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Old 07-06-2011, 09:39 PM   #1
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Questions about subpanel 240V


I bought a house that already had a 30A30A 240V breaker coming from the main box with a 10-3 Romex to a circuit board in a shed.

One of the power wires in the Romex broke and I need to run new wiring to the shed. I also want to upgrade the power with the potential of drawing 56 Amps at 120V.

I am getting told two different things. Group A is telling me the 30A30A 240 V breaker can draw a max of 60A at 120V if I split it evenly between the lines and that I shouldn't go over 80% of that. Group A says I need to upgrade and either use 8-3 Romex (or run 3 #8 THHN wires and 1 #10 THHN for ground) and upgrade the breaker at the main board to 2, 120 V 40A breakers or 1, 240V 40 A breaker

Group B says that since the breaker is 240V I am just fine running another 10-3 Romex and that I can draw 56 Amps at 120V with no problems.

Anybody out there who can help clarify?

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Last edited by mulltron; 07-06-2011 at 09:41 PM.
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Old 07-06-2011, 09:59 PM   #2
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Questions about subpanel 240V


First, can’t run ‘Romex’ outside at all, so in a since, neither are right.

What is you want to run from the shed? In other words, what is the load calculation? Why do you need 56 amps? With that, we can give you proper ideas and suggestions.

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Old 07-07-2011, 01:43 AM   #3
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Questions about subpanel 240V


Thanks for responding. Let's forget the term romex if that is the problem in this equation.

I bought a house that has both a main circuit board and an external circuit board in the shed running off a 240V 30A breaker from the house. The shed is quite large. The shed run to the house is about 55 feet.

I calculated the load by adding up the Amp rating on each device in the shed. No device in the shed is over 9A. No circuit in the shed is over 9A. The box in the shed has 8 - 120V 15A breakers with the load split between them. I need 56 A total all devices could easily run for 12 hours straight at the same time if I am working. I think that if the 240V breaker is rated at 30A then I have 60A total to work with, but good practice is to only go up to 80% of the total load for each side, which is 24 Amps per side. This led me to believe that I should go with wire rated at 40A or more to make sure I am not running any wires near max and to make sure that my total load is under 80% of the max.

The research I have done says 8 gauge wire can handle 40+ amps. I thought if I had 2 of those coming from a 240 breaker that I would have 80% of 80Amps I could use which is 64 amps and well above the 56 amps I need.

I have had a few people telling me I don't understand something about 240V and that I can easily draw what I need with 10 gauge wires. I am hesitant to believe that because 10 gauge wires are rated around 30A. I don't think they understand that my power needs are much more than average. To me, I am not using any 240V devices and think of it as 2, 120V circuits.

Does it matter if I use 1, 240V breaker on the main box to send 2 power wires to the shed box or does it do the same thing if I were to run through 2 separate 120V breakers?

Do I need 8 gauge power or is 10 gauge power ok?

If Romex and THHN wire are both innapproptiate to run outside, underground through 3/4" PVC conduit, what other wires exist that should I use? MC cable isn't going to fit.

Thanks for anyone and everyone who responds with helpful advice!!!!!
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Old 07-07-2011, 03:21 AM   #4
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Questions about subpanel 240V


Let me try and tackle this, and the pros can fix my errors.

AS IS: Your 30-amp Double-Pole breaker from the house will provide 30 amps of current to the shed (this should currently be ran w/ 10 AWG UF for the outdoor portion of the run). From the shed, you simply use the 15-amp breakers to provide several circuits.

Indicating that you want to provide 54-amps at 120v doesn't make sense at first, but I know what you want to do, I think.

What you should do is change the Double Pole breaker in the house to 60 amps, run three (Black, White, Red) #6 THWN-2 and a #8 bare
underground in conduit to the subpanel in the shed for your service, and then you can use the 15-amp breakers for your branch circuits, using 14 AWG (inside). Or, for the branch circuits, you can replace some of your 15-amps breakers with 20 amps, ran with 12 AWG Romex (inside).

Hope this helps.

Last edited by sirsparksalot; 07-07-2011 at 03:24 AM.
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Old 07-07-2011, 07:12 AM   #5
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Questions about subpanel 240V


Quote:
Originally Posted by sirsparksalot View Post
Let me try and tackle this, and the pros can fix my errors.

AS IS: Your 30-amp Double-Pole breaker from the house will provide 30 amps of current to the shed (this should currently be ran w/ 10 AWG UF for the outdoor portion of the run). From the shed, you simply use the 15-amp breakers to provide several circuits.

Indicating that you want to provide 54-amps at 120v doesn't make sense at first, but I know what you want to do, I think.

What you should do is change the Double Pole breaker in the house to 60 amps, run three (Black, White, Red) #6 THWN-2 and a #8 bare
underground in conduit to the subpanel in the shed for your service, and then you can use the 15-amp breakers for your branch circuits, using 14 AWG (inside). Or, for the branch circuits, you can replace some of your 15-amps breakers with 20 amps, ran with 12 AWG Romex (inside).

Hope this helps.
I agree with this, but you also have to install ground rods for the subpanel.
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Old 07-07-2011, 08:39 AM   #6
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Questions about subpanel 240V


With 10-3 cable you can draw up to 30* amps at 240 volts or two allotments of 30* amps at 120 volts or any combination in between. If you draw 15 amps from one side you cannot draw 41 amps from the other side to get to a total of 56 amps at 120 volts. This is the same as thinking of the 3 conductor (black, red, white) feed as two separate 120 volt circuits.

