Blew a 250 amp fuse the other day. How to avoid this

[Skwidward]

... I am now, as I type, in the process of ensuring this does not happen again...

Just to clarify, I am not in the process of ensuring my failsafes do not work again.

I am, instead, in the process of ensuring that they do work and work faster at less of a current/draw and without a scary burning smell and very small but still very heart-stopping amount of smoke.

[Driggs-Dave]

I'm a bit confused about the 250 amp fuse blowing.

RV air conditioners usually draw about 10-12 amps AC which would be about 120 amps at 12 V, so why did the fuse blow?

I'd check all the connections around the fuse block to make sure they're sound. Some of the heat could have been caused by a bad connection at the fuse block and the excess heat caused the fuse to fail.

Do have a low voltage alarm and disconnect to save your batteries?

[Skwidward]

I'm a bit confused about the 250 amp fuse blowing.

RV air conditioners usually draw about 10-12 amps AC which would be about 120 amps at 12 V, so why did the fuse blow?

I'd check all the connections around the fuse block to make sure they're sound. Some of the heat could have been caused by a bad connection at the fuse block and the excess heat caused the fuse to fail.

Do have a low voltage alarm and disconnect to save your batteries?

I’m a bit confused myself, to be honest, by your post.

Air conditioners have a starting surge—for a second or so when the compressor first kicks on they use way more power than they do when just running normally. This is something of which you should be aware and for which you should account, especially when sizing a generator you plan to use to power your air conditioner.

No, the connections at the fuse block are not bad. They’re sound.

Yes, I do have both a battery disconnect and a low voltage alarm.

[neweclipse]

Do you understand the application of the mentioned relay to instantly disable the running of the A/C upon loss of shore power?

[Skwidward]

Do you understand the application of the mentioned relay to instantly disable the running of the A/C upon loss of shore power?

I think so. There’s one part I’m not sure about, though.

Sub panel with two circuit breakers. Shore power in. Shore power out and Air conditioner out. This way the air conditioner is dependent on “true” shore power; shore power is lost, air conditioner receives no power.

What I’m not too sure about is:

one hot (black) wire from shore power is wired into the main breaker (of this new subpanel), one neutral (white) wire from shore power is wired into the neutral bus bar, and one ground (green) wire from shore power is wired to the ground bus bar. Coming out the other side of this subpanel would be one hot (black) wire from the “bottom” of the circuit breaker, from the neutral bus bar one neutral (white) wire, from the ground bus bar one ground (bare copper, as I used UF). These 3 wires would combine to form my 10/2 UF that would run to my inverter’s ac in— I’ve got this part.

So inside this new subpanel sitting next to the main circuit breaker with the shore power will be the circuit breaker for the air conditioner. Out of the “bottom” of this breaker will be one hot (black), from the neutral bus bar there will be a second neutral (white) wire (the shore power being the first), from the ground bus bar will be a second bare copper ground (the shore power being the first). These 3 wires will combine to form the 12/2 romex that runs to the air conditioner.

Will I need to put a jumper in the “top” of the air conditioner’s circuit breaker from the “top” of the shore power “main” circuit breaker, or will the air conditioner’s circuit breaker receive “hot” power from the bar it clicks onto inside the subpanel next to the shore power circuit breaker?

[neweclipse]

You missed the relay entirely.

The relay alone could drop the A/C by itself (no sub-panel required) or for more loads/circuits, drop the whole "Shore Power ONLY sub-panel" and any circuits that are provided by it.
Two different approaches as for electrical hardware needed goes.

[Skwidward]

You missed the relay entirely.

My apologies. The sub panel idea made a lot of sense to me, and I was grateful for the suggestion; as I would not have been able to come up with such a plan on my own. However, I am unsure about the jumper or no jumper part I mentioned above. Evidently this "relay" is something entirely different.

The relay alone could drop the A/C by itself (no sub-panel required) or for more loads/circuits, drop the whole "Shore Power ONLY sub-panel" and any circuits that are provided by it.
Two different approaches as for electrical hardware needed goes.

I am completely ignorant of the relay. I'd be super grateful if you wouldn't mind explaining how to go about setting it up.

[jflorey2]

I am completely ignorant of the relay. I'd be super grateful if you wouldn't mind explaining how to go about setting it up.

