How many cables fit on a busbar?

[Fuss]

Originally Posted by s/v Jedi

So here’s me considering to put 16 cables onto a 4-stud busbar.... thoughts?

I would be wanting to put a double nut on them, I'm not sure I would be happy with just one.
Maybe modify to have longer studs.

[Bycrick]

Jedi’s correct about it being legal and within specs, except for the fact that I, personally, don’t like the aesthetics. But it’s not my boat, and I don’t have to work on it.

As to not being able to get a washer on some terminals combinations, that I think IS a problem. However, the simple solution is to replace the hex-nut/lock washer combination with a serrated, flanged nut, as Blue Sea says there’re noting with new shipments.

[dfelsent]

Originally Posted by Factor

Well then why did you bother asking?

Highly skilled practitioners know that review and discussion of their work product by other highly skilled practitioners can result in new insights an a better quality result.

[Manateeman]

This was an excellent thread. I just invested in a Temco crimper. I’ve looked at a number of lugs and would appreciate recommendations from the readers of this thread. I also asked Rolls battery where they get their lugs and the extra thick shrink tube they use but they just say custom.
I noticed Blue Sea has gone to flanged serrated nuts...is this the best available?
Next ...the big fuse issue. I always learn something when I see how other carefully built boats lay out things from the battery bank. I’m sure photos will be of interest to others.
Happy trails to you
Mark and his “float point” manatee friends

[Frankly]

https://www.cruisersforum.com/forums...cture24349.jpg

https://www.cruisersforum.com/forums...cture24348.jpg

[Bycrick]

I’ve been using lugs/terminals from FTZ. They are easy to deal with if you can handle their minimum order requirements. I also use their big crimping tool. Their "heavy duty" terminals are massively lovely compared to the usual "flared starter" terminals. But they have problems in tight spaces because they’re longer. I’d be surprised if you couldn’t get an equivalent to the Rolls heat shrink from McMaster-Carr, Digikey or Mouser.

Blue Sea says they’re now shipping with the flanged, serrated nuts rather than a hex nut/lock washer combination. I haven’t seen them, but I’d expect that the single part would be thinner than the pair. It’s anybody’s guess whether this just reduces parts count, or prevents people from skipping the lock washer or holds better.

Big fuse issue. The amp rating of the fuse is based on protecting the wire. A lot of people overfuse circuits. The not-often-looked at spec is Amp Interrupt Capacity, which is the amount of current that the fuse is guaranteed to blow and stay open. The ANL fuses have an AIC around 2000A. The MRBF box fuses around 10K amps. The Class T fuses are 20k amps. Why might you need a big fuse? Because my Group 31 Optima starting battery can put 3000a into a short circuit. Imagine the peak current from a big AGM bank.

[s/v Jedi]

Originally Posted by Bycrick

Big fuse issue. The amp rating of the fuse is based on protecting the wire. A lot of people overfuse circuits. The not-often-looked at spec is Amp Interrupt Capacity, which is the amount of current that the fuse is guaranteed to blow and stay open. The ANL fuses have an AIC around 2000A. The MRBF box fuses around 10K amps. The Class T fuses are 20k amps. Why might you need a big fuse? Because my Group 31 Optima starting battery can put 3000a into a short circuit. Imagine the peak current from a big AGM bank.

Indeed. My starting batteries are Odyssey TPPL AGM and can do 5kA each. I will use 300A MRBF fuses on those. The new Lithium bank gets two 400A class-T fuses (24V and 12V center tap).

The picture is a teaser of the new cells I just received from China

[Bycrick]

You didn’t say how big your motor was and what current is required. But the 300a MRBF fuse will carry 600a for over 90 seconds. 2/0 wire has a maximum ampacity of about 200 a. What size wire are you planning on using?

[s/v Jedi]

Originally Posted by Bycrick

You didn’t say how big your motor was and what current is required. But the 300a MRBF fuse will carry 600a for over 90 seconds. 2/0 wire has a maximum ampacity of about 200 a. What size wire are you planning on using?

