Inverter circuit breaker - where and how big

[2WheelJon]

I'm installing a 1500 watt inverter in my 2104S TT. It will connect via 4 ga wire to two Weize 100 Ah LiFePO4 batteries. The wire run will be less than 6'. It will see limited duty, running the TV, charging some cell phone/Ipad/toothbrush batteries and perhaps running the electric coffee pot. No microwave, no AC, no frig. We've determine our usage after just completing six weeks on the road, and not from conjecture.



Where should I put the circuit breaker in the line between the batteries and inverter and what amperage should it be?

[TitanMike]

Originally Posted by 2WheelJon

I'm installing a 1500 watt inverter in my 2104S TT. It will connect via 4 ga wire to two Weize 100 Ah LiFePO4 batteries. The wire run will be less than 6'. It will see limited duty, running the TV, charging some cell phone/Ipad/toothbrush batteries and perhaps running the electric coffee pot. No microwave, no AC, no frig. We've determine our usage after just completing six weeks on the road, and not from conjecture.



Where should I put the circuit breaker in the line between the batteries and inverter and what amperage should it be?

Remember that you are protecting the wire, not the appliance. Max recommended fuse/circuit breaker for #4 awg wire is 150 amp. Your 1500 W Inverter will draw ~120 amp max while running and depending on it's spec's anywhere from 50% to 100% more "surge" current for a few seconds.

I would go with a 150 Amp circuit breaker and just install it in a location between battery and Inverter where you can reach it should you trip it with a heavy load.

[jimmarako]

Originally Posted by TitanMike

Remember that you are protecting the wire, not the appliance. Max recommended fuse/circuit breaker for #4 awg wire is 150 amp. Your 1500 W Inverter will draw ~120 amp max while running and depending on it's spec's anywhere from 50% to 100% more "surge" current for a few seconds.

I would go with a 150 Amp circuit breaker and just install it in a location between battery and Inverter where you can reach it should you trip it with a heavy load.

Wouldn't a 1500W inverter draw much closer to 150A under full load assuming the invertor is about 80% efficient? Also I'm not sure I would run 150A through #4 wire even for 6'.

Jim M

[TitanMike]

Originally Posted by jimmarako

Wouldn't a 1500W inverter draw much closer to 150A under full load assuming the invertor is about 80% efficient? Also I'm not sure I would run 150A through #4 wire even for 6'.

Jim M

Based on OP's description of his use/loads I wouldn't worry about the breaker size or efficiency of an Inverter. All potential loads are small and/or resistive do surge current won't be an issue.

FWIW, the 80% efficiency you state is not exactly what today's Inverter manufacturers are stating. More like ~90% and my Renogy 2,000 watt Inverter runs right at that number. When running my 1350 watt microwave 100% efficiency would yield 108 amps @12.5 volts. Actual measured current (on Victron BMV-712) is 120 amps @12.5 volts or right on the mark of 90% efficiency.

A 1500 watt Inverter with similar efficiency would only draw 134 amps at 12.5 volts.


On the "12.5 volts" both the OP and I are using LiFePo4 batteries which will have less voltage "sag" under load. Lead Acid batteries may only provide ~12 volts which could increase the current draw another 5 amps or so to produce 1500 W.

[Bob K4TAX]

I would not install a circuit breaker in the line for the inverter. The inverter will automatically shut down on an overload. Turn the unit OFF at the unit to reset it. And some will automatically reset when the overload is removed.

In a 12V system, it adds too much resistance, and at 100 plug amps, the voltage drop becomes significant. The inverter is protected by several internal fuses in parallel to the 12V line.

Also, you state the wire run is 6 ft. Is that the combined length of the POS and NEG cable, or just the distance being about 6 ft. If so, the cable run is closer to 12 ft. It is a round-trip thing.

Bob

[NXR]

Originally Posted by Bob K4TAX

I would not install a circuit breaker in the line for the inverter. The inverter will automatically shut down on an overload. Turn the unit OFF at the unit to reset it. And some will automatically reset when the overload is removed.

In a 12V system, it adds too much resistance, and at 100 plug amps, the voltage drop becomes significant. The inverter is protected by several internal fuses in parallel to the 12V line.

As TitanMike noted, a breaker or fuse is not intended to protect the load but rather protects the power source, the RV itself in this case.

If the inverter suffers an internal failure and, for example, the power transistors short circuit you can have a fire if the power source is not protected.

No fuse or breaker that is working properly will introduce electrical resistance. That's a non-issue.

The breaker or fuse should be placed as close to the power source as possible. If the cable from the power source to the inverter chafes to ground you need the circuit protective device to cut the power at the source.

Ray

[PhilFromMaine]

Originally Posted by 2WheelJon

I'm installing a 1500 watt inverter in my 2104S TT. It will connect via 4 ga wire to two Weize 100 Ah LiFePO4 batteries. The wire run will be less than 6'. It will see limited duty, running the TV, charging some cell phone/Ipad/toothbrush batteries and perhaps running the electric coffee pot. No microwave, no AC, no frig. We've determine our usage after just completing six weeks on the road, and not from conjecture.