From before:

If you want to run the A/C only when shore power is present, you might consider a 120VAC relay connected to shore power that enables the air conditioner. Easiest way to do that is via the thermostat line (if you have a separate thermostat.) You can also switch the power to the A/C itself.

More detail:

Take a 120VAC DPDT relay. Wire the coil to the incoming (shore) power. You can connect it right there at the input, so close that there's effectively no chance it will short to anything. If you use anything other than a short wire, use a 1 amp fuse between hot and relay to protect the hot wire.

Now the relay will click ON when there is shore power available, and OFF when there is no shore power available. To be more specific, the common contacts of the relay will connect to the NC (normally closed) contacts when there is no shore power, and connect to the NO (normally open) contacts when there is shore power.

You now have two choices.

The easiest is if you have a separate thermostat. If you do, cut one wire of the thermostat lead and connect the two ends to the relay - one to a common pin and the other to the associated NO pin. Now the thermostat will be disconnected when shore power is not available and the A/C will not start, thinking the trailer is already cool.

The second way is to run the power for the A/C through the relay. Take hot and neutral from your panel's A/C circuit breaker and put them on the common pins of the relay. Connect the A/C itself to the NO pins of the relay. Now the relay will physically disconnect power to the A/C when there is no shore power. If you go this route ensure that the relay contacts are rated for whatever current the A/C pulls.

[Skwidward]

So here’s an easier fix to avoid blowing any fuses if you have a system like mine:

Turn the inverter to “charge only” when plugged in.

It’s that simple.

[Mike90250]

I'm very uncomfortable trying to instruct someone how to construct a transfer switch with a couple relays.

If you DO NOT know how to work with Grid power, 240 V AC circuits, instructions over the internet are not the best way to learn.

[Skwidward]

I'm very uncomfortable trying to instruct someone how to construct a transfer switch with a couple relays.

If you DO NOT know how to work with Grid power, 240 V AC circuits, instructions over the internet are not the best way to learn.

100% agree

[Driggs-Dave]

I'll try another approach.....you have two fuses in series, one at the battery and one under the bed, they will have the same current flow.
Do they look the same?
Or is one holder blackened and the other like new?

If you only smoked one of them I'd replace that fuse holder, it could have been defective.

Just another thought, most travel trailers will charge the house battery from the tow vehicle, did you remove the battery connection to the 7 way plug?

[Skwidward]

I'll try another approach.....you have two fuses in series, one at the battery and one under the bed, they will have the same current flow.
Do they look the same?
Or is one holder blackened and the other like new?

If you only smoked one of them I'd replace that fuse holder, it could have been defective.

Just another thought, most travel trailers will charge the house battery from the tow vehicle, did you remove the battery connection to the 7 way plug?

Both fuse holders are the same and look the same. The cover on the one that blew has an area on it that looks like a lighter had been held under it. I posted a video of an ANL fuse blowing. Neither fuse holder is “blackened.”

I am replacing the fuse that blew with a circuit breaker, as I mentioned before.

The fuse holder is actually a piece of acrylic with two 5/16” studs. The ANL fuse sits on the studs and is held in place with nuts. It was not and is not defective-it’s two nuts and two bolts. The fuse was not defective. The fact that the fuse blew when the inverter tried to run the air conditioner off the batteries indicates that it was not defective.

My battery bank is not connected to the trailer connector. Apparently it never was. I found that strange.

Either way, the concern I had in the OP is solved as easy as me flicking a switch on the inverter’s remote panel. No need to add a sub panel. No need to try and figure out how to rig up a relay. Just flick a switch and done.

[neweclipse]

So you are content with your manual/human dependent interaction over an automatic no-brainer solution, hope you sleep well.
Can't imagine it fun to wake up dead...nothing new, but mis-calculations sometimes can cause bad things...

BTW: The sub-panel alone was never a solution to your problem...relay or relay plus sub-panel was.

[Ampster]

..........BTW: The sub-panel alone was never a solution to your problem...relay or relay plus sub-panel was.

The problem was that the inverter was overloaded by the Air Conditioner when shore power was disconnected. Why do you think a sub panel was not a solution? My understanding was that the proposed sub panel would be powered by shore power and AC would only run from subpanel.