240v added to barn panel

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Old 06-22-16, 04:12 PM
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240v added to barn panel

Hi - I have a sub panel in my barn that is fed by a 30A circuit from my home main panel. The cable has hit, neutral, and ground to the sub panel and I have a few circuits I the sub panel.

I would like to add 2 240v outlets for a table saw and jointer in the barn. My father in law is a licensed electrician from Puerto Rico but he is getting on in age and I'd like to check his thinking and code compliance.

He is telling me that he can add a new sub panel in the barn that will pull 2 120v lines from the existing sub panel. He is showing me the 2 places in the existing sub panel he will connect from. While they are 2 separate buses, they are connected by a jumper wire. So i don't know how he thinks he will get 240 magically from a single 120 line.

Question - is it possible to create a 240 circuit off a sub panel box that is fed 120 from a main panel. Or do I have to run a new 4 wire cable from the panel to the sub?
 
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Old 06-22-16, 04:32 PM
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Question - is it possible to create a 240 circuit off a sub panel box that is fed 120 from a main panel.
No.
Question - is it possible to create a 240 circuit off a sub panel box that is fed 120 from a main panel.
Yes. Is the feed in conduit? If so and it is at least 1" conduit I suggest you pull three #6 and reuse the current ground if #10. Hopefully you can use the two wires you are removing to pull in the three #6. (Assumes conduit is PVC not EMT.)

Not to disparage your father-in-law but many posters here think 240 is obtained by using two 120s which of course is false. Perhaps your father-1n-law is also under that impression.
 
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Old 06-22-16, 05:23 PM
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Thanks ray! I have an electrical engineering degree but I'm no electrician. Based on my research you get 240v using 2 120 nodes that are 180 degrees out of phase. The only way to get that is for the main panel.

So I have neutral, ground, and a single 120 line running to the sub panel now. Why can't I run the second 120v from the main panel using a 6g single wire? Seems that you are suggesting I have to run 3 new wires and find a proper ground?

Btw - I will need to investigate but the line was most likely run in the early fifties. Any guess if that is pvc or if I should be prepared to dig up and run fresh UF lines?
 
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Old 06-22-16, 05:50 PM
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Early fifties would probably be EMT if it is individual wire. EMT would have long since rotted away. If your very lucky it is rigid steel conduit and that may still be good.

Sometimes when I write I'm not clear. My instructions were for upgrading to a 60 amp feed because I felt a 30 amp feed might be enough.
Based on my research you get 240v using 2 120 nodes that are 180
No. Your house is supplied with 240 volts plus a center tap from the secondary of the supply transformer. The 120 is derived from the center tap wire (grounded conductor - AKA neutral) and either of the 240v legs.
So I have neutral, ground, and a single 120 line running to the sub panel now.
No, you have one leg of the 240, neutral and ground. You need the second 240 leg.

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should be prepared to dig up and run fresh UF lines?
Best practice is not to run cable in conduit except to sleeve it for protection where it enters and leaves the ground. In conduit it is easier to use individual conductors (wires) such as THWN.
 
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Old 06-22-16, 06:33 PM
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You raise a good point - if I'm using all the lights and one of the 240v circuits I could get close to blowing the 30A breaker that feeds the barn from the main panel.

Assuming it will be a big project to run or pull new cable....and I wanted to see if I could get by with the 30A service to the barn....

Can I get away with using the neutral wire to carry the scold 120v line? Then at the sub panel I can connect the barn neutral to ground and then connect the sub panel ground and neutral bus to a pipe that is buried in the ground?

Just trying to avoid running a new cable out to the barn. But if I have to then it sounds like I need to get 6 or 8 gauge UF cable to replace the 10 gauge that is run through the conduit today. Correct?
 