Where should I put the circuit breaker in the line between the batteries and inverter and what amperage should it be?

The breaker should be just after the battery since it protects the wire from over heating.

Just on the side of safety, I would go with a thicker wire although I have tested 4 gauge wire at 2 feet and 130 amps and it didn't overheat. However, the added safety margin assures happier camping.

Your loads, IMO, are very optimistic at least in my experience. On more than one occasion, we have inadvertently overloaded our inverter by inattention. So, IMO, all wiring should be designed for the maximum capacity of the inverter plus some.

[TitanMike]

Originally Posted by PhilFromMaine

The breaker should be just after the battery since it protects the wire from over heating.



Just on the side of safety, I would go with a thicker wire although I have tested 4 gauge wire at 2 feet and 130 amps and it didn't overheat. However, the added safety margin assures happier camping.



Your loads, IMO, are very optimistic at least in my experience. On more than one occasion, we have inadvertently overloaded our inverter by inattention. So, IMO, all wiring should be designed for the maximum capacity of the inverter plus some.

I agree that a larger wire size would be better but not so much for safety. Voltage drop is a more likely issue especially when high current devices are involved.

Big difference between what's allowable and what's desirable. Design criteria can range from 10% drop down to 2%. Not a big deal on higher voltage circuits but in a 12v DC circuit it can be a big deal.

Most will go with a wire that's just large enough due to cost. In the larger scheme going larger and paying the extra price can often end up far cheaper. There may come a day when a larger Inverter is desired and now larger wire and fuse/circuit breaker need to be installed. Building twice is always more expensive.

Common statements made by many in years later usually start out with "I wish I'd gone with-------- (insert the more expensive option here)".

[Bob K4TAX]

In regard to "If the inverter suffers an internal failure and, for example, the power transistors short circuit you can have a fire if the power source is not protected." I will emphatically state the fuses inside of the inverter are providing this protection.

As to the large cable wiring, proper installation avoiding sharp metal edges, and use of grommets along with proper sheathing, when transitioning a wall or panel are required.

Law of Physics; any connection has resistance and any conductor has resistance regardless of how small. The system is operating in a low voltage/high current environment. Any resistance will cause a voltage drop at the inverter. Regardless of battery voltage sag, the inverter can likely show a low voltage condition and automatically shut down as designed. The math is E = I x R where E= voltage drop, I =current in amps, and R = resistance in ohms.

As to the protection of the RV, the LiFePo batteries provide their own internal current limiting. I just don't see adding a 2nd level of current protection, in view of added path resistance, as viable.

Bob

[PhilFromMaine]

Originally Posted by Bob K4TAX

In regard to "If the inverter suffers an internal failure and, for example, the power transistors short circuit you can have a fire if the power source is not protected." I will emphatically state the fuses inside of the inverter are providing this protection.

As to the large cable wiring, proper installation avoiding sharp metal edges, and use of grommets along with proper sheathing, when transitioning a wall or panel are required.

Law of Physics; any connection has resistance and any conductor has resistance regardless of how small. The system is operating in a low voltage/high current environment. Any resistance will cause a voltage drop at the inverter. Regardless of battery voltage sag, the inverter can likely show a low voltage condition and automatically shut down as designed. The math is E = I x R where E= voltage drop, I =current in amps, and R = resistance in ohms.

As to the protection of the RV, the LiFePo batteries provide their own internal current limiting. I just don't see adding a 2nd level of current protection, in view of added path resistance, as viable.

Bob

Good engineering demands redundancy.

[NavyLCDR]

Originally Posted by Bob K4TAX

I would not install a circuit breaker in the line for the inverter.
Bob

I would install the inverter according to the instructions provided with the inverter. If it says to install a fuse or circuit breaker in the DC input, then install a fuse or circuit breaker in the DC input.

Who do you trust more? The manufacturer of the inverter, or Bob from the internet?

[2WheelJon]

Thanks for all the suggestions (thanks for your thoughts, Mike). We just came off a six week trek, including four weeks boondocking (good to see you, Mike). During this time the ONLY thing we needed an inverter for was watching TV (we did it with a temporary hookup using a 400 watt portable inverter). We decided to do it right and permanently install an inverter



We will not be using the inverter for AC, microwave, toaster oven, etc. The biggest load it may see, and then for only a couple minutes, is the 120 VAC coffee maker. If it turns out to be a problem, we'll go back to using our stove top peculator which is how we make coffee when boon docking.



Yes, I know folks will say you should "overbuild" a circuit because later you'll regret not going "big enough." We've been camping for decades, know what we want and our electrical needs are very modest.



All this is to say that I think 4 ga wires running to the inverter should work fused per the discussion here.



Thanks again.