctly the neutral should never carry more than 50 amps. Each of the two legs of the system should be connected to a phase that is 180 deg opposite to the other one. To test:

Hot to hot = 240 volts correct
Either hot to neutral or ground = 120 volts correct
Hot to hot = 0 volts incorrect do not use

If the neutral becomes disconnected then a condition known as open neutral will exist. This can cause extensive damage and even a fire. This applies to a two leg 50 amp service.

Any outlet showing less than system voltage when connected hot to neutral should not be used.

Keith

Re: Burnt Neutral

Reply #4 – July 29, 2015, 02:49:37 pm

Food for thought.
Number 6 wire is rated up to 75 amps depending on grade, the lowest (cheapest) is rated 55 amps, most hot, burnt wire is due to poor electrical/mechanical connections.
Might be worth opening the boxes and retighten some screws, I do when have one open.

Re: Burnt Neutral

Reply #5 – July 29, 2015, 03:19:56 pm

Quote from: Keith and Joyce – July 29, 2015, 02:36:30 pm

Yes,

Brett is correct. If wired correctly the neutral should never carry more than 50 amps. Each of the two legs of the system should be connected to a phase that is 180 deg opposite to the other one. To test:

Hot to hot = 240 volts correct
Either hot to neutral or ground = 120 volts correct
Hot to hot = 120 volts incorrect do not use

If the neutral becomes disconnected then a condition known as open neutral will exist. This can cause extensive damage and even a fire. This applies to a two leg 50 amp service.

Any outlet showing less than system voltage when connected hot to neutral should not be used.

Keith

Slight correction.

Properly wired 50 amp hot to hot= 240 VAC
Improperly wired 50 amp hot to hot= 0 VAC

Re: Burnt Neutral

Reply #6 – July 30, 2015, 01:28:59 pm

Thanks Brett, Should proof my post better!!!

Keith

Re: Burnt Neutral

Reply #7 – July 30, 2015, 02:24:09 pm

Quote from: wolfe10 – July 29, 2015, 01:49:56 pm

'snip
Properly wired, the neutral carries only the difference between amps on L1 and L2. .... snip'

Brett can you explain the physics behind what you have said here.

By my understanding (And I am not an electrician but have a pretty solid grounding in the fundamentals) the two legs of a properly wired 50 Amp service are out of phase. The neutral being shared between the two. I can see how the amperage is never more than the max on a single leg but cant quite understand why the amperage is the difference...

(And even as I type this out the "i" based vector calculus from decades ago is trickling back into the old brain and I can see what you are talking about...)

Complex Numbers - HMC Calculus Tutorial

Wow that brought back some not so fond memories!!!!

An even better visualization here.

Phasor Diagram and Phasor Algebra used in AC Circuits

Crawling back into my corner now...

Cheers

Chris

Re: Burnt Neutral

Reply #8 – July 31, 2015, 11:10:36 am

My understanding is that what Brett said isn't entirely correct. It's not about sum or difference but instead is about 240v vs 120v loads. Any 120v load connects to one of the 50 amp leg and the neutral. If you had literally 100 amps of 120v loads, both using the 50 amp capable neutral you could easily smoke it. Even if the neutral is undersized for the reality that most 50 amp rv's are filled with only 120v loads, most rvs only really consume 50-60 amps max. In mine, the worst case scenario is about 75 amps and even an undersized neutral can handle that load temporarily.

But it would make sense that the neutral is the first conductor to burn.

Re: Burnt Neutral

Reply #9 – July 31, 2015, 12:39:22 pm

The two phases are supplied by a center tapped transformer. The utility provides three phase power from the generating station, hence the main supply poles carry three wires. Thus there are three phases at 120 deg. That is not the case with what is supplied as 50 amp power in RV parks. Only one phase is tapped at the pole and the center tapped transformer is what gives 2 phases at 180 deg.

There should never be more than 50 amps present on the neutral as 180 deg opposite phase is subtractive IE: leg 1 running at 20 amps Leg 2 running at 15 amps will show 5 amps on the neutral. Wire both legs to the same phase and it is additive and you will have 45 amps.

Keith

Re: Burnt Neutral

Reply #10 – July 31, 2015, 02:43:22 pm

Quote from: Keith and Joyce – July 31, 2015, 12:39:22 pm

The two phases are supplied by a center tapped transformer. The utility provides three phase power from the generating station, hence the main supply poles carry three wires. Thus there are three phases at 120 deg. That is not the case with what is supplied as 50 amp power in RV parks. Only one phase is tapped at the pole and the center tapped transformer is what gives 2 phases at 180 deg.

There should never be more than 50 amps present on the neutral as 180 deg opposite phase is subtractive IE: leg 1 running at 20 amps Leg 2 running at 15 amps will show 5 amps on the neutral. Wire both legs to the same phase and it is additive and you will have 45 amps.

Keith

This sounds more intelligent and more correct than what I wrote. But if the campground happened to wire both legs to the same phase....

Re: Burnt Neutral

Reply #11 – July 31, 2015, 03:12:14 pm

If you use a 50 to 30 amp plug adapter, both hot legs are wired to one hot ( one phase) side within the adaptor.

Re: Burnt Neutral

Reply #12 – July 31, 2015, 04:32:19 pm

Quote from: piku – July 31, 2015, 02:43:22 pm

This sounds more intelligent and more correct than what I wrote. But if the campground happened to wire both legs to the same phase....

Then, it is wired INCORRECTLY. PERIOD.

Re: Burnt Neutral

Reply #13 – July 31, 2015, 04:33:24 pm

Quote from: Keith and Joyce – July 31, 2015, 12:39:22 pm

There should never be more than 50 amps present on the neutral as 180 deg opposite phase is subtractive IE: leg 1 running at 20 amps Leg 2 running at 15 amps will show 5 amps on the neutral. Wire both legs to the same phase and it is additive and you will have 45 amps.

Keith

CORRECT.

Re: Burnt Neutral

Reply #14 – August 03, 2015, 01:39:47 pm

Quote from: wolfe10 – July 31, 2015, 04:32:19 pm

Then, it is wired INCORRECTLY. PERIOD.

But what about when running the generator? It's additive on my RV, verified with clamp fluke. Breakers on the generator are 35 each (8kw). Hot to Hot is 0 VAC

Re: Burnt Neutral

Reply #15 – August 03, 2015, 03:22:08 pm

Krush, with 0v hot-hot, your 8kw gen is wired correctly as it came from the factory, and if it was wired 240v hot-hot, it would also be wired correctly. There are two ways to wire our 8kw Power Tech gen and each way has advantages and disadvantages, so either way is a compromise.
From my limited knowledge, all of the many 8kw Power Tech gen's were built with zero-volts hot-to-hot, and also maybe some 10kw generators.

Re: Burnt Neutral

Reply #16 – August 03, 2015, 03:59:09 pm

We recently experienced an open neutral condition in a campground in Whitney, TX. It is very scary to see your power monitor suddenly shoot up to 134 volts on L1 when one of the ACs pulls the other leg down to 116 volts. The CG sent out two guys with their little voltmeter who tested each leg at the pedestal, said there is nothing wrong, and walked away, telling me the problem was in my coach. I tried explaining that they had a serious situation, but I was ignored. We ran our generator the rest of our stay. Royce

Re: Burnt Neutral

Reply #17 – August 03, 2015, 05:51:55 pm

Quote from: Barry & Cindy – August 03, 2015, 03:22:08 pm

Krush, with 0v hot-hot, your 8kw gen is wired correctly as it came from the factory, and if it was wired 240v hot-hot, it would also be wired correctly. There are two ways to wire our 8kw Power Tech gen and each way has advantages and disadvantages, so either way is a compromise.
From my limited knowledge, all of the many 8kw Power Tech gen's were built with zero-volts hot-to-hot, and also maybe some 10kw generators.

There is one flaw to the generator being wire as only 120V and that is the load on the neutral (white) the sum of the load on each phase conductor (Black & Red) In my coach they ran #6 NM cable to the transfer switch from the generator. #6 is only good for 55 amps. If you take a 10K generator it can put out 83 amps @ 110v before the breaker would trip (I am not sure what breaker the generator has on my coach) I know the odds are not great that I would be using more then 55 amps total on generator since I have a 36' with only 2 AC's but I could see it happen.

2 AC's @ 13.2 = 26.4A
Battery charger +/- 8A
Microwave = 15A
TV & Misc = 6A

and I am at 55 amps

Re: Burnt Neutral

Reply #18 – August 04, 2015, 07:56:59 am

I agree on Turbojack's one "flaw". So why does Power Tech and Foretravel sell coaches with only 120 volts? Could it be that if a generator is mostly run with only one of its two internal legs loaded, the life of the generator could be shortened?

It appears that generators are built with two 120 separate legs which can be owner-changed to both be in same sinewave 'phase' as in 120v or different phases as in 240v.

Same phase generator windings equally share in power generation. With different phase, 100% of the load could be on just one winding.

Maybe same phase could stress neutral wire and different phases stress generator windings?

Re: Burnt Neutral

Reply #19 – August 04, 2015, 11:25:10 am

55 Amps on the #6 wire (neutral) will not BURN the wire unless the connections are POOR, that will produce the super hot condition, if properly connected, no heating.

Re: Burnt Neutral

Reply #20 – August 04, 2015, 02:20:00 pm

Quote from: Dave M – August 04, 2015, 11:25:10 am

55 Amps on the #6 wire (neutral) will not BURN the wire unless the connections are POOR, that will produce the super hot condition, if properly connected, no heating.

#6 wire is rated for 55 amps in the 2014 National Electrical Code. The wire can safely carry 55 amps. 83 amps on #6 can cause problems up to and including a fire.

Barry, if a 10KW generator is set up for 240 volts then the max per phase would be 41.66 amps. If everything is set up in the coach correctly then I do not think that you would go over 41.66 amps per phase but that might have been Foretravels concerns for wiring it up as 120v only. On a side note I had mine powered at the house on a 30 amp 240v circuit (with a neutral) and never tripped the 30 amp breaker feeding the coach. (this was the same as 50 amp RV plug but only had 10 gauge wire going to it on a 30 amp breaker )

As was said before if you are wired with a 110v and something on either wire goes wrong, nothing bad happens except you loose power. If you are wired 240V and you loose a neutral, things can get bad quick. The circuit that draws the most current has the lowest voltage and the other has the higher voltage. Such as say based on the load, one side can be at 70 volts and the other at 170 volts.

Edited: Added 240V 30 amp circuit had a neutral per Brett's recommendation

Re: Burnt Neutral

Reply #21 – August 04, 2015, 02:31:20 pm

Quote from: turbojack – August 04, 2015, 02:20:00 pm

On a side note I had mine powered at the house on a 30 amp 240v circuit and never tripped the 30 amp breaker feeding the coach.

Probably needs some clarification so as not to mislead others into doing a 3 wire, 30 amp 240 VAC outlet (standard house dryer wiring) and LETTING THE SMOKE OUT OF THEIR COACH ELECTRICAL SYSTEM.

If this was 4 wire, wired the same as 50 amp RV service, but with 30 amp breakers/wire instead of 50, all is right with the world.

So, a functional 30 amp 240 VAC RV outlet would have:

2 hots (L1 and L2) from opposite sides of the main feed
1 neutral
1 ground

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