Voltage Drop Calculations

Hey Forum,
Not sure if I'm posting in the correct category but thought I'd start here...
I have a question on using the voltage drop calculator (I'm currently using the one on AZW&S's site).

I'm trying to figure out voltage drop on an AC circuit that will be 200' long (one direction). I'm unclear if I should be plugging 120V or 240V into the calculator because I'm not sure if it's asking me the circuit voltage or the conductor voltage. Also, I'm not sure if I should say my distance is 200' (one way) or 400' (the length of the whole circuit).

Is it standard to want to stay under 3% or do folks try to reduce voltage drop even more than that?
Thanks for any thoughts!

Comments

  • westbranchwestbranch Solar Expert Posts: 5,142 ✭✭✭✭
    Re: Voltage Drop Calculations

    there are a couple of them, which calculator are you using? LINK>>>>?
     
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  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Re: Voltage Drop Calculations

    Here is one:

    http://www.calculator.net/voltage-drop-calculator.html
    * Please use one-way distance to the load. Not round trip distance.

    200 foot one way run (some you have to put round trip length).

    Voltage is across the wire as measured by a volt meter (in the US, usually 120 VAC or 240 VAC). Note that 12 VDC is equal to 12 VAC for resistance and current calculations (simple resistance, not impedance).

    So, for example 5 amps @ 120 VAC and 200 foot one way run, ~3% drop:

    14 AWG:
    Voltage drop: 5.05
    Voltage drop percentage: 4.21%
    Voltage at the end: 114.95

    12 AWG:
    Voltage drop: 3.18
    Voltage drop percentage: 2.65%
    Voltage at the end: 116.82

    10 AWG:
    Voltage drop: 2.00
    Voltage drop percentage: 1.67%
    Voltage at the end: 118

    8 AWG:
    Voltage drop: 1.26
    Voltage drop percentage: 1.05%
    Voltage at the end: 118.74

    In general, voltage drop is energy lost... P=V*I=V2/R=I2R

    You look at the working voltage needed at the end of the wire run. You look at surge current (too much resistance, not enough current to start a motor), too small of wire it will over heat (NEC tables for acceptable current), etc.

    5% is probably the maximum you want. 3% is usually what we look for maximum drop with AC wiring. And 1% is about the bottom end (spending too much money on copper for little improvement).

    The one problem sometimes seen with "too heavy of copper wire"--An induction motor can take a lot of extra starting current and fault the inverter (inverter shuts down from over current). Sometimes using a thinner copper cable has enough resistance that the surge current is reduced.

    Another handy rule of thumb for North Americans... Every ~3 AWG change in a factor of 2 (if going lower in AWG) or 1/2 (if going up in AWG) in wire cross section.

    For example, from 8 AWG to 14 AWG is 1/2 * 1/2 = 1/4 the cross section. 4x the resistance and ~1/4 the cost of wiring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gww1gww1 Solar Expert Posts: 963 ✭✭
    Re: Voltage Drop Calculations

    bill
    Am I correct that it doesn't matter if you use 120 volts or 240 volts in the caculator that you posted cause it will know that if you put 120 volts you will only be sending 3 wire (hot, neutral and ground) and if you put 240 in it will know it is split phase and you are running 4 wire?
    gww
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Re: Voltage Drop Calculations

    Voltage drop is Vdrop=Iwire*Rwire ... So, you are correct that line voltage does not matter here.

    However, you do need to understand the voltage drop too... a 3% drop on 120 VAC wire is 3.6 volt drop--Running your loads at 116.5 volts is fine.

    However, a 3.6 volt drop on a 12 volt battery bank gives you 8.4 volts at the load--Well below the ~10.5 to 11.5 volts minimum that most DC loads need to operate.

    120/240 VAC Split Phase power (North America) is the output of a three terminal transformer:

    == ===== Line A / Black
    )||(
    )||(
    )||(===== Neutral (usually grounded)
    )||(
    )||(
    == ===== Line B / Red

    For:

    A:Neutral = 120 VAC
    B:Neutral = 120 VAC
    A:B = 240 VAC

    Neutral to White: If bonded Neutral+Green Wire ground ~0 volts. If floating Neutral, may be anything from 0 to 120 VAC or so.

    What you need to know is the voltage across your loads... If it is 120 VAC loads, then 120 VAC (A:neutral, B:Neutral).

    If it is a 240 VAC load, then A:B for 240 VAC line.

    Note, you can have a mix of 120 VAC and 240 VAC loads on a circuit... You have to understand where the current flows in A:B:Neutral to understand the voltage drop.

    For 240 VAC loads, the current flows in A & B.

    For 120 VAC loads, the current flows in A&Neutral or B&Neutral.

    For mixed loads... The neutral current is the difference between A current B current:

    5 amps A
    10 amps B
    5 amps Neutral

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gww1gww1 Solar Expert Posts: 963 ✭✭
    Re: Voltage Drop Calculations

    Thank you bill, I thought that was part of the posters question and I know that I ask and got answered that same question some time ago and being me, I couldn't remember what I was told.
    gww
  • AenergiesAenergies Registered Users Posts: 3
    Re: Voltage Drop Calculations

    Thanks for the answer but I guess I am still confused about if I should be saying 120 or 240. The circuit is a 240V circuit, but each wire is only carrying 120 V in it. I get different answers for the wire gauge if I put 120 or 240 and so I'm not sure if it is asking me the voltage of each wire or the voltage of the whole circuit...
  • gww1gww1 Solar Expert Posts: 963 ✭✭
    Re: Voltage Drop Calculations

    If you use the link bill posted and your amps don't change, you leave it at single conductor, It doesn't change when you put 240 volts or 120 volts in the space for volts. I put in 30 amps at 100 feet and it came out as the same voltage drop. All caculators may not be like his. I have always been confused my self so I just tried it and that is how it worked for me.
    Good luck
    gww
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Re: Voltage Drop Calculations

    For the 240 VAC--It is the voltage between the two "Hot" wires that we care about... Voltage to neutral (120 VAC) is a "don't care" with 240 VAC motor.

    Only care about the 120 VAC drop if your 120 VAC load is between the Hot and Neutral.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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