Electrical math help...lol

PhotowhitPhotowhit Solar Expert Posts: 4,765 ✭✭✭✭
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, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol

Comments

  • EstragonEstragon Registered Users Posts: 3,016 ✭✭✭✭✭
    Seems to me it should work fine, FWIW.
    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
  • BB.BB. Super Moderators, Administrators Posts: 28,144 admin
    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
  • PhotowhitPhotowhit Solar Expert Posts: 4,765 ✭✭✭✭
    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, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • BB.BB. Super Moderators, Administrators Posts: 28,144 admin
    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
  • PhotowhitPhotowhit Solar Expert Posts: 4,765 ✭✭✭✭
    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, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • EstragonEstragon Registered Users Posts: 3,016 ✭✭✭✭✭
    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
  • PhotowhitPhotowhit Solar Expert Posts: 4,765 ✭✭✭✭
    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, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • BB.BB. Super Moderators, Administrators Posts: 28,144 admin
    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
  • mcgivormcgivor Solar Expert Posts: 2,401 ✭✭✭✭✭
    edited December 2 #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 bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • PhotowhitPhotowhit Solar Expert Posts: 4,765 ✭✭✭✭
    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, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • mike95490mike95490 Solar Expert Posts: 8,011 ✭✭✭✭
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
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  • NANOcontrolNANOcontrol Registered Users Posts: 87 ✭✭
    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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