That cable is 2/0. Ancor 2/0 has an ampacity of 330A

[Manateeman]

I understood there was a rule of thumb you could use for AIC...that you multiply the amp hours x 10. Also this number is very close to the short circuit rate for the battery? I know there are formulas to calculate AIC, but are the above shorthand methods accurate?
My new house bank is 700 amp hours at 24 volts so I was just going with T class fuses. Correct?
Mark and the calculator challenged manatees.

[Bycrick]

Anchor 2/0 does say 330 ampacity outside the engine room. Inside 280 a

Still, that fuse will allow the wire to carry 200% of rated current for at least 90 seconds. I think the fuse is too big for the wire. On my Yanmar 4JH, I draw 260-280a when cranking for a few seconds. I fused at 200 a and have never had a problem. I might have used 175 a but they weren’t in stock.

Manatees rule-of-thumb might work somewhere but not on Optima batteries. Mine is rated about 100 ah and is speced at 3000 amps short-circuit current.

[Bycrick]

Also, the AIC is related to battery voltage. At 24v the AIC is a lot lower than at 12v.

[Bycrick]

AIC is really important with big, low-internal-resistance batteries. If it isn’t high enough, when you get a dead short circuit, the fuse doesn’t just open; it vaporizes. The metal vapor is conductive and the "open circuit" arcs over. The arc has less resistance than the fuse did, and in fact is a negative resistance: the higher the current, the less circuit resistance. So the current goes to maximum until something else burns open or the battery voltage drops enough to extinguish the arc. Usually the wire insulation will catch fire long before that happens.

[s/v Jedi]

Originally Posted by Bycrick

Anchor 2/0 does say 330 ampacity outside the engine room. Inside 280 a

Still, that fuse will allow the wire to carry 200% of rated current for at least 90 seconds. I think the fuse is too big for the wire. On my Yanmar 4JH, I draw 260-280a when cranking for a few seconds. I fused at 200 a and have never had a problem. I might have used 175 a but they weren’t in stock.

Manatees rule-of-thumb might work somewhere but not on Optima batteries. Mine is rated about 100 ah and is speced at 3000 amps short-circuit current.

You can fuse the cables up to their ampacity rating. The fuse may take 90 seconds or even more to blow, but the cable can take much more before it becomes hot enough to burn. This is why a single 4/0 cable can be fused at 400A. For the engine room, cables are de-rated, but that is for actual amps going through; you are actually allowed to not fuse the engine start circuit at all: straight from battery to engine, no fuse.

[Dsanduril]

Originally Posted by Manateeman

I understood there was a rule of thumb you could use for AIC...that you multiply the amp hours x 10. Also this number is very close to the short circuit rate for the battery? I know there are formulas to calculate AIC, but are the above shorthand methods accurate?
My new house bank is 700 amp hours at 24 volts so I was just going with T class fuses. Correct?
Mark and the calculator challenged manatees.

Short answer - no.

At the first order level available short circuit current is dependent on two things; voltage and internal impedance (resistance) of the battery. Internal impedance is in turn dependent on two things; battery chemistry and battery size. FLA has a higher impedance than AGM/gel which in turn is higher than lithium chemistries. If you look at the "fast charge" capabilities of a battery you'll see the same trend, for the same reasons. The lower the internal impedance the easier it is to charge at high currents without creating too much heat.

There is some intrinsic resistance due to chemistry and then some additional resistance that depends physical characteristics, i.e. size. As you get larger plates and greater surface area, and/or lower plate spacing then the internal impedance drops with size. But it's not linear. As an example, a Concorde U1 AGM battery (33Ah) has a short circuit current of 1719A (way more than your 10x). The Concorde 8D battery in the same line is 255Ah (7.7x the U1) and has a short circuit rating of 6605A (3.8x the U1).

Long answer, you really do need to look at your batteries specifically to evaluate available fault current. Also remember that fault current will almost always be reduced somewhat by the external connections. Especially on low internal impedance batteries the external connections at the positive and negative posts, even if extremely well made, may be nearly as high in resistance as the internal characteristics of the battery.