Well, you’ve certainly spent some time with this. Nice work. A couple of general comments:
10ga wire is good for 25-30A (depends on ambient temp, stranded vs solid, allowable V loss, etc). You may not need wire that heavy. And, it’s best to use stranded rather than solid in a mobile environment.
For when you actually install your fuse holders, the best approach is to put them as close to the source as possible rather than the load. One of the main purposes for fuses is to protect the wiring and the fuse can’t do that unless the short is “downstream” of the fuse. Yes, put 'em in!
And a question: What is the black wire you are showing which comes from the switch? Are you planning on using a SPST or DPST switch?
I’m so glad you answered Rick.
I spent the time because I wanted to clearly illustrate my plan so that I’d get some good feedback. Electricity scares the life out of me - scared is good when an RV fire is at stake.
I already pulled the 10AWG wire from the fuse box to the equipment closet (10’). It is stranded.
I will definitely put the fuses near the sources, as close as I can get the splice.
The switch is a 10amp SPST switch (it is lighted) so you see the piggyback on the ground.
Please take a fresh look at my post since I’ve edited recently. My main question is how should I select the fuse amperage? Should I assume 48v since they’re downstream from the step-up converters, or should I assume 12v since the circuit source is 12v? Also, do I have the fuses in the right places? Should there be additional fuses somewhere?
Ahhh . Lighted switch. Good. Your diagram is fine then.
The optimum fuse amperage depends on a number of factors. We’d use a slow-blow type for motor circuits and a super-fast blow type for electronic circuits. So, what’s the risk here? I’d venture to say it is not that your router is going to ever “short-circuit.” The real risk (as you have suggested) is damage to the wiring and a resultant short circuit. In such a case you’ll have a brief current surge pretty much equal to whatever the source can supply.
I think the general locations of your fuses is fine – just get 'em as close to the source as possible. Example: For the 12-48VDC converter I’d want them at the output as close to the converter as possible.
As regards the amperage rating, I’d use as small a fuse as reasonably possible allowing for consistent operation and low voltage drop. (Smaller fuses exhibit more V drop as load increases.) My gut says your selection vis-a-vis A rating is fine. Fuses are really cheap. Buy a few extras. If you find the fuse blows use the next greater size – assuming the wiring is OK and there are no issues with the load. (V rating is not at all critical – just ensure the V rating is at least as high as the circuit’s V.)
Thanks again.
I will actually shorten the 16ga wire coming out of the step-ups and shoorten the leads that are attached to the fuse boxes to get the blade fuses really close to the step-ups.
I still have a question about the fuse amperages. The amps drawn by the router on 12v would be 4.58. The amps drawn by the router on 48v would be 1.15. I chose to work with the 4.58 EVEN THOUGH the draw is downstream of the step-up converter. If I used the amp draw at 48v, I would use a 5, or even 3 amp fuse, however I chose a 7.5 amp fuse based on a 12v calculation. This is what I’m still puzzled about. What voltage should I use? Based on the 12v source (fuse box), or the 48v source (step-up converter)?
Also, should I put a fuse between the input of the step-up converters and the switch (16AWG splice to 10AWG)? Or, will the 10amp fuse at the fuse box protect the wires between step-up and fuse box.
I can easily put a fuse between the switch and the step-ups if this will protect the 16AWG that comes out of the step-ups. I’m not so worried about the 10AWG to the switch and spliced to the 16AWG downstream of the switch.
I guess I’m worried about the 16AWG wire of the step-ups since I can’t change them. They are waterproof integrated.
ALL of my inline splices will be soldered and shrink insulted. I will also shrink-insulate my blade splices.
Use a fuse that is rated for not less than the voltage passing through it. A 120V fuse would even be OK. Fuses don’t “blow” based on V. If the expected “draw” of the router is 1.15A @ 48VDC I’d probably use a fuse of about 3A or so. But someone could make argument for, say 2, 2.5 or even 4.
I think you are OK just as you have drawn it.
Don’t. You are doing fine
Smart move.
Comment: You are being more diligent about all of this than most folks (including a couple who have worked for me.) You’re doing fine.
Great Rick. I’ll leave the fuse boxes exposed ( not behind the wall) and start with 3amp fuses on both the Nvidia Shield and the Peplink router. I have a variety of spares to use if I need to ramp up the fuse amperages.
Is there anything I can do to protect the 16ga on the input side of the step-ups? Will the fuses on the output side of the converters protect the wire on the input side?
