Electrical math help...lol

Photowhit
Photowhit Solar Expert Posts: 6,011 ✭✭✭✭✭
in In the Weeds--Member's Choice #1
Okay I can't make my mind work this morning.

I want a momentary foot switch for my scroll saw, It draws 1.4 amps at 120v.

Lots of imports that are rated at 10 amps at 230 volts. Am I correct in assuming 10 amps at 230 volts will handle about 1/4 that power on 120 volts?

That's my thoughts from resistance loads, but since this involves contacts and I can't make my mind work, I'm wondering if I'm safe?
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    Seems to me it should work fine, FWIW.
    Off-grid.  
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #3
    Yes, no problem... We are talking about VAC in the rating and your usage voltage....

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,011 ✭✭✭✭✭
    #4
    Thanks, Yes all voltages are VAC. So is it the same rules about 1/4 the voltage? I guess I might like one for my spindle sander, Guess it's about 400 watts. So above the 1/4...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #5
    Less than rated voltage (vac in this case). Good.

    Less than rated current (aac). Good.

    You have met both requirements. You are golden.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,011 ✭✭✭✭✭
    #6
    BB. said:
    Less than rated voltage (vac in this case). Good.

    Less than rated current (aac). Good.

    You have met both requirements. You are golden.
    Thanks, it has tiny wires, I've see the 22 gauge connecting the outlets in a 500 watt inverter, these just look so tiny. I guess if it will handle 10 amps at 240 it will handle 10 amps at 120...


    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #7
    I was likewise surprised by the small AC wiring in the guts of my diesel genny designed to carry ~17amps/leg out.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • Photowhit
    Photowhit Solar Expert Posts: 6,011 ✭✭✭✭✭
    #8
    Estragon said:
    I was likewise surprised by the small AC wiring in the guts of my diesel genny designed to carry ~17amps/leg out.
    Thanks, it really does help! I'm always worried about under sized wiring.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #9
    OK, now you are getting into custom "engineered" installations/applications.

    22 AWG cable cannot safely carry 10 amps directly... The two questions are "what is the max continuous current" and what is the "branch circuit current rating"...
    • 500 Watts / 240 VAC = 2.1 amps
    If you run a 500 Watt inverter at 120 VAC:
    • 500 Watts / 120 VAC = 4.2 amps
    So 120 vs 240 VAC are different questions if both are 500 Watt rated inverters.

    Then the question of what is the minimum wiring used on a 15/20 amp AC branch circuit... I have never seen any calculations, so this is a guess, but (for example) you have a 80 Watt power brick for your laptop, and find that it uses 18 (or even possibly 22) AWG cable on the 120/240 VAC input line.

    From the NEC table, we see that they require 14 AWG for a 15 Amp circuit--But the fine print says that 14 AWG can carry 20 Amps (why the derating, I have never found out).

    I believe the code issue is the ability of the small AWG cable to conduct enough fault current to trip the upstream protective device...

    For example, if you can use 8 (insulated?) or 6 (bare?) AWG cable to ground bond a 200 amp service (I do not have an NEC book--numbers from vague memory decades ago)... From this chart, we find that:

    https://www.powerstream.com/wire-fusing-currents.htm

    8 AWG has ~473 amps of "fusing" current... (6 AWG fusing current for copper is ~668 amps).

    I guess is that the cable needs to survive just "long enough" to trip the upstream breaker... If 8 AWG is OK for 200 Amp service (sake of argument, I do not have code to reference), then ~2x fusing current gives enough headroom to trip the upstream breaker/fuse.

    22 AWG copper has a ~41.5 amp fusing current, or a little bit over 2x the 15/20 amp branch circuit you find in most home branch circuits (wiring, outlets, etc.). The 22 awg serving a 2.1 amp load is "safe" and OK to use on a 15 amp branch circuit (assuming GT inverter here) because if there is a short circuit somewhere, the 22 AWG wiring will conduct enough current to trip the upstream breaker/fuse.....

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited December 2018 #10
    Photowhit said:
    Okay I can't make my mind work this morning.

    I want a momentary foot switch for my scroll saw, It draws 1.4 amps at 120v.

    Lots of imports that are rated at 10 amps at 230 volts. Am I correct in assuming 10 amps at 230 volts will handle about 1/4 that power on 120 volts?

    That's my thoughts from resistance loads, but since this involves contacts and I can't make my mind work, I'm wondering if I'm safe?

    The best method  would be to use a motor rated relay with the foot pedal  controlling the relay coil current only, this would prevent the foot pedal wiring from becoming a fuse. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
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  • Photowhit
    Photowhit Solar Expert Posts: 6,011 ✭✭✭✭✭
    #11
    I'll likely make a box stirrup for the pedal to prevent accidentally starting and have an electric box and switched outlets on the back side. I'll look inside and see if I can replace the wiring with some heavier gauge wire.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #12
    If you can't find a "motor start relay" a conventional well pump Contactor is about the same thing. 
    https://www.platt.com/platt-electric-supply/industrial-automation-controls/motor-starters-and-motor-contactors/search.aspx?q=well+pump+control+panel+240volt&sectionid=5&groupid=69
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  • NANOcontrol
    NANOcontrol Registered Users Posts: 277 ✭✭✭
    #13
    Well, it is from China.  Contacts have a current rating for resistance and motor loads. It drops considerably for motor loads. You will likely have no problem given its minimal use.  Adding a RC network snubber would increase the life of the switch. These are available commercially or one can be built with a 47 ohm and 0.22uF capacitor of suitable voltage.
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  • notes
    notes Solar Expert Posts: 52 ✭✭
    #14
    Look for a, Rigid model 300 remote foot control. Used for a pipe threading machine. Our 2 machines have a switch rating of 20fla @ 120 volts. These machines take a beating, never replaced a switch in 30 years, just the cord. 
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  • Photowhit
    Photowhit Solar Expert Posts: 6,011 ✭✭✭✭✭
    #15
    notes said:
    Look for a, Rigid model 300 remote foot control. Used for a pipe threading machine. Our 2 machines have a switch rating of 20fla @ 120 volts. These machines take a beating, never replaced a switch in 30 years, just the cord. 
    Looks like a $200 solution looking for a problem. 

    I went with a different foot switch, still an import with same 10 amp, but the cord is roughly 18 gauge. Should be fine for the 120 watt scroll saw and my small shop vac (400 watt? 1 gallon)

    Thanks for everyone's help!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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