FUSES FOR 24VDC - ATC, blade-type or Square D QO?

I'm going to be building a small off-grid house. I plan to run 24vdc for all the lights. I'd like to use the BlueSea ST Blade fuse block 5029 (takes up very little space, fuses are cheap), but I'm concerned that the electrical inspector might balk at that, so I'm wondering if I should use a Square D QO (like I'm using on the AC side) just to be "safe." Does anyone have experience with ST blade fuses and inspectors???

I actually wrote to the inspector in my town, and her reply was:

"...as long as you can convince me that the installation will be as safe as a conventional system, I would likely approve your substitution request."

Thoughts? Thanks all!!

Comments

  • H2SO4_guyH2SO4_guy Posts: 212Solar Expert ✭✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    I think you would be happier in the long run with breakers as they will serve as a much easier disconnect. You might also want to consider Midnite (from NAWS) DC rated breakers for about $10 or so. You can mount them on an inexpensive rail for a couple of bucks (Also from NAWS), but they might have to be in an enclosure.

    Skip
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    I think you'd be happier in the long run if you skipped the 24 VDC lighting and went with all 120 VAC as all the wiring and components would be conventional and easily available. Since you mention having AC anyway, might as well make it easy on yourself.
  • openplanetopenplanet Posts: 34Registered Users ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    My two reasons for going with 24vdc lighting: (1) more efficient (more lumens per watt), and (2) I like the idea of being dependent on one less piece of gear for lighting. If my inverter fails, I'll still have lights! Likewise for the 24vdc SunDanzer I plan to use.
    But I'm always open to counterarguments!
  • BB.BB. Posts: 27,588Super Moderators admin
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    How big of battery+AC system overall?

    If you stay small and can use a 300 watt TSW inverter and the DC loads are less than ~1,200 watts (and you don't have any continuous 24x7 loads that drive up the size of the battery bank AH capacity), the MorningStar 12 VDC 300 TSW inverter (or even a couple of them) would be a nice fit.

    In the end, we aim for "balanced" system design--And that means having a good handle on your loads/power needs, matching the battery bank to those needs, then defining the charging sources to support the loads+battery bank.

    Low voltage DC power systems can be a pain in the wiring... A 24 volt system takes about 5x the current as similarly powered lightning does at 120 VAC. And many times, the AC versions of the lighting is cheaper (and more easily sourced from local hardware stores these days). As well as other AC appliances being cheaper/more available/more repair options (like AC vs DC refrigerators, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • openplanetopenplanet Posts: 34Registered Users ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    450ah in the form of 4 6v Surrette's in series.
    I have a Samlex 2kw PSW inverter. i anticipate minimal ac loads though. Vacuum cleaner, and some wall warts for network stuff (ironic, right? just back to dc...).
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    I've known some cabins wired for DC lighting.
    I've also known them to get re-wired for AC when the DC turned out to be inefficient and problematic due to V-drop over wire runs and difficulty finding components.

    A vacuum cleaner is a monster power user better suited to firing up the generator for the time it is used, rather than building a system that can handle it if there are no other significant loads. But since you already have a 2kW inverter and 450 Amp hours of battery (5 kW hours) I wouldn't worry about that. I certainly wouldn't worry about a few lights at night.

    I have about half as much battery capacity as you, and it runs everything including an electric 'frige and water pumps. A few CFL's for light is so far down the list of power consumers they don't even make a blip in the usage.
  • BB.BB. Posts: 27,588Super Moderators admin
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    That size of battery bank is just about the absolute limit of a 12 volt system... You are probably better off going to 24 volts. Then comes the issue of finding a small/efficient 24 volt AC inverter (still waiting for the rumored next MorningStar inverter).

    If you are already 24x7 for powering AC networking equipment--Then the AC power for the lights is pretty much "free" (at least in terms of running the inverter and its "wasted" idling current).

    Any plans for an 120 VAC fridge?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    He's already said 24 Volts, Bill.

    Just as an exercise:

    120 Volts @ 1 Amp over 30' of 14 AWG wire is a V-drop of 0.13%

    24 Volts @ 5 Amps (same Watts) on the same wiring is a V-drop of 3.24%

    To reduce the Voltage drop on 24 Volts to the same % as the 120 Volt system you end up switching to 1 AWG wire.

    I'm not saying these are the numbers that will be in place for your particular system, just pointing out that getting the same amount of power across the same distance at lower Voltage requires larger wiring. As such low Voltage is not always more efficient.
  • openplanetopenplanet Posts: 34Registered Users ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    I'm planning to home run all lights with awg 10 stranded. Max 1a draw per light...most will be closer to .5a.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?
    openplanet wrote: »
    I'm planning to home run all lights with awg 10 stranded. Max 1a draw per light...most will be closer to .5a.

    As opposed to doing a single circuit route with 14 AWG wire using 120 Volts?
    I do not see any advantage to your plan at all.
  • openplanetopenplanet Posts: 34Registered Users ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    Advantage of a home run for each and every load (lights, outlets, fridge...) is that every wall switch in the house will be a N\O pushbutton that also home runs to the electrical panel using 2 conductors of a CAT-5 cable. At the panel it will be connected to an Arduino microcontroller that will toggle a MOSFET-based solid-state relay that will toggle the load on or off. This makes controlling a given light (or outlet, or any load for that matter) from multiple locations extremely easy, because the power-carrying wire doesn't have to daisy chain from switch to switch. By having every load in the house controllable by the Arduino I can do things like dim lights (through PWM) that are otherwise not dimmable, set separate time schedules for any load in the house, and potentially control any load remotely via the 'Net.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    And you're concerned about the efficiency of the lights themselves? :confused:
  • openplanetopenplanet Posts: 34Registered Users ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    Nope...guess I like the aesthetics of minimal inverter dependency. (BTW, the Arduino and MOSFETs use minute amounts of power.)
  • BB.BB. Posts: 27,588Super Moderators admin
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    DC electronic relays will have to be larger (higher current at 24 VDC--And make sure they are rated >32 volts maximum). AC relays may be more common and cheaper.

    By the way, how have you addressed the fusing/breaker per home run wiring?

    Say each is 1 amp circuit. And you get 10 amp fuse/breaker * 1/1.25 NEC derating = 8 amp load --- Or a maximum of 8 home runs per 10 amp fuse/breaker. And the DC relay should be rated for >10 amps (if there is a short, you want the fuse to pop vs the relay melting down).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • openplanetopenplanet Posts: 34Registered Users ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    I plan to fuse each home run individually. Each solid state relay really just consists of a MOSFET (20A current rating) whose gate is toggled by an logic level Arduino output.
  • ChrisOlsonChrisOlson Posts: 1,807Banned ✭✭
    Re: FUSES FOR 24VDC - ATC, blade-type or Square D QO?

    I guess it's already been mentioned, but DC lighting and circuits in a residence that has to pass inspection for Code are basically a non-starter. That's RV stuff. I don't know where you're going to get fixtures and switches that are UL listed for a residence. The other problem is that DC lighting, powered directly from a battery bank, is like in a RV - they get dim when the bank is down on voltage and bright when the battery is absorbing.

    120VAC lighting requires 1/5th the size for wiring, the voltage at the load never changes, and everything is legal for Code and inspection.

    Limit the DC stuff to your utility room to get power in from RE sources and power to the battery and inverter. Wire the rest of the house with standard house wiring at 120/240V (or usually just 120V for a cabin). When you're off-grid and worried about the inverter failing - well - that's what the generator (or two or three) is for.
    --
    Chris
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