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Class T fuse holders?

Daddy Tanuki said:
the fuse holders I use are made by bussman/eaton my t classes are all 80 amp fuses one for each bank. the most I have drawn to date with one inverter was about 43 amps. since I installed the second inverter I have seen 52 amps, but both amps were running under full output. (maybe 2/3rds?)

as my house and shop running flat out do not draw but 52 amps any one of the fuses/banks could power my system i chose to go with the smaller class t fuses on each bank. the holders that bussman/eaton make are a composite that reminds me a lot of Bakelite without the brittleness. blue sea only offered the massive fuses and I want that fuse to blow if there are any problems. fuses are cheaper than the cabling, cheaper than the inverters, cheaper than my shop.... and yes I have a 5 pack of spares just in case.

to give you an idea, I blew two of the fuses when I applied power to my system without a resistor to pre-charge the system. that was three morningstar SCC's and two Magnum MS-PAE 4448 inverters and the inrush popped two of them. learned my lesson....again.
Click to expand...


Yup... it comes down to the heat wasted by the class T... they have internal resistance... the lower the amperage value the higher the internal resistance. If your wires and fuses are sized right they may get warm but should never be hot. If they are hot, you either have a bad connection, bad crimp, or an undersized fuse/wires.

To give you an example of class T internal resistance
300amp - 0.25milliohms
200amp - 0.40 milliohms
100amp - 1.46 milliohms

And this is how much heat is wasted using ohms

1730910854946.png

Now assume you have a bad connection at the fuse that is 1, 5, 10 milliohms at 3/4 current

1730911002708.png

It doesn't take much for a "bad connection" to get red hot at those current levels... a bad crimp or connection a tiny amount loose.... this is why the NEC calls for using torque wrenches to tighten the bolts/nuts on things.

If the battery is a 48v with built in fuse/breaker the ideal place to fuse is just before the bus bar to protect the wire in case there are parallel packs. If it is just one battery it doesn't matter much where the fuse is.

And if the battery doesn't have any built in protection I would fuse at the terminal with a MRBF and a class T at the bus bar... overkill, probably, but maybe not depending on length of wire and if there is a possibility of it being shorted in the middle someplace. A dropped object causing a short at 2ft with the fuse at the terminal will heat things up quick, a short right at the terminal not so much if the fuse is at the bus bar.

it comes down to resistance in the wire and distance and mass of the wire. At 2 ft there is enough resistance in the wire at high current to melt it if there are multiple other battery packs dumping their rated current into it... The house burned down thread has the examples and math....

In a longer wire like 6ft the mass of the wire and the heat dissipation balance enough that the short may get things hot without melting anything.
 
Last edited:
SparkyJJO said:
People have dead shorted T class and besides the pop noise the fuse had remained fully intact.

I slightly modified my Blue Sea holders to not crack when removing the covers. Bad design IMO but wasn't hard to remedy.
Click to expand...
How did you modify them?
 
SparkyJJO said:
I just trimmed the edges that clip to the base so they were not as pronounced and slightly rounded the edges to not catch as tight.
Click to expand...
I'll give that a try, but hopefully once I put them on I won't have to take them off very often...
 
I much prefer the ignition safe version of the blue sea mount myself.

Cracking isn't an issue, and you don't need to fix it. They just have levers to open. And If you only need a few the price difference isn't horrible.... if you need 12 or something they might add up though.
 
robbob2112 said:
I much prefer the ignition safe version of the blue sea mount myself.

Cracking isn't an issue, and you don't need to fix it. They just have levers to open. And If you only need a few the price difference isn't horrible.... if you need 12 or something they might add up though.
Click to expand...
Thanks, I was wondering what the issue was because I had no issue.
Now I know why
 
Without filing down the clip edges a bit you have to apply so much force to the sides to get it off you you can slip and crack it easy.

You also need a bit of space on the sides to get a good grip where the ignition safe version only needs about 1/4" to 3/8" gap between if you are mounting multiple...

And of course the most important factor --- the ignition safe ones have a lot higher coolieo quotient!!
 
On a related topic, I have to remove a couple of the sections on the cover to accommodate right-angle cabling. Do I use a Dremel or is there a better way to snap off the sections?
 
I have 4 of these type fuses and had to remove some of the tabs …I use a Fordham flexshaft and a small carbide wheel on the hand piece ….I have also used dremel and style drywall side cutting bits/blades

it’s a good idea however you remove them to take a file and roll the sharp edges or kerfs that will be left where it was cut.. it’s easy…

fairing the sharp edges smooth helps them not cut into wire insulation if it’s a tight fit.

Filing the little side locking tabs down a tat on the sharp edge will help make them much easier to open in tight quarters.

The two metal arm locking top version for 110 to 200a are much nicer to work with..

J.
 
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