Need some electrical education

[brirene]

Finally managed to get a site with only 50 amp service in my 30 amp fiver. Fortunately, I had purchased a 50 to 30 dogbone a couple of years ago for just this situation, so I was pretty pleased with myself. That is, until after I disconnected the following morning, and had nothing 12v. Everything worked fine on shore power. Found the 30 amp fuse at the battery fried, replaced it, and all is good to go. So...I thought the dogbone only allowed 30 amps, so why the fried fuse? Do I need to do something else? Is there some other problem? Thanks for helping me figure this out!

[wa8yxm]

brirene wrote:
I thought the dogbone only allowed 30 amps, so why the fried fuse? Do I need to do something else?

Yes, You need to study the RV electrical SYSTEMS (Note the use of the plural)

There are, at the minimum, two different electrical systems in your RV, one is 12 volt, one is 120 volt. They have one (or two) common points.. The Converter or "inverter/converter" option and possibly a control computer.

The 30 amp dogbone is for the 120volt system, this system includes the Microwave, Air Conditioners, electric side of fridge and water heater, Televisions and other entainerment electronics (with one or two exceptions) and outlets scattered about.

The 12 volt system is batteries, lights, water pump, furnace, control power for selected devices listed last paragraph (A/C, Firdge, and Water heater) and if you have an energery manager that can shed loads. control power for it.. Also possibly a radio.

There are two possible things that happend to your 30 amp fuse.

ONE: An intermittent short, likely popped the fuse.

TWO: And this applies big time if you have upgraded batteries and converter as many have... The batteries were down so far (State of charge that when you plugged in they sucked 30+ amps (*Mine can hit 100+) and popped the fuse.

Recommendations: Post battery bank specifications (Size and number) Converter make and model so we can advise if TWO is possible.

Recommendation 2: Carry a spare fuse just in case, But if the new fuse does not blow then it might have been issue 3 that blew it.

Issue 3: Sometimes, for no good reason, a Fuse just goes bad.. I have seen it many times in my 60+ years on this earth.. I actually DO know the reasons for it happening,, but Well they are not GOOD (Fixable/fault) reasons, its just the nature of the fuse.. Blade type fuses are LESS prone to this.

[cochise49]

Coach battery switch/disconnect? Ours is in the step well. Someone is frequently switching it or the chassis battery off by accident. Poor design but common.

[aruba5er]

I always ( when available) use the 50 amp outlet with the adapter. the only way my cord would melt would be that the 30 amp breaker would be bad. And a short would blow faster than you can blink your eyes. Probably never in a lifetime is that going to happen. The 50 amp outlets are hardly ever burnt like the 30 amp one that almost always are. No bearing between the 12 volt and the 120 volt.

[ksg5000]

Your battery fuse is is on the DC system - your shore power is on the AC system. I suppose it's possible your converter went bonkers - worth a look - but often as not fuses sometimes blow and you never know why - if the replacement fuse blows then you need to check out the DC system.

[wilanddij]

I thought the dogbone only allowed 30 amps,

The dogbone only changes the incoming power by dropping one side of the 50A hookup. It does nothing to limit it to 30A. If your rig is pulling over 30A, the park power pole with continue to feed it until the breaker on it trips at >50A.

[eHoefler]

The dog bone only uses one leg of the 50 amp service, and allows the full 50 amp to flow through to your 30 amp cord. The main breaker in your coach is the device that will limit the power to your coach. The dog bone has nothing to limit the amount of load that is placed on it. If the load is there you can actually melt your 30 amp cord before the 50 amp breaker in the pedestal trips.

[tvman44]

Just a coincidence. 🙂

[2oldman]

brirene wrote:
Found the 30 amp fuse at the battery fried, replaced it, and all is good to go. So...I thought the dogbone only allowed 30 amps,

30a battery fuse has nothing to do with 30a electrical service.

[Golden_HVAC]

There is probably another problem. Such as you might have a battery that did not get water added to it on a regular basis, and now is shorted out. Or another reason that the charger put out more than 30 amps when you plugged into the shore power.

Chances are that you might have a 45 - 55 amp capable 12 volt charger/converter in the RV - near the fuse panel. It can put out amperage that will run the lights, and also charge the battery. If the charger is putting out say 14.1 volts, and the battery is only at 11.5 or less, then the charge circuit might carry more than 30 amps, melting the fuse.

Two things to check. Measure the voltage at your battery. If it is less than 11.5 volts, then you might have a problem with the battery. But don't replace it just yet. First check to see if it needs water. Only add distilled water.

I would also suggest replacing the 30 amp fuse with a automatic reset circuit breaker. http://www.napaonline.com/Catalog/CatalogItemDetail.aspx/Circuit-Breaker-30-Amp-Threaded-Stud/_/R-BK... You can find that at any local auto parts store.

A automatic reset circuit breaker will shut off power if it exceeds 30 amps for about 3-5 minutes, then will cool off (so will the wire) and then will re-set and allow 30 amps to flow again. After a while, the battery will have enough charge, then the charge rate slows to less than 29 amps, and will stay charging.

If you have a portable battery charger, I would charge the battery with it for a while, then plug in the shore power, and check the 30 amp fuse. It probably is bad again - if you plugged in the RV now, while the battery was dead. Yet after the battery has charged for a while, then it should be accepting less than 30 amps from the RV charger, and then the fuse should not blow again.

So first thing is add water to the battery.

Fred.