Replies

1. 
   Guest_ | Dec 19, 1998 09:09am | #1

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   There is a voltage drop calculator on the net. Try an Alta Vista search. That's how I found it. I have it bookmarked at another location and will post it when I get there.

1. 
   Guest_ | Dec 20, 1998 08:26am | #2

   *Marco, I think Mike's chart disappeared into cyberspace but I've got lots of info we use in wiring heavy motors around the mill and maybe some of that will help you. First, I'm assuming that your motor is the only thing feeding off of your subpanel?? To answer your question specifically, I would have to have the info off of the motor tag. I'll give you some info right off of the charts and you can use it for whatever your situation is. This is for single phase 230v motors and the distance for the feeder is 200'. A voltage drop of 2% is considered maximum acceptable and these charts assume no more than 1 1/2%. (Running amps of motor / size wire required) 12amps / #8 17amps / #6 28amps / #4 40amps / # 2 50amps / #2 This takes all NEC codes into account and allows for a line rating of 125% of motor amps. Voltage Drop = amps x ohms ( of resistance in the feeder wires) If you want to play around with figures, I'll give you the ohms /1000' for different wire sizes. ( You would figure 380' for your application) #8 / .641 #6 / .410 #4 / .259 #2 / .162 I would be very conservative when running the new wiring as you never know what you might hook up out there at a later date. Low voltage and dirt are the main killers of large electric motors. Hope this helps you out.....

2. 
   John_Wells | Dec 20, 1998 03:24pm | #3

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   Marco,

   You are correct to worry about voltage drop. The code doesn't care. They are concerned that you don't melt the insulation off the conductors.

   Need more data - such as the pump amperage, probably actual load, the rating of the wire, etc.
   Is this going to be an underground feeder? As to a shop building? Just the pump?

   Using 50a for the whole panel, 75dg wire, single phase, etc., my ElectiCalc 96 comes up with #4CU or #2AL using their stock 3% V Drop. They calculate actual VD as 2.4%. #4CU is both spendy and a bit heavy as you are not (we hope) going to be pulling 50A on that panel at one time. What is a realistic load?

   Code says: #4AL in type UF (60dgC) is rated at 55A (Table 310-16: Allowable Ampacities.) So, for underground feeder, don't get under that.

   If underground, check out the prices on AL triplex in the commonly stocked sizes. I like to over size feeders so there is room for the future. I have had to deal with enough undersized feeders sized with no consideration for the voltage at the far end. Some of our projects have 4/0 UF feeders to deliver 100A at the far end owing to the distances.

   Send me the data and I will send you the VD calcs for any desired drop (2-3% is common.)
   Look on the bright side. RFGC has been running trap motors off #12 romex with runs of 350'. (You don't want to even look at this one - but they have not burned out any motors. Perhaps because the use is momentary.)

   j wells

3. 
   Guest_ | Dec 20, 1998 09:03pm | #4

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   I put "Voltage drop calculator" into Alta Vista and got it. I don't know why you couldn't find it. Click to go to it.

   Voltage
   Drop Calculator
   
   

1. 
   Guest_ | Dec 20, 1998 09:09pm | #5

   *Marco;Some good advise so far. Though I'd add my thoughts.Code allows 5% voltage drop. This is 6V for a 120V circuit or 12V for a 240 Volt circuit. You specified 50A at 240V, so I assume this is not going to a subpanel where you may need 120V at high currents.The equation is:Voltage Drop = Amps x (DistancexConductivity/1000)The "1000" is because conductivitiy is in ohms per 1000 feet.In your case:12V = 50A x (DistxCond/1000)You need to use the round trip distance (380ft). This is because you drop voltage both going to the load and coming back.12V = 50A x 380 x cond/1000or conductivity = 0.631 MINIMUMlooking at the tables for Copper (Personally, I try to stay away from Al.) You need to use #6 MINIMUM. Based on the table, #6 has a conductivity of 0.491 ohms/1000. (I don't have my tables handy, so I'm not sure what type of Cu this is for. I believe it is 7strand.)We can determine what the actual drop will be if you use this:VD = 50A x 380 x 0.491/1000or9.33 volts or 3.88%If you want to be real conservative, #4 will get you 0.308 ohms/1000:VD = 50 x 380 x 0.308/10005.85 volts or 2.4%personally, I always use conduit, but I'm finicky. 190 foot is a long pull.Hope this helps.

1. 
   Guest_ | Dec 20, 1998 09:30pm | #6

   *Adam, As you and John correctly point out, the figures are slightly different depending on who puts out the information. Our charts are all prepared by the manufacturers of the motors, and tends to be much more conservative. Other common tables are prepared to cover requirements of the NEC. If Marco absorbs all of this, I'm sure his pump will hum along..

1. 
   Guest_ | Dec 21, 1998 02:45am | #7

   *Sorry Mike, Didn't read very well. I was looking for something on the Breaktime board. I did look that site up and it's very handy. Got any other gems like that ??

4. 
   John_Wells | Dec 22, 1998 12:55am | #8

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   Too true. I suspect what we'd like to know is how many amps are required to start the motor vs. run it.

   The motor makers are well advised to be conservative - and hope that they get it. Probably a lot different in a mill where they are used to this and know the score vs. a homeowner well pump installation. I wonder what other electical requirements are needed in this outbuilding. Area lighting, the usual outlets, etc.

   I used to do the VD calcs by hand, but it's more fun with the ElectriCalc - and saves paper. :-)

   j wells

1. 
   John_Wells | Dec 22, 1998 01:00am | #9

   *Mike,Neat VD calculator! Thanks. I've marked the URL so I can play with it. Got any more sites like that?j wells

5. 
   marco_ | Dec 22, 1998 01:00am | #10

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   I have to run a 50a-240v feeder to a sub panel supporting a water pump. The problem is it will be about 190'.
   My code book allows #8 wire (hot, neutral) - My understanding is that that's valid for up to 100'. Then voltage drop comes into play. Any electrician out there know what size wire I'd need - and how to calculate both amount of drop, and how to calculate wire size? Thanks in advance.