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Old 08-13-2017, 12:48 PM   #1
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Replace inverter fuse with breaker?

Hey electric gurus: My 1500w inverter has a big inline fuse between it and the batteries. The fuse label is hard to read, but it looks like it says "200A." Can that be right, a 200 amp fuse on a SMB system?

Anyways, there's nothing wrong with it, but for simplicity's sake I was thinking of replacing it with a push-reset breaker. Any issues that I'm not thinking of if I go that route? What size breaker should I put

Our SMB is all electric, with a 1500 watt inverter and a 315ah battery bank (3 group 31 AGM batteries), standard 30 amp shore power inlet. No solar.

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Old 08-13-2017, 01:11 PM   #2
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I see no problem with your idea.
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Old 08-13-2017, 01:39 PM   #3
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Fuses are sometimes specific to a device. The relay curve varies from fuse to fuse so if it's designed around the inverter I'd want to talk with the manufacture. Maybe Greg can chime in here. If you could find a breaker that reacts the same as the fuse I wouldn't see a problem. I've never seen a fuse like that associated with an inverter/converter but they are common as high amp fault protection from the coach battery.
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Old 08-13-2017, 02:57 PM   #4
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What you have is a Class T fuse, which is code compliant and used in proper installations. A circuit breaker could fail to open in a high fault current condition, as the contacts have been known to weld together. They usually have a smaller arc interupting rating too. I would leave it just as it is.
49 Class T- DC Fuses & Fuse Blocks

Class-T fuses
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Old 08-13-2017, 05:50 PM   #5
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Agree totally with arctictraveller, leave it as is. Yes it is a 200 Amp.

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Old 08-13-2017, 06:30 PM   #6
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Thanks guys! I'll leave well enough alone then.
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Old 08-13-2017, 07:20 PM   #7
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As an aside, you should never have to worrry about replacing the fuse. About the only time it will blow is if you develop a direct short in the positive battery cable.
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Old 08-14-2017, 08:10 AM   #8
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This is actually a good place to Hijack this thread, most of the sportsmobiles I have seen use a class T fuse between the inverter and the battery only. And that is because of the recommendation of the inverter company. What sportsmobile does not provide is a fuse that is just for your battery bank. In many cases our batteries are some number of feet away from the inverter or fuse panel, etc.

I highly recommend that you have a Class T fuse close to your battery (7" if possible) as your primary "Main" circuit protection.


In Brian case his battery bank is over 2500 amps of short circuit capacity, This is where the term interrupt capacity or Arc Interrupt Capacity, Class T fuses have a AIC of 20,000 amps (older ones at least 10,000) amps. It is pretty common to go with 2X your short circuit rating.

You will also find that most breakers that can handle 200 amps (will also state not to use as a switch), which is the reasons many say they want a breaker, most breakers have a AIC of 3000 to 5000 amps if rated for AIC. I would generally recommend a breaker only in a "Branch" circuit not a "Main"

As a side note many times there is equipment that wants you to wire directly to the battery (voltage measurements, etc) , I have a couple of these for voltage measurements, or sense voltages of battery monitors or solar charges. I tie these on the terminal after after the Class T fuse. It should still be close enough to get a the proper measurement you want and still be protected.

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Old 08-14-2017, 08:27 AM   #9
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Great info Greg and others! I never knew about Class T fuses.

My van, as set up from SMB, has the positive wire from the inverter to the battery bank fused with the Class T fuse shown. But, there is at least one smaller wire going directly to the battery bank (I think it goes directly to the 12v fuse panel). Should all positive wires drawing from the battery bank be relocated to the lug on the Class T fuse block?

My battery bank is very close to the inverter and fuse panel (about one foot, maybe).
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Old 08-14-2017, 10:31 AM   #10
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[QUOTE=BrianW;209612] But, there is at least one smaller wire going directly to the battery bank (I think it goes directly to the 12v fuse panel). Should all positive wires drawing from the battery bank be relocated to the lug on the Class T fuse block?QUOTE]

A fuse or circuit breakers main mission in life is to prevent a fire. Any wire passing current will begin to heat up, the larger the wire, the smaller the heat gain for any given current. The problem is, if the wire is conducting too much current, it may overheat and cause a fire. The solution to this issue is two fold, one, the wire must be sized for the max current the circuit may draw, and two, it must be fused for the same max allowable current. There are tables that help you size the wire so that it won't overheat under normal current loads. Circuit Wizard - Blue Sea Systems

If properly sized, the wire will never overheat, but the problems start under abnormal conditions such as a short. That's where the fuse comes in, it limits the ammount of current to that of the max allowable for any given wire size. When a short happens, the fuse blows. So, the max fuse size is dictated by the wire size. If you have an unfused wire connected directly to a battery, a short could cause it to turn into flaming balls of molten copper. It could be the same thing with a wire with an oversized fuse. So, if your little unfused wire connected directly to the battery is good for say 20amps and it shorts, and the battery could provide 2500 amps, instant molten copper. If it's connected to your 200A class T fuse, it's still not protected and can catch fire. You need to determine what size the wire is, and then insert a properly sized fuse between the battery and the wire, as close as possible to the positive terminal. The only place where a fuse is not required between the battery and the load is the starter motor, all other wires must be properly protected.
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