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Installing a sub-panel....


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It depends on the wire gauge and type that feeds the outlet, and the size of the breaker. Can you see the wire type and gauge anywhere on the jacket of the wire? I believe Chuck has a table somewhere on his regular PlanetChristmas site that tells what each wire gauge and type is rated for. If you can't find it I can easily look it up for you. Also, some 220 lines don't include ground wires. If the current receptacle only has three slots and you can't see a ground wire stubbed inside the junction box I would shy away from using the line to feed a sub panel.

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In this case, if I was going to run a sub panel, I would probably start at the breaker, and run new wiring to a sub panel just to make sure it was correct for the load, that it was properly grounded, and that there were no splices and such between the breaker and the sub panel. Also the shorter the run between main panel and sub panel the better.

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I did just this the other day to power up my AL stuff with 2 seperate 30 amp breakers. I started down the path of a sub panel, but for the short term, I took out the double pole 220 amp breaker and replaced with 2 30 amps single pole. The catch is that the hot from the 220 was going to a electric dryer plug. Inside it had 2 hots and a netural. I just pulled through another neutral back to the panel and then had exactly what I needed for 2 30amp GFI outlets. I changed the box ring and put on my new plugs. Under $25 for a safe quick fix. Now for next year, I will upgrade to a full subpanel or maybe a new 220amp mainpanel.

A couple of things to remember. Turn power off to the panel, either on the side of the house, or in the main panel. The sub panel you add can only be as large as the power left on your main panel. For me, I had 80amps left, but only replaced the 60amp double pole with 2 30s just to be safe.

-jds

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Marty Slack wrote:

It depends on the wire gauge and type that feeds the outlet, and the size of the breaker. Can you see the wire type and gauge anywhere on the jacket of the wire? I believe Chuck has a table somewhere on his regular PlanetChristmas site that tells what each wire gauge and type is rated for. If you can't find it I can easily look it up for you. Also, some 220 lines don't include ground wires. If the current receptacle only has three slots and you can't see a ground wire stubbed inside the junction box I would shy away from using the line to feed a sub panel.

Marty,

Great answer for the OP, just a minor clarification. There should be four electrical connections in the existing installation: Two hot (black wires), one neutral (white) and one ground. If the installation is in all metal conduit the ground wire can be omitted and the metal used instead.

What I've seen as common practice for an electric dryer (or room size air conditioner) is just two hot wires (both black), as you pointed out.

To convert the existing 240V dryer/AC receptacle for 120V service it is necessary to pull a neutral wire (white). One black and the new white are circuit A, the other black and the same white are circuit B. Both new receptacles should be grounded through the metal conduit and workbox or a separate ground (green) wire.

At this point I'd opt to make each new receptacle a CGFI (Current Ground Fault Interrupter) for added safety when Christmas lights are placed outdoors.

Lastly, if the dryer/AC receptacle was rated to 30Amps each of the new 120V circuits will be good for 30Amps (as pointed out by Zac), but standard outlets are rated for 15Amps (14AWG wire) or 20Amps (12AWG wire) so the old breakers need to be downgraded to either 15Amps or 20Amps each. For the 220V service the original two breakers should have their handles locked together, when the circuit is split for two 120V circuits the handles also need to be separated.

This detail is for safety, please don't skip it. If in doubt have a certified electrician check your work. Not every electrician is certified - many journeymen (junior electricians) rely on their boss' authority to sign off on their own work. Alternatively you can ask your local city or county building inspector to visit. Doing so will likely expose you to a building permit (for the permanent electrical work), and many weekend warriors avoid contact with inspectors. Personally, I like to sleep at night...

Why is this important? Because if you have an accident or fire that can be traced back to badly installed wiring the insurance company will not cover the loss. The larger Christmas light displays noted on PC definitely up the safety issues due to the amount of raw electrical power in the project. "Safety first", not "Safety third"!

Comments Welcome!

Peter

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In Peter's post he talks about two black (hot or positive) wires. With many if not most wire bundles one of these is red but is still a hot. So, what you would likely have is one black (+), one red (+) one white (- or neutral), and one that is either bare copper or green (ground.) In it's simplest form each of the hots, paired with the neutral, can make up one circuit. If the wire size is sufficient these could each be split to make more separate circuits, if each has its own breaker.

Now that I've said too much let me add a very important disclaimer. Unless you completely understand the residential electrical wiring and NEC codes do not even mess with this stuff. If there is any doubt please contact a qualified electrician. There also may be local codes in addition to the NEC that need to be followed (such as neutral bonding and grounding issues.) Please don't attempt something you are not qualified to do. It's just not worth the risk.

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joshschaf wrote:

...The sub panel you add can only be as large as the power left on your main panel.

