Correct way to fuse parallel conductors with Quattros and Lynx

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jonathan-lee asked Jul 3, '19

Correct way to fuse parallel conductors with Quattros and Lynx

I've noticed Victron often recommends using 2 conductors each for negative and positive DC input to inverters, and it's unclear to me whether each conductor should be fused separately. I'm looking at battery fuse and cable sizing recommendations in the manual here: https://www.victronenergy.com/upload/documents/Manual-Quattro-5k-8k-10k-15K-100-100A-230V-(firmware-xxxx4xx)-EN-NL-FR-DE-ES-SE-IT.pdf.

What's confusing to me is for the 48 V, 8 kVA Quattro, it recommends a 300 A fuse, and 2x 50 mm^2 conductors for each positive and negative. I believe a 50mm^2 conductor can carry 235 A, so it seems to me it's recommending to use a single fuse for both cables. This makes me uneasy though; I thought each conductor should be protected by a fuse rated for its ampacity.

So if the answer is that the two wires share a single fuse, when connecting to a Lynx Distributor, does that mean that I would be fixing both conductor terminals to a single bolt (1 of the 4) on the distributor?

Thanks for your help.

MultiPlus Quattro Inverter Charger fuses

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stavarda commented · Mar 19, 2020 at 06:34 PM

do you have any pics of your set up ?

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wkirby answered · Jul 03, 2019 at 06:11 AM

When the two conductors are in parallel, being used as one conductor then it is only necessary to have an appropriately sized fuse for the overall cable CSA, which would be 100mm2 if two 50mm2 cables are being used in parallel.

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jonathan-lee commented · Jul 03, 2019 at 05:35 PM

Thanks @WKirby. Do you know is this rule jurisdiction specific? I was reading a post here: https://shop.pkys.com/Installing-your-Victron-MultiPlus-Inverter-Charger_b_88.html (scroll down to "Main Battery Cables" and the footnotes) that says "If doubled up cables are used the ABYC says each individual cable must be capable of carrying the entire load by itself and they must share a single fuse. " The "ABYC" is the American Boat and Yacht Council.

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wkirby ♦♦ jonathan-lee commented · Jul 04, 2019 at 08:29 AM

I would say it is jurisdiction specific.
You should do what ABYC say if they govern your jurisdiction.

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Answer 1 · 2019-07-03T07:58:17Z

mose answered · Jul 03, 2019 at 07:58 AM

You should never use two fuses in parallel! Small variations in characteristics can not be avoided in production so even if both fuses state 100A they are never 100A exactly. In parallel wiring one gets always more load than the other. This can lead to unexpected behaviour. In a fault condition both blow anyway. In your case both wires are used as one conductor, this means one fuse for both together.

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jonathan-lee commented · Jul 03, 2019 at 05:16 PM

Thanks! I would think the same logic applies to the conductor, right? That one of the parallel conductors will be carrying more current than the other. Normally I’d size a single conductor to carry 1.25 times the current rating of the fuse. So how to size each parallel conductor to ensure that neither conductor could carry too high of a current before blowing the fuse? I see WKirby’s answer says add the areas of the parallel conductors, but I just want to confirm.

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sean jonathan-lee commented · Jul 03, 2019 at 06:28 PM

A note for clarity, and accuracy - paralleling a number of fuses is a reasonably common industry practice - obviously all manufacturers design guidelines for the connection of their products in parallel (in terms of possible derating of enclosures, and other performance characteristics) must be adhered to.

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Answer 2 · 2019-07-03T11:15:55Z

kai answered · Jul 03, 2019 at 11:15 AM

Also the thing about conductor(s) being protected by a fuse by its ampacity - recall that the underlying principle is that you don't want the conductor heating up to a point where its a fire hazard. Hence you protect with a fuse that blows if the current goes above the ampacity. That doesn't mean you can't use a fuse that blows before the ampacity limit. its just a little more... twitchy (for a given ampacity). Or conversely safer (for a given fuse rating).

Under normal (non-fault) circumstances though you would never get close to the limit.

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Answer 3 · 2019-07-04T05:40:45Z

ben answered · Jul 04, 2019 at 05:40 AM

I suggest putting a fuse on each conductor, but prove to yourself that your parallel interconnects are of equal resistance afterwards by testing with your meter under load. With long enough wire, or quality enough fuses, you can get balance.

(If you find your fuses are varying a lot, you should pre-test additional replacement pairs. In my setup, the first six I tried all were very close and interchangeable.)

A single big fuse with undersized dual conductors fails to protect you against a fault in one conductor. (It could open, overloading the other conductor, and/or it could fault semi-short and overload itself.)

ABYC sometimes takes the belt-and-suspenders approach. You may not even be able to physically fit that in a larger Quattro, which I suspect is part of why they provide dual terminals on the big ones.

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stavarda commented · Mar 18, 2020 at 09:01 PM

for point of clarity to me on the position of the fuses? these would at the battary end or the Inverter end of the system? and would it be a T class fuse?

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kai ♦ stavarda commented · Mar 19, 2020 at 11:40 AM

battery end. Fuse type/class would depend on the prospective shortcircuit current. A UL class T fuse would (I think) be the gold standard for most battery systems being discussed in this forum, including the various lithium chemistries.

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