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Old 04-06-2011, 01:49 AM   #1
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Electric Run, Voltage Drop Issues. Eventual upgrade of system this year.


Just so you folks know I am not the person who originally installed this electrical setup. I am just trying to use the current system until I have to upgrade it this fall due to a workshop I am building. The original system was installed in an age that as long as it worked it didn't matter and code was not an issue.

In the past, electrical use has been limited to four or five incandescent light bulbs that were turned on randomly (one to two times a month for ten minutes at most) and a 43 watt dusk to dawn light that is on every night.

Keep in mind this only has to work for me till the temperature reaches the 60F range which should be in the next 2-4 weeks. I am not looking for large investments till next fall.

Recently, due to a small tobacco greenhouse we have had to add a small 1500watt quartz heater due to the oddly cold April we are currently having here. The heater is heating around 400 cubic feet area as far as volume goes. The heater does not run constantly and only kicks on if temperature inside the float bed greenhouse falls below 65 degrees.

Now to the setup:

The electrical supply has its own meter and Service Entrance at a metal building that I am going to call Point A.
The breaker is 30 amps on the line running from point A to point B. There is a 10-3 wire running the distance of 275' -325' (never measured it) from point A to the next junction box at Point B. When the wire gets to point B it enters an old style circular bus fuse box with a time-delay 20 amp screw in type fuse. The power then proceeds into the barn itself.

There is an 8 inch long 10-3 wire running from the bus fuse box at point B to a GFCI outlet (replaced old receptacle with GFCI recently because the greenhouse is a water float bed system).

The following setup is mine. From the GFCI outlet at point B I ran a 50' 10-3 extension cord which I had to connect to a second 40 foot 14-3 cord. At the end of this 90 foot run is the 1500 watt electric quartz heater and a small 7" personal fan to keep the air circulating. (I have no idea what the watt requirements are for the fan even after researching the specs on the internet). I actually need to be using a larger (80 watt) oscillating fan but I do not want to do so until I know if it can handle that.

The heater has a thermometer and does not run all the time. The brand name is Holmes and the model number is HQH319-U. Also, from the distance of point A to point B there is a dusk to dawn light which requires 42 watts for operation.

I need to know if the system can handle the heater and the dusk to dawn light. There is nothing else on the system at night 99% of the time.

Do the breakers have the correct amperage?

Occasionally a small incandescent barn light may be on for a maximum of ten minutes to feed the cows.

An additional detail that may be helpful is the fact that I do not have to use 90 feet of extension cord. The 10-3 line that goes to Point B proceeds another 200 feet to a second barn and a Junction Box with circular time delay fuse of 20 amp fuse also. There is a old farm installed power pole near the 100 foot mark within 20 feet of the greenhouse. I could very easily install a GFCI on this 10-3 wire and use a 20 foot 10-3 extension cord to power the greenhouse but I don't see where I would gain anything other than losing the 40 foot 14-3 extension cord and having 10-3 all the way through to the heater.

The only thing on the system at Point C in the barn, if it is ever on, are three 40 watt incandescent bulbs and occasionally a battery charger for one of the tractors. Currently Point C is turned off at the Breaker Box at Point B. So the 200' of line from point B to C is not powered at this time.

A neighbor down the road suggested that I run a second 10-3 wire along with the current one and make one hot and one neutral then utilize the ground wire in both wires as ground. He actually has a setup like this from what he was telling me but I don't like the idea. I have heard bad things of parallel lines.


The power company will not install service to the barn. I have already asked. They gave me a quote of $13,000 because I already have a utility line through the other shop.

I am considering a small inexpensive buck-boost transformer if I absolutely have no choice.

Also for the record, there is not code enforcement in this rural area. I appreciate any advice regarding code, but I would also appreciate honesty in what my current system can handle. I am not even required to get a building permit for the workshop. When I asked about electrical code and the permit at the county seat they looked at me like I was crazy. I was then told just do whatever and have a good day.

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Last edited by lawnphysics; 04-06-2011 at 01:52 AM. Reason: spelling
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Old 04-06-2011, 07:49 AM   #2
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Electric Run, Voltage Drop Issues. Eventual upgrade of system this year.


The 10 gauge wire will support 30 amperes intermittent or 80% (here, 22-1/2 amps) continuous no matter how long it is. Except when you draw all 30 amps, there can be dimming of lights and less heat from the heater due to voltage drop. I haven't done the math but I kinda' guess you would get 15 amps before the voltage dropped too much given the distance.

Going the extra 100 or so feet to point C ini order to lose the 14 gauge extension cord won't help much and may actually hurt (again I haven't done the math). Replacing the 14 gauge extension cord with 10 gauge will help a little. Replacing both extension cords with 8 gauge ones will also help. Despite the fact that the main feed is smaller 10 gauge.

Equipment with electronics and motors should not be used during times of significant voltage drop. A battery charger may stop charging although it is still "on" during times of significant voltage drop. You can take your chances that the fan won't wear out from undervoltage since many motors will draw more amperes when fed insufficient volts.

Use of heaters should take into account the continuous draw amperage limit. You may use the larger (intermittent) figure when considering lights, cow milkers, televisions, small appliances etc.

The continuous draw (it's based on heat buildup in the wire) does not go down proportionately with voltage drop but rather is "picked up and taken down" for example if voltage drop limits the draw to 20 ampsand the continuous rating was 22-1/2 then the continuous becomes 20. IF the draw is limited to 23 due to distance and the continuous used to be 22-1/2 the continuous stays 22-1/2.

You may only have one feed between the two buildings. WIth the 10-3 cable you have two allotments of 30 amps (practically about 15 amps given the distance) with the neutral shared and the two hots on opposite sides of the 120/240 volt line when breakered at the house main panel. The fuse box with the single 20 amp fuse can be replaced with a subpanel with two 20 amp breakers, one using the red feed and one using the black feed. (Individual lights and receptacles need breakering at no more than 20 amps. no matter what the distance) You can install a larger subpanel for now awaiting a bigger feed cable.

A 240 volt heater drawing 7 amps will suffer much less voltage drop compared with a 120 volt heater drawing 13 amps (both 1500 watts). Just the amperes drawn will govern what the voltage drop will be.

For more power you need to decommission the old cable as you put in a new, fatter, one.

A big disadvantage of a buck boost transformer is that the output voltage will vary with the current draw (maybe in the 115 to 135 volt range as the current draw varies and makes the input voltage vary in the 95 to 115 volt range).

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Last edited by AllanJ; 04-06-2011 at 08:41 AM.
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Old 04-06-2011, 12:01 PM   #3
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Electric Run, Voltage Drop Issues. Eventual upgrade of system this year.


Revisiting the question, I did a quick calculation and estimate the voltage drop to be around 12-15 volts when the heater kicks on. This will leave the voltage for the heater and fan in the 100 to 105 range. I believe that the fan will still work acceptably and the heater will still give reasonable heat.

The maximum draw for 120 volts and normally tolerated voltage drop is around five amperes continuous or intermittent.

(Approximately 800 foot round trip from house to first outbuilding plus extension cords)
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Old 04-06-2011, 06:14 PM   #4
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Electric Run, Voltage Drop Issues. Eventual upgrade of system this year.


The main problem seems to be the long cable runs.
Common practice is to derate the cable by 50% for long runs.
Using a boost transformer is not the best option,
Better to use larger cables.
If finance is tight, then replace one run at a time.
The first section should be the largest cable.
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Old 04-06-2011, 07:24 PM   #5
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Electric Run, Voltage Drop Issues. Eventual upgrade of system this year.


To keep the routing the same (your A to B and B to C, about 500' one way) the voltage calculator I used came up with O gauge aluminum cable for 40 amperes @ 240 volts using two hots and neutral and with 120 volt loads more or less balanced between the two hot lines and 3% voltage drop. The required 4'th conductor for the ground can be somewhat smaller, I don't have the size yet.

Added later -- Subpanel feeds up to 60 amps can use a 10 gauge copper grounding conductor. At 40 amps the O gauge aluminum current conductors suffer a round trip voltage drop around 3%, you can push it to 60 amps with a voltage drop of 6%.

The 10 gauge ground wire (500' one way) will suffer a voltage rise of 5 volts for a fault current of 10 amps or a voltage rise of 15 volts for a fault current of 30 amps (latter current implied as reasonable worst case for a 60 amp subpanel given the #10 wire). Not sure what kind of derating (larger size) is needed for long distance grounding conductors given that you need a grounding electrode (ground rod etc.) at each subpanel out there.

If we wanted to arbitrarily derate the ground wire 50% the desired size would be 6 gauge copper.
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Last edited by AllanJ; 04-07-2011 at 08:47 AM.
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