-jds

Can you elaborate? Our house was built with a 200-amp panel with a 200A main breaker... but the 120v breakers add up to 210 amps and the 220s add up to 230. Assuming an even split between phases I'm already at 230 amps/phase. Apparently the plan is that not every circuit will draw full load at the same time.

BTW I've gone even further over this by using the twin CH breakers... two 15s or two 20s in one package, the size of a standard (original) breaker. Very handy!

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Marty Slack wrote:

It depends on the wire gauge and type that feeds the outlet, and the size of the breaker. Can you see the wire type and gauge anywhere on the jacket of the wire? I believe Chuck has a table somewhere on his regular PlanetChristmas site that tells what each wire gauge and type is rated for. If you can't find it I can easily look it up for you. Also, some 220 lines don't include ground wires. If the current receptacle only has three slots and you can't see a ground wire stubbed inside the junction box I would shy away from using the line to feed a sub panel.

Marty, The wire doesnt have any writing on it that I can see. The outlet has 30A 250V printed on the front. The wire is fairly large gauge with a yellow fabric sheath.

I believe the breaker it is wired to is rated for 40 amps. I would feel safer though repl;acing the wire all the way back to the main box I think. Erin

post-203-129570946706_thumb.jpg

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fmsjr wrote:

joshschaf wrote:

Can you elaborate? Our house was built with a 200-amp panel with a 200A main breaker... but the 120v breakers add up to 210 amps and the 220s add up to 230. Assuming an even split between phases I'm already at 230 amps/phase. Apparently the plan is that not every circuit will draw full load at the same time.

BTW I've gone even further over this by using the twin CH breakers... two 15s or two 20s in one package, the size of a standard (original) breaker. Very handy!

FMSJR,

The purpose ofeach branch breaker is to protect the circuit down-stream. Nothing more. The purpose of the main breaker is to protect the feeder from the street (and the meter and the buss bars in the box). Because each branch is unlikely to be fully loaded at the same time there can be numerically more total amps (adding up the breaker handle ratings) in the branches than the main breaker rating.

Christmas lights, particularly if they are steady burning, will be an exceptional case where the circuit(s) could be loaded much higher than average uses (for a non-Christmas decorated home).

The point made earlier about a sub-panel is that it can't be larger than the main power rating (i.e. you can't add a 100Amp sub-panel to a 60Amp meter). Also, if it is to be useful and not cause nuisance tripping of the main breaker it should be smaller than the difference between the main breaker and the average load on all the existing circuits.

For example: A 200Amp service with a 200Amp main breaker that is loaded to 100Amps could support a new 100Amp sub-panel. The same service with 150Amps load would only support a 50Amp sub-panel addition. There's not much stopping the installation of a 100Amp sub-panel in this case, but the results would be irritating if the new panel's load caused nuisance tripping of the main breaker.

The key is to know the average existing load to determine the spare capacity before choosing a sub-panel. One way is to measure the current with an ammeter, another is to add up the typical loads by exploring how electricity is used by the rest of the home, and finally, a clue about consumption could be calculated from the previous utility bills (that charge for kW-Hr).

A good example of an audit would be to consider if heavy A/C loads are operated only in the summer. If so, then the A/C power demand is 'free' for use in the winter for Christmas lights. Another seasonal heavy use is swimming poolsand/or hot tubs.

If in doubt have a certified electrician do an audit of the existing system. One thing to watch for is that new circuits may have been added after the home was built, or older circuits were illegally 'upgraded' by swapping the breakers to larger handle rating versions (and thus weakening the protection for the original wiring).

Comments Welcome!

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Erin_King wrote:

Marty, The wire doesnt have any writing on it that I can see. The outlet has 30A 250V printed on the front. The wire is fairly large gauge with a yellow fabric sheath.

I believe the breaker it is wired to is rated for 40 amps. I would feel safer though repl;acing the wire all the way back to the main box I think. Erin

Erin,

The “30A 250V” refers to the maximum rating of the receptacle, it doesn't tell you much about how its being used here.

To consider upgrading this for multiple 120V branch circuits you will need to do a couple of things. Firstly, find out if the cable has a dedicated neutral wire (look inside the panel and count the number and colour of the conductors). Secondly, follow the wiring to a breaker (likely two with their handles tied together) and read the current rating.

For your upgrade the breaker needs to be split (i.e. no link on the handles) and the current rating selected for your new wiring size. The maximum you can have is 20Amps set by the new duplex receptacle's rating, and if the old breakers are higher then new breakers must be used.

The good news is that you do not need to remove and replace this cable (unless it does not have four conductors - two hot, one neutral, and a ground)!

I would consider saving the existing cable and receptacle in case you need to restore the original circuit. One day you may have a 240V dryer or sell the home.

Comments Welcome!

