Fuse types

[jungpeter]

A previous owner installed a Heart ECS 1800 inverter/charger on my Tolly 48 in the early 90's. The installation does NOT contain an inline fuse on the DC output side, despite Heart (and others) recommendation. I would like to clean up this installation by installing proper fusing on this circuit.

The Heart callout for fusing on the DC output of the inverter is a 200 amp Type-J slow-blow fuse. I'm unable to locate a suitable marine fuse holder for this type of fuse. And Type-J fuses don't appear to be common stock items in marine supply stores. Fisheries Supply, for instance, lists ANL, T, AMI, AMG, T, yada, yada, yada, but no Type-J.

Some marine professional electricians recommend a Type-T fuse for this application, which is not a slow-blow fuse.

So, I be conflicted on many fronts. Why fuse an inverter/charger on the DC output leg at 200 amps, when the rated output of the inverter is 65 amps DC? Shouldn't that be more appropriately fused at something like 100 amps, which is about 150% of output? And why a slow-blow fuse? Obviously to limit nuisance tripping, but ???

And, does anyone have a lead to a Type-J fuse holder for marine use? Maybe I'm trying to pick the fly poop out of the pepper here, and use of a readily-available Type-T fuse and holder is just fine. But inquiring minds wish to know....

Regards,

Pete

 

[O C Diver]

That wire is supplying power to your inverter from the batteries, hence the need for 200 amps. Is your battery bank fused?

Ted

 

[fractalphreak]

We found something similar on our installation, there was a fuse on the inverter side but not one on the battery side of the DC wiring. So the wire was protected when charging but not when providing current when inverting. We ended up adding a class T fuse on the battery side sized to protect the cable.

My understanding is the overcurrent protection is based on protecting the wire or cable, not what the output might or should be...

I can offer no opinion on the fuse type. My inverter manual recomended the class T and i could find no reason to not use it. We sized it to the inverter manual recomendation which coincided with the ampacity for that sized cable (4/0).

 

[O C Diver]

The fuse all the left is the one for my inverter. It was the size required by the manufacturer and works fine.



Ted

 

[tiltrider1]

First, your fuse should be determined by wire size. Second, your inverter is calling out minimum specifications. If your wire can handle a 400amp T fuse then use a 400amp T fuse. Slow burn fuses are designed to take a larger load momentarily, this is commonly used with motors that take a momentary large draw to start.

Do not use a 400amp fuse if your wire is not sized for it.

Just because the inverter is rated at 65a. Dosnt mean it’s not capable of larger spikes

 

[jungpeter]

Thanks to all for your responses. And yes, the battery bank(s) are fused, although not correctly in my opinion. But that's another story, for another time.

Looks like a Type-T, fused to match the gauge of wire currently feeding the DC side of the inverter, is in my future. Thanks again.

Pete

 

[twistedtree]

The important requirement is that there be a fuse in the inverter DC cable in close proximity to the battery. ABYC calls for no more than 7" which is often impractical to meet, but you get the idea. The battery is the "nearly unlimited" source of current, and that's what you are protecting against.


It's not uncommon to see a second fuse in close proximity to the inverter itself, but it's really not required or necessary. The reason is that the inverter is not an unlimited source of current. It is inherently limited to it's charging capacity, and even in a fault condition can produce more than that. If your wires are big enough to handle it, which they should always be, then you are safe. The same is true for alternators.


Now all this said, if having the second fuse makes you feel better, by all means put one in. Unless it's got a bad connection on it, it can't do any harm.


As for fuse types, Class T fuses are most common for this sort of application. My recollection is that even though they aren't designated as slow blow, you have to overload them for a while before they let go.

 

[syjos]

The size of the battery cable and fuse is usually specified by the inverter manufacturer based on the AC surge output capability of the inverter.

The maximum DC amperage while charging is usually less than during inverting. And no surges during charging.

 

[twistedtree]

The size of the battery cable and fuse is usually specified by the inverter manufacturer based on the AC surge output capability of the inverter.

The maximum DC amperage while charging is usually less than during inverting. And no surges during charging.

Agreed. But what implications does that have on the fusing? Do you feel that necessitates a second fuse located at the inverter? Or is it equivalent to just have the fuse located at the battery, which you should have no matter what. And of course all assuming properly sized wire and fuses.


Until recently I always thought it correct to place a second fuse right at an inverter/charger and an alternator because they are sources of power, and you are supposed to fuse at the source of power. But then I learned about this carve out for devices that are inherently power limited, and no need for fusing provided the wiring can handle their max power, which also makes sense.


It's definitely a gray area, especially if you look at other examples. If an alternator is inherently power limited, wouldn't a generator be too? Yet we fuse a generator at it's output. And the shore power cord coming into your boat is power limited by the breaker at the power post. Yet we fuse them again shortly after the wiring enters the boat.


So in the end I think you need to do what you are comfortable with.

 

[O C Diver]

It's definitely a gray area, especially if you look at other examples. If an alternator is inherently power limited, wouldn't a generator be too? Yet we fuse a generator at it's output. And the shore power cord coming into your boat is power limited by the breaker at the power post. Yet we fuse them again shortly after the wiring enters the boat.


So in the end I think you need to do what you are comfortable with.

When rewiring parts of my boat, I started with fusing the house battery bank as a huge potential of power. All items that consume power (such as the inverter) must have an internal or external fuse. While the house bank fuse protects it to some degree, the fuse size is atleast 50% larger than what the inverter calls for. Only thing not fused is the alternator as it doesn't have a source for store power and the battery bank fuse protects that wire (wire is 4/0).

One potential problem scenario for shore power is the power cord adapters. While extremely rare for boats with 50 amp service, it's common to see boats with 30 amp service using an adapter from a 50 amp power pedestal plug to 30 amp power cord. Obviously there's no protection till you get to the main breaker panel on most boats. This is one of my pet peeves with ABYC. This is such a common occurrence that there should be a circuit breaker where the power inlet enters the boat and 50 amp rated wire between the two. Actually they need to do away with the 30 amp POS inlet and make all new boats use a 50 amp plug. Then the breaker inside would limit the amperage and determine 125 or 250 volts. Guess it's a good thing I'm not in charge.

Ted

 

[DDW]

I agree that fusing at the source of limited output devices (most charging circuits) is merely adding a point of failure with no benefit. For an inverter in particular that requires 200A input inverting and puts out 65A charging, you would need a 200A+ fuse and this will never blow from a 65A charging device even if shorted indefinitely. The 200A fuse belongs at the battery end, and the battery has virtually unlimited current output.

The best way to fuse a battery is with the Buss MRBF fuses as they can be mounted on the battery terminal, so about 1/2" between battery and fuse. Only available to 300A though.