Certain kinds of usage (air conditioner, heater) are considered "continuous" and certain kinds of usage (lights, most power tools) are considered "intermittent". For continuous usage you are limited to 80% of the wire size amps rating; intermittent usage can use the full rating. For very long runs you derate the number of amps available to prevent excessive voltage drop. But you derate only once; if both continuous/intermittent and voltage drop are issues, you pick the one derating that is more severe.

The breaker at the main panel must be a double wide breaker with handles joined so if one side trips the other side is forced off. It must also be rated for 240 volts. You are not permitted to run two or more separate circuits of similar kind* between any two buildings so two 120 volt circuits is not an alternative. (*120 and 240 volt circuits are of similar kind; 3 phase and 480 volt circuits are different kinds)

It is okay for the breakers in the subpanel to exceed the feed amperes (wire size max amps) for example two 20 amp breakers on each side of a 10 gauge 30 amp 120/240 volt line.

Using 8 gauge wires you get 40 amps on a side, using 6 gauge wires you get 60 amps on a side.

Many experts suggest running at least 6 gauge wires for a shed that might someday become a workshop.
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Last edited by AllanJ; 07-07-2011 at 09:00 AM.
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Old 07-07-2011, 10:38 AM   #7
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Questions about subpanel 240V


Thanks Allan, Sirs, and NJ!

Most of the power hungry devices are room A/C units, lights and fans, all 120V and continuous. I know if I used a 240V I could get away with less amps and stick with 10 gauge, but I already have 3, 120V AC units, and 8 500W lights installed and they work fine.

Thanks for letting me know the 240 V breaker is definitely the way to go. I don't thing 6 gauge wires will fit through the 3/4" conduit, but I don't ever see the need for 60A per side. I estimate pulling 28.6A from one side and 27.4A from the other. After reading everyone's responses, I think a 40A 240V breaker on the main box, and running 8-3 is the way to go.

I have one last question about what kind of wire to use. The branching circuits in the shed are all MC 12-2 wire. The main power to the shed is running through underground 3/4" pvc conduit. I am not sure how many bends are in the run. I read in a few places to go with a stranded wire for an easier feed since I will be using most of the conduit space.

I am getting reports of two different kinds of wire to use
#8 AWG UF or
#8 THHN

Is either acceptable? What are the differences? Is the UF wire just bundled while the THHN come as individual wires? Which do you recommend for attaching to the old wires and pulling them through the conduit?
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Old 07-07-2011, 10:52 AM   #8
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Questions about subpanel 240V


One last question

Will 8-3 UF-B slide through 3/4" conduit?
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Old 07-07-2011, 11:32 AM   #9
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Questions about subpanel 240V


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Originally Posted by mulltron View Post
One last question

Will 8-3 UF-B slide through 3/4" conduit?
It might but it won't be code compliant.
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Old 07-07-2011, 12:10 PM   #10
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It might but it won't be code compliant.
Thanks for letting me know! I am having a very hard time finding what wire to use.

So far I have been told that Romex is a no go, that UF-B is a no go, and I haven't gotten a clear cut answer on running 4 THHN wires. Is the only way to be safe to run 8-3 MC cable through a PVC conduit? I don't think it would fit in 3/4" conduit. It runs under quite a bit of cement, and I want to avoid running another underground conduit. I am not worried about inspection, but I want to be safe.
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Old 07-07-2011, 12:15 PM   #11
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Questions about subpanel 240V


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It might but it won't be code compliant.
Why Not?
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Old 07-07-2011, 12:19 PM   #12
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Questions about subpanel 240V


Quote:
Originally Posted by mulltron View Post
Thanks for letting me know! I am having a very hard time finding what wire to use.

So far I have been told that Romex is a no go, that UF-B is a no go, and I haven't gotten a clear cut answer on running 4 THHN wires. Is the only way to be safe to run 8-3 MC cable through a PVC conduit? I don't think it would fit in 3/4" conduit. It runs under quite a bit of cement, and I want to avoid running another underground conduit. I am not worried about inspection, but I want to be safe.
Code says you can put 5 #8 thwn/thhn wires into 3/4 pvc conduit.
With one #10 and 3 #8's you should make it.
Make sure the wire is rated with THWN for a wet location.

MC cable would not be allowed inside the pipe.
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Old 07-07-2011, 12:27 PM   #13
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Why Not?
The cable is too large. 8-3 UF-B is 0ver an inch wide.

Last edited by brric; 07-07-2011 at 12:33 PM.
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Old 07-07-2011, 01:25 PM   #14
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Questions about subpanel 240V


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Code says you can put 5 #8 thwn/thhn wires into 3/4 pvc conduit.
With one #10 and 3 #8's you should make it.
Make sure the wire is rated with THWN for a wet location.

MC cable would not be allowed inside the pipe.
Thanks JB! Very helpful! I think that is the way to go!


To to be clear for the future, is UF-B just fine going through PVC conduit as long as it fits? Brric, you were just pointing out that 8-3 UF-B wouldn't fit through 3/4" conduit, not that UF-B is generally against code? Is that right?
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Old 07-07-2011, 01:39 PM   #15
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Questions about subpanel 240V


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Originally Posted by mulltron View Post
Thanks JB! Very helpful! I think that is the way to go!


To to be clear for the future, is UF-B just fine going through PVC conduit as long as it fits? Brric, you were just pointing out that 8-3 UF-B wouldn't fit through 3/4" conduit, not that UF-B is generally against code? Is that right?
It is fine as long as the pipe is proberly sized for the size of the cable.

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