So I’m a bit confused about the wiring of devices that say to use 2x wires (eg 2x 2/0 for a multiplus). I get that by using the two wires you are doubling the current carrying capacity (and surface area to dissipate the heat). My question is on the fusing side..
Normally you place a fuse on the positive wire close to the power source (eg battery) to protect the wiring (both positive and negative wires as the same current will flow). If something happens you can never have more than the fuse allows in either wire.
Now if you have 2x wires and you fuse them both, all is good for the positive wires - if there is a short in a wire, the fuse blows to protect the wire… BUT if you have an open (or bad connection) on one of your 2x negative wires, the other negative will then be carrying 2x the current - not good!
Why is this not seen as a problem.. or is it?
Is there a common solution (Fuse the Negatives as well?)
If this has been answered before, please point me to that post (which I didn’t find..)
If the Crimps are well made, and the bolts are subject to a regular inspection for tightness (6 - 12 months is standard) then this is unlikely to be an issue.
Let me see if I can explain the fusing on the negative side with my limited understanding. The positive and the negative cables create one circuit. The negative side is grounded already. So a short on the negative side will just create another chassis ground in most cases. The positive side is not grounded. Breaking the current on the positive side disables the entire circuit.
@TherealKT The problem is not an accidental short on the negative side, but an unintended open or high resistance on the negative side, causing the remaining cable to carry higher than intended current…
Maybe best practice would be to just use one sufficiently larger cable whenever possible.
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
9
I think MikeD is correct. Now, if you’re selling to a DIYer, then it probably makes more sense to steer them to higher DC buss voltages to eliminate the need to use two 2/0 AWG cables to an inverter in the first place. The more we can get people away from using 12V anything the better!
very few people have the capacity to crimp up 300mm^2 and 450mm^2 cables!!
Biggest install that I’ve seen was 6 x 450mm^2 in parallel, per phase at 240/415V.
I thought we were talking about 2/0(70mm2) cable not 300 - 450! Yes I agree. But I would much rather use a single 4/0 than two 2/0 cables any day.
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
12
I’m not saying you are wrong, but I will say I don’t agree with you. Early on in my business, the most common MEGA fuses available had a straight plastic section which required shaving down the end of a 4/0 lug. Nowadays those larger MEGA fuses have a curved section that doesn’t require that. But, here’s the primary reason I disagree with you: the cross-sectional area of two 2/0 conductors is quite a bit larger than a single 4/0 and 2/0 lugs fit better with a Lynx distributor. I did the math a few years and concluded that a single 4/0 DC cable to a 12V 3kVA MultiPlus under continuous maximum operating (3 hours per NEC) current, like you would encounter when recharging a large 12V lithium bank, exceeded my comfort zone with 4/0. So when we could only run a single pair of 12V cables to a 3kVA inverter, I would use 250MCM instead of 4/0 along with Blue Sea e-series 9003 or HD switches. The combination of 4/0 and m-series or even e-series just got too hot for my comfort level. (You can search the community for my post measuring the internal resistance of various Blue Sea switches and Victron’s 275A switch).
Of course, the better solution is to not use 12V, but I didn’t come to that conclusion back then. I was just happy to have a customer, even if they insisted on a 12V installation. Today, I won’t install 12V for anything above 2kVA, and if a customer insists on it then I will sell them a design, wiring diagram, and parts and tell them to do it themselves and to sign a waiver. We haven’t installed a 3kVA or higher system running 12V for several years now.
I had Claude re-run the math because I’m lazy. See here:
I follow your logic and like it.. now to my specific install:
I was going to go 12V, but decided to go 24V. (I’d prefer 48V, but it’s not realistic as I can get may 24V parts for the boat, not 48V (yet)).
The MP II 24/5000 says:
2 x 50mm (AWG 1/0) up to 5m
2 x 70mm (AWG 2/0) 5-10m
I’ll be in the 5m range.. so 2x50mm would be 100mm. With this math a 4/0 would be fine, however if you do the current carrying 4/0 (445A) is lower than 2x 1/0 (2x 285A). My understanding is this is due to the ability to shed the heat due to the resistance being large when you have two wires.
I can’t realistically go higher than 4/0 .. what would you do?
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
14
Don’t let 48V scare you. You can use 48V to 12V DC-DC converters or a DC-DC charger along with a 12V lead acid battery for heavy current items such as a windlass.
Yes, I agree re multiple cables having better current carrying capacity : - they also have a larger surface area for cooling, and are more flexible. Bend radius for a cable should not be less than 6x diameter of the cable.
Heating due to resistance is just one of the reasons. You want to keep your DC ripple low for inverter efficiency. Victron’s Wiring Unlimited booklet has a good explanation of this. See here: