Fuse Question for 48V System

[wpduhe]

I've built a 48V battery and am wondering about fusing branch circuits. If it is required that a class T fuse be used for 48V systems, does that apply to every branch circuit?

Example: I have a 48V to 12V 360W buck converter that will pull (360W / 48V) * 1.25 = 9.375A. Does this mean I need a class T 10A fuse?

 

[John Frum]

I've built a 48V battery and am wondering about fusing branch circuits. If it is required that a class T fuse be used for 48V systems, does that apply to every branch circuit?

For lead acid, MRBF fuses are fine.
For 12 volt LFP systems, I like MRBF fuses.
For 24 or 48 volt LFP systems, I would use a class-t fuse as the battery fuse.
The branch circuits don't need the same breaking capacity as they are further away from the angry pixies.
Plus the battery fuse is there as a backup.

Example: I have a 48V to 12V 360W buck converter that will pull (360W / 48V) * 1.25 = 9.375A. Does this mean I need a class T 10A fuse?

The upper bound for the fuse size depends on the wire metal, awg and insulation temperature rating.

The lower bound is 125% max continuous current rating.

360 watts / .9 efficiency factor / 48 volts low cutoff = 8.333333333 service amps
8.333333333 service amps / .8 fuse headroom = 10.416666667 fault amps.

I've never seen a class-t fuse smaller than 100 amps.

Now the question is, does the buck converter have pigtails?
The pigtails will likely determine the fuse size.
If its a cheap buck converter the pigtails may not even be copper.
I don't recommend cheap buck converters.

Please provide a product link for you main busbars and for you buck converter.

 

[wpduhe]

Please provide a product link for you main busbars and for you buck converter

Battery Bank is LFP EVE LF280K

Buck Converter:

https://www.amazon.com/dp/B09ZNW2HSG?ref=ppx_yo2ov_dt_b_product_details&th=1

Input wires are 14 AWG rated for 105C (printed on the wire)
ABYC reports that gauge ampacity as 35A

Bus Bars:
Mofeez Power Distribution Block Bus Bar, 4 x 5/16" Posts, 6 x #8 Screws Terminals, Max 48V 250A (Pair, Red & Black)

https://www.amazon.com/dp/B09J23SRCW?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

[John Frum]

Battery Bank is LFP EVE LF280K

Buck Converter:

https://www.amazon.com/dp/B09ZNW2HSG?ref=ppx_yo2ov_dt_b_product_details&th=1

Input wires are 14 AWG rated for 105C (printed on the wire)
ABYC reports that gauge ampacity as 35A

That is a cheap buck converter.
I would not fuse 14 awg pure copper with 105C insulation higher than 15 amps.
Actually I would not even go that high because I think the wires are copper clad aluminum.
The ends look silvery.
I just doubt like heck that they are tinned copper.

I don't use any produdct that has copper clad aluminum wires.

Bus Bars:
Mofeez Power Distribution Block Bus Bar, 4 x 5/16" Posts, 6 x #8 Screws Terminals, Max 48V 250A (Pair, Red & Black)

https://www.amazon.com/dp/B09J23SRCW?psc=1&ref=ppx_yo2ov_dt_b_product_details

Those busbars are rated for 20Amps per connection.
That is really whimpy.
What inverter will you be using?
Will you have any pure 48VDC loads?

 

[John Frum]

This is a quality buck converter

Victron Energy ORI481240110 Orion-Tr 48/12-30A Isolated DC-DC converter

Typically for 48VDC systems all the power distribution is done in alternating current.

 

[wpduhe]

@John Frum

Inverter: https://www.amazon.com/dp/B07JBC9Q5G?ref=ppx_yo2ov_dt_b_product_details&th=1
3000W continuous / 6000W surge. I'm trying to size everything for surge current.

I do plan on having a pure 48V load. An electric motor for propulsion. Hopefully, some other components if I can find them (fridge hopefully).

