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druidwood 09-16-2018 01:08 PM

fuse versus breaker for dc
Thought I had it all figured out then rereading NovaKool brochure see that I need a breaker as well as a fuse. I thought DC just used fuses, and my AC/DC power center has no place for DC breakers, only AC so guess the combo one was a bad idea. So it calls for a 20 amp breaker and a 15 amp fuse. Why are both required? Do they both do something slightly different? Oh and can I use an inline fuse versus getting a whole breaker box?

I will be installing a battery turn off switch, but should I be adding an inline fuse for the battery as well? I won't be pulling more than 20 amps DC.

Sadly I knew nothing about voltage drop and my household battery and items it is running (fridge, toilet fan, h20 pump) are at the other end of the bus. Seems like my DC panel is for small wires, but wire gauge calculator says I need a 6 gauge to run 40 feet, and that is a huge wire.

brokedown 09-16-2018 02:15 PM

Wire thickness is based on amps, distance, and how much power you're willing to lose (as heat, of course) along the way. You can save half your money by having your power center in the middle of the bus so none of your runs are actually 40 ft.

I use DC for basically everything, but I do have an inverter for infrequently used things. it has fuses on the input side so it's connected directly with a nice thick cable.

For the rest of my build, I have a fuse panel:

I tie my start and house batteries to a shared chassis ground, so I'm only running positive wires.

I don't see a good reason to have a breaker combined with a fuse, but if I did it anyway, I'd use a higher rated fuse and a lower rated breaker. Note also that most household type breakers are NOT suitable for DC power.

john61ct 09-16-2018 06:13 PM

Fuses for sure for high current rates.

In general simpler cheaper.

CBs more difficult to spec, major quality problems with generic cheap stuff, better to pay for quality, buy from known trusted sources.

Research delay issues, many variables.

Don't assume long lifespan if frequently used as a switch under loads.

Iceni John 09-17-2018 12:03 AM

Carling makes their Type C circuit breakers with DC ratings as well as for AC. (They're the sort used by Blue Sea and Paneltronics for marine and commercial use.) Square D also make their QO breakers with a DC rating, but they're a lot clunkier than the Carlings. (Just don't use Square D's Homeline breakers - they're not DC-rated.) I'll be using the Carling breakers for both my AC and DC breaker panels: I can make two 11-breaker panels smaller than one QO 8-breaker panel box.

If it's critical to stop an over-current RIGHT NOW, then fuses are best. This is why I have two 300A Class T catastrophe fuses for my battery banks - they are very fast-acting. My inverter has a 250A ANL fuse for its DC supply, but its outputs are breakered. Elsewhere I use a mixture of ATC and ATO fuses for general-purpose low-current protection, several dozen self-resetting Cole-Hersee miniature breakers for chassis loads, an AGC fuse for each of the solar panels, and Carling Type C breakers for the charge controllers' inputs and outputs. There's no one single protection system that's best for everything, so you'll need to pick and choose what's best for each application.

Remember, circuit protection is there to protect the wiring, not the load. It presupposes you are using appropriate sizes of wire or cable in the first place. Aim for 1% voltage loss under the heaviest anticipated load, and if in doubt go bigger on the wire sizes. Short and fat is best!


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