Hi. I'm trying to upgrade my electrical service in my barn from 120 to 240. It is 290 feet from my main service panel, and I'd like to take 50 amps out to a subpanel that's already there. I've done the calculations for voltage drop (I hope correctly) and I think what I need is #4 (copper) wire (I plan on putting it in buried PVC). Also, can't I just run three wires--two hot and one neutral--and just ground the sub panel since the main panel is already grounded? Thanks for letting me know if I've figured this out correctly.

Do you only want 240 VAC only or will you actually need 120/240?

My rough calculation calls for #2 AWD conductors.

You would need 3 (120/240) #2 conductors THHWN (90 celsius) (two ungrounded and one Neutral return). I believe that you will also need a #10 ground at the Barn Panel, You cannot use the ground provided by the main panel as part of the EGS at the barn.

Just a few clarifications,

The insulation is THHN/THWN.

The green wire will need to be #8, if # 2 wire is used. The equipment ground is to be proportional to th line conductors when increased in size.

My calcs agree with #2 copper, assuming a conservative amount of inductive load, such as motors, to arrive at less than the max recommended 3% of Vd.

How did you arrive at #4?

Unless there would not be a large motor load, or a welder, and a steady 50 amp load would be unlikely, and considering the cost of copper, I would not hesitate to install #2 aluminum with XHHW insulation, with a #4 equipment ground. If there is any chance the load will increase due to expansion, stick to the copper.

By the way , the conduit will need to be min 18" deep. Not required, unless by local code, but having a warning tape 6-12 in below grade above the conduit is wise in the event digging happens there.. Using metallic tape will allow a metal detector to locate the path easy later on.

Quote:

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Originally Posted by tkrussell

Just a few clarifications,

The insulation is THHN/THWN.

The green wire will need to be #8, if # 2 wire is used. The equipment ground is to be proportional to th line conductors when increased in size.

My calcs agree with #2 copper, assuming a conservative amount of inductive load, such as motors, to arrive at less than the max recommended 3% of Vd.

How did you arrive at #4?

Unless there would not be a large motor load, or a welder, and a steady 50 amp load would be unlikely, and considering the cost of copper, I would not hesitate to install #2 aluminum with XHHW insulation, with a #4 equipment ground. If there is any chance the load will increase due to expansion, stick to the copper.

By the way , the conduit will need to be min 18" deep. Not required, unless by local code, but having a warning tape 6-12 in below grade above the conduit is wise in the event digging happens there.. Using metallic tape will allow a metal detector to locate the path easy later on.

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I agree with TK and Don---seeing that we have no idea on the load planned in your remote structure.



To add: It would be nice to see people show how they came up with their numbers. Anyone can go to some renky dink site and use their VD calculator--- later coming on here as if they did the calculation work! :rolleyes:

here's how I came up with my calculations for 4 AWG wire:

A: The formula for voltage drop is: Vd = 2K x L x I / Cm

Vd = Voltage Drop
I = Current in Conductor (Amperes)
L = One way Length of Circuit
Cm = Cross Section Area of Conductor (Circular Mils)
K = Resistance in ohms of one circular mil foot of conductor
K = 12.9 for Copper Conductors @ 75 degrees C
K = 21.2 for Aluminum Conductors @ 75 degree C
/ = Divided by

using the above formula, I came up with


Vd = 2 x 12.9 x 290 x 50 / 41740 = 8.9 volts
8.9 volts / 240 volts = 0.037 = 3.7%

am I to take it from your response, that this is a rinky-dink calculation? And therefore wrong?

Strickly speaking yes and no, because you said 240V, but you really mean 120/240V.

While true 8.9 / 240 = 3.7% (taking your word here) is > 3%, so you have to try again. Last time I checked 3.7 is greater than 3.

Second, your loads also contain some 120 V, those loads are going to get jipped a lot more than 3.7%. More like double 3.7% or 7.4%.

If it were strickly a 240 volt load like a water heater, you can use 240V to compute the voltage drop in %.

While it is true that N carries the difference current of the two hots, that difference will not always be zero.

Thank you for your response. I don't claim to be the expert. That's why I'm here. You're right. I need 120 and 240 service. I've got overhead lines now that I'm trying to get rid of. I've got to take the service 290 feet from the main service panel to the sub panel at the barn. I'd like to put it underground. What's the best way?

If the only reason you want to change to underground is for aesthetic purposes, personally, I'd leave it alone.

If the service you already have is meeting your needs, save the money you would spend. 290 ft. of copper will cost a considerable amount of dollars!

I want to address this question too:

Quote:

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Originally Posted by WmInChilton

Also, can't I just run three wires--two hot and one neutral--and just ground the sub panel since the main panel is already grounded?

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The answer is no. The connection to a ground rod at the detecahed structure is for lightning protection. The ground connection to the utility is for a reference. The ground an neutral busses will remain separate and isolated from each other in the barn.

See if everyone else arees:

URD drirect burial aluminum wire. You'll have to recalculate the required conductor size with K for Aluminum. 18" depth.
Aluminum URD Cable- 600 Volt- Quadruplex-90°C For Direct Burial on Custom Cable Corp.


Donf:
I believe part of the reason for the upgrade is to bring 240 into the barn.

EDIT: Added link. Removed reference to tracer wire, but will add link here: Kris-Tech Wire Company: Why You Should Always Use The Appropriate Tracer or Locating Wire Instead of THHN for Direct Burial

Donf:

I believe part of the reason for the upgrade is to bring 240 into the barn.

Ron -

The poster already has 240 to the barn, supplied via overhead. See post #8.

Donf:

In post #1:

Quote:

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Originally Posted by WmInChilton

Hi. I'm trying to upgrade my electrical service in my barn from 120 to 240.

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