I believe the silver color on the converter pigtails is some solder that was put on the wire. To be sure, I cut and stripped wires and took a picture. The conductors look similar to the Ancor Marine wire I have on hand.

 

[wpduhe]

This is a quality buck converter

Hopefully, this is not my ONLY option.

 

[John Frum]

@John Frum

I believe the silver color on the converter pigtails is some solder that was put on the wire. To be sure, I cut and stripped wires and took a picture. The conductors look similar to the Ancor Marine wire I have on hand.

Ancor marine grade wire is top quality.
That buck converter is bottom quality.
Its not impossible that they used top quality wire on a ghetto buck converter but I really doubt it.

 

[John Frum]

3000 ac watts / .85 inverter efficiency factor / 40 volts low cutoff = 88.235294118 service amps
Add 10 amps for the buck converter
Add $n amps for your 48VDC loads
That busbar will be running grossly out of spec.

 

[Rocketman]

I looked at the bus bar-
Isn’t the 20a spec for the small screws terminals- (something that would only be rated as 10 to 20a anyway),
And the big (3/8th?) terminals 150a or 250a.

They are using tin plated BRASS for the bus bar - so it will heat up more than tin plated copper.

Be sure and look at a failure analysis with your setup. For example, if the 48v/12v converter died, can you make it back to shore. Maybe you need a backup navigation device… and/or a backup converter on board.

Good Luck

 

[John Frum]

Copper clad aluminum would appear copper colored.
My bad.

 

[wpduhe]

So, in conclusion, branch circuits do not need to be protected by class T fuses. Will automotive fuses suffice or is there another type that's recommended? I'm most interested in the higher current branches like the inverter and the motor.

 

[John Frum]

Will automotive fuses suffice or is there another type that's recommended?

Mini fuses are rated to 58 volts.
They top out at 30 amps which means you run up to 24 amps continuous on the branch.
I'm unable to find a fuse block rated over 32 volts.
I think secondary dc distribution is going to be a challenge.

I'm most interested in the higher current branches like the inverter and the motor.

For loads that exceed 24 amps continuous you will likely want to come off the main busbars.
How many battery(feeder) circuits will you have?
How many primary branch circuits will you have?

 

[wpduhe]

How many battery(feeder) circuits will you have?

I'm thinking one feeder to the main distribution bars. On the main bus bars' large terminals, 1 terminal will be the input from the battery, 1 terminal output to the motor, and 1 terminal output to the inverter (only 3 in use). Using the smaller screw terminals, 1 will be output to the buck converter (~10A), and 1 will be charge input from a solar charge controller (500W solar @ 48V maxes out around 10A). This will be all of the branches off of the primary power source, unless I can locate a 48V fridge.

Mini fuses are rated to 58 volts

I should have been more specific. I meant the Littlefuse MEGA, bolt-on fuses that can be purchased from an automotive store; pictured below (with appropriate amperage ratings for each load). If these can be used, that would be great. I'll not attach them directly to the main bus bar, but to the load device. Would the smaller loads be better facilitated by inline fuses?

 

[Vigo]

So unfortunately, the simple answer is yes, every branch circuit needs its own fuse that is appropriately rated for the voltage used, AND appropriately rated for the current the conductor can carry. Unfortunate because appropriately VOLTAGE rated fuses for 48v dc are much much more expensive than for <42v systems, even if you can find them in small amp ratings.

But the voltage rating is important because a fuse with too low of a voltage rating may allow an arc (think welding) to form and continue allowing current to cross the fuse even after it has blown. Even if you have one large appropriately rated fuse near the battery, like a 100a+ class T fuse, it would still allow current under that to melt and ignite things in or near the branch circuit with the fault condition, until the battery drained! You could have a 70a welding arc going for.. minutes? hours? and never blow the big fuse.

One thing i think is viable as an alternative is to use Dc-rated circuit breakers rather than fuses for the branch circuits. This may only be because the cheap ones are not held to any rigorous testing standard to prove anything, but circuit breakers rated for >60v dc seem to be cheaper than fuses in that voltage range. Hopefully if i'm wrong someone will come along and point out all the cheap >60v fuses i've been missing.

 

[wpduhe]

One thing i think is viable as an alternative is to use Dc-rated circuit breakers

I thought about using circuit breakers rather than fuses. In the case of the inverter, would it allow surge current (6000W @ 48VDC) to pass through without tripping the breaker? I've been sizing everything according to surge current (notably the wire), perhaps I shouldn't?

 

[John Frum]

I'm thinking one feeder to the main distribution bars. On the main bus bars' large terminals, 1 terminal will be the input from the battery, 1 terminal output to the motor, and 1 terminal output to the inverter (only 3 in use). Using the smaller screw terminals, 1 will be output to the buck converter (~10A), and 1 will be charge input from a solar charge controller (500W solar @ 48V maxes out around 10A). This will be all of the branches off of the primary power source, unless I can locate a 48V fridge.


I should have been more specific. I meant the Littlefuse MEGA, bolt-on fuses that can be purchased from an automotive store; pictured below (with appropriate amperage ratings for each load). If these can be used, that would be great. I'll not attach them directly to the main bus bar, but to the load device. Would the smaller loads be better facilitated by inline fuses?
View attachment 114040

Victron has 58 volt mega fuses rated from 125 amps to 300 amps.
So that battery gets a class-t fuse
Mega fuses are fine for the branch circuits off of the main busbars.
125 amp fuse needs 4 awg wire.
300 amp fuse requires 2/0 or larger.
Its the next level down that is the problem.
Hopefully the mini fuse will either open or sustain enough of an arc to blow the 125 amp mega fuse before something ignites.
Eaton calls this selective coordination.
Littelfuse calls it fault isolation.

 

[John Frum]

So unfortunately, the simple answer is yes, every branch circuit needs its own fuse that is appropriately rated for the voltage used, AND appropriately rated for the current the conductor can carry. Unfortunate because appropriately VOLTAGE rated fuses for 48v dc are much much more expensive than for <42v systems, even if you can find them in small amp ratings.

But the voltage rating is important because a fuse with too low of a voltage rating may allow an arc (think welding) to form and continue allowing current to cross the fuse even after it has blown. Even if you have one large appropriately rated fuse near the battery, like a 100a+ class T fuse, it would still allow current under that to melt and ignite things in or near the branch circuit with the fault condition, until the battery drained! You could have a 70a welding arc going for.. minutes? hours? and never blow the big fuse.

One thing i think is viable as an alternative is to use Dc-rated circuit breakers rather than fuses for the branch circuits. This may only be because the cheap ones are not held to any rigorous testing standard to prove anything, but circuit breakers rated for >60v dc seem to be cheaper than fuses in that voltage range. Hopefully if i'm wrong someone will come along and point out all the cheap >60v fuses i've been missing.

Circuit breakers and fuses have to overcome the same problem.
Class-t fuses are filled with sand to help quench the arc.
Breakers use various tricks to divert the plasma stream in order to break the arc.

 

[John Frum]

In the case of the inverter, would it allow surge current (6000W @ 48VDC) to pass through without tripping the breaker?

That inverter has no real surge capacity.
Probably 2x surge for .02 seconds(1 ac cycle) which is only meaningful to the marketing department.

 

[wpduhe]

That inverter has no real surge capacity.
Probably 2x surge for .02 seconds(1 ac cycle) which is only meaningful to the marketing department.

Correct. It's marketed as 6000W (2x) surge current. I could test it by hooking up my table saw. But I'm pretty sure I watched an independent review video of someone doing that with this unit already. In terms of sizing, I've planned my fuse and wire size on that 6000W surge. Am I putting too much caution into that circuit?