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Old 06-22-16, 07:37 PM
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at the sub panel I can connect the barn neutral to ground and then connect the sub panel ground and neutral bus to a pipe that is buried in the ground?
No. Earth ground resistance is too variable and usually too high to be used for neutral and has never been allowed. In all subpanels neutral and ground can not be connected together (under modern code). Also you are confusing the types of ground. You must have a GEC (Grounding Electrode Conductor) to help manage atmospheric charges. The GEC is connected to one or two eight foot half inch ground rods. In addition you must have an EGC (Equipment Grounding Conductor) back to the main panel that provides a low resistance path to trip breakers if there is short of a hot to the metal case of equipment. Note also at the subpanel you have a separate ground bar. The ground bar is bonded and the neutral is isolated.
I wanted to see if I could get by with the 30A service to the barn....
That is why I suggested conduit with individual wires. You can always replace with larger wires if needed. Cable is normally direct buried so replacing a cable would be more work. (Yes, code allows cable in conduit but it is difficult to pull and the larger the cable the more difficult the pull.)

Not discussed yet is if you will need to replace the subpanel at the barn. We would need to more about it to answer that.
 

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Old 06-23-16, 01:43 PM
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Several decades ago it was allowed to feed an outbuilding a 240/120V service using only two insulated hots and a bare shared neutral/ground. Over time, it's clear that practice had significant safety implications which can result in shock or fires, and as such has been removed from the code. Your father is probably thinking of this method which is now forbidden. Outbuildings must be fed with separate insulated hots, insulated neutral and an equipment ground wire and also have an earth grounding electrode system to comply with modern safety standard.
 
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Old 06-23-16, 03:02 PM
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One thing you could try, although I do NOT recommend it, is to install a transformer at the barn that had a 120 volt primary and center-tapped 240 volt secondary. The cost would likely be high and the total power is still only 3,600 watts.
 
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Old 06-23-16, 04:14 PM
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Thanks all for the replies - very helpful.

I'm absolutely clear on the ground and neutral. However I came home today and my father in law proudly showed me a second sub panel on the wall with 2 240 circuits. I'll have to take a picture when he isn't around to show.

He claims the voltage tester shows 240 in the second panel. I think he took 120 from the first subpanel and connected it to 2 different buses on the second subpanel. Then he installed a double breaker to get to 240.

I know it's not right and I need to run 2 120v lines plus neutral and ground to my barn from the main. However - assuming he is right and the voltage tester works - how is that possible?

Maybe I'm just missing something about how the 240v circuit works..
 
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Old 06-23-16, 04:47 PM
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You have to use a multimeter not a voltsge tester to determine the voltage. Post some pictures of these subpanels with the covers off so we can see the wiring. http://www.doityourself.com/forum/li...rt-images.html
 
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Old 06-23-16, 04:48 PM
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You stated that you are an electrical engineer, do you understand how transformers work? I have known many electrical engineers, most with a PE license, and meaning no offense to you, most of the EEs I have known couldn't plug in a toaster and do it correctly.

Are you familiar with the method that Edison invented using three wires to transmit two different voltages? This method is used to this day in supplying residences with dual voltage electrical service.

To state again what Ray posted, 240 volts services are NOT made up of two, 120 volts circuits. The 120 volts is derived from the 240 volts by means of a center-tapped transformer.

Rather than using a center-tapped transformer we could have a single tap at 1/4 from one end of the transformer secondary coil which would make the same tap 3/4 from the other end of that same coil. Measuring between the end closest to the tap and the tap itself we would get 60 volts. Measuring from the other end to the tap we would get 180 volts. Would you then try to tell us that 240 volts is the result of having a 60 volt line and a 180 volt line?
 
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Old 06-24-16, 08:35 AM
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He either pulled another wire or changed the ground conductor to a neutral. If he pulled another wire, I'd be amazed that conduit was still in good shape, but hey it's done. If he shared the ground and neutral that should really be redone correctly as it is a safety issue.
 
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Old 06-24-16, 04:17 PM
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So I went to take pictures and even this lowley EE knows he did it wrong and is getting too old (82) to be doing this any more. I don't know how he got his indicator to show 240 but the MM shows exactly what I thought - 0 volts. Because all he did was pull 120 to each bus in the second subpanel - from the same bus on the first subpanel! So the voltage between 2 nodes that are already connected to each other should be zero.

I'll do some more research and start a separate post to confirm what I need to do to make this work.

Appreciate all the help....
 
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Old 06-24-16, 04:52 PM
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I don't know how he got his indicator to show 240
I'm guessing he really was using a voltage indicator. AKA non contact tester, not a multimeter. A non contact tester can be used for real testing. It can be fooled by induced or capacitive voltages that have no real power.
 
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