Sure you can add fuses there. But normally, as I pointed out, to protect the entire “system” fusing is done as close to the supply as possible. However, in your case you are using a 10A fuse which is fine for the wire size. If you wanted to add a smaller fuse in front of each converter that’d be fine.
Gratuitous comment: We have never performed an installation in a RV or marine craft. But we’ve done more fire engines, police cars, special purpose vehicles than I could ever count and that experience goes back several decades. In my experience one key to resilience and safety that many folks miss during the course of an installation is to ensure that the wiring/cabling does not move – and is safe from physical damage. The worst enemy (and one that can be difficult to troubleshoot) is chafing due to repetitive movements. So, we tend to put much of the wiring in loom and secure it with cable ties and various types of mounts. Sometimes we use the cheap-o plastic split loom; sometime fabric loom and sometimes the more expensive high-temp stuff. All depends on where it is going. Regardless, the objective is that wires/cables that should not move will not. Perform a “solid” physical installation and you’ll find things are more safe and reliable – and fuse don’t blow.
Thank you Rick. I ordered some split wire loom. I’ll also cover the circuit that the RV dealer installed when I purchased.
Between the fuse box (10amp fuse) and the 48v step up converter (10ga wire), how much amperage do you think flows 1) when there is a load (the router after the step up), and 2) when there is NO load (router unplugged)? I’m trying to determine fuse amperage for the additional fuse upstream of the converter on its 16ga wires.
There is an SPST switch on the circuit between fuse box and step up converter (20amp) attached directly to the 12ga wire.
Last question I believe - we’ve covered about everything with your fantastic help. Much appreciated.
Thank you for you help Martin. I’ve over-engineered my circuit, as you can see. I still don’t understand current flow, before and after a step-up converter. I’ll be testing different fuse amperages starting small and increasing until stable. It’s going to be a pain to keep plugging and unplugging the router.
Rick has really been a great help as well - I’m not quite as scared of this as when I started conceptualizing. Step-up converters are really somewhat of a mystery - I can’t even find what gauge wire they use but I’m assuming small @ 16AWG. This is kind of a sign that they consider their internal protections to be robust enough to prevent overheating the wires, like you mentioned.
@MartinLangmaid
The new circuit is up and running. I have noticed that the Max Transit Duo, powered via the terminal block at 48v, runs MUCH hotter than using the barrel plug power. The wiring is all checked with multimeter and no shorts or anything. Is this to be expected? It’s a lot concerning! Thanks.
I also have a barrel plug 12v connection that I used to power the TST in the past. It sounds like it’d be better/cooler to use that. But, even though the TST is spec’d for 48v, the amps are very low and the router doesn’t require many watts. I’m still confused why it runs so hot. 48v low amps should be cooler than 12v higher amps since less current is needed. This is absolutely true of the wires and connectors. Why not the router?
What @jvarnell1 said is correct. The router needs a lower voltage to operate. You’re feeding it 48V. So there is almost certainly a step-down module in there which would use either or both of the chips John cited. The step-down process involves the production of heat and you’re seeing that thermal gain at the case of the router. I may also add that it’s common to use a metal case such as Peplink uses as a heat sink – although I don’t know if that’s what they are doing with this model. So, your analysis vis-a-vis “48v low amps …” does not hold in this case. And, your use of the multi-meter would not be helpful in this situation.
I have not torn into one one of that model (yet) but when I do I’ll bet I find I am correct. (If anyone else has – kindly let me know.)
I used the multimeter to confirm that only 48v is present since the router spec says OK via the terminal block. It sounds like a 12v connection would stay cooler. I am surprised and worried how hot it is.
Without looking at the populated PCB and probing a bit I cannot be certain but I’d strongly suspect that feeding it 12VDC would be much more efficient. As to the heat, I’d trust Peplink’s engineers to do it right.
Personally, I’d go have a beer and find something else to worry about.
Yes the 48vdc is a max and in most of the models I see a 10vdc min. So unless you need to use a full RS-232 your device should work the same at the min. voltage. Normal circuit boards use 5v and 3.3v circuits. The processor can be either of these. The cell circuits may run on 3.3 because of chip selection. Amplifiers use all range of voltages but if I design something I will use the same voltage I have on all other places on the board so most likely the RF amplifiers are designed to use the min. voltage source.
Nice work. A couple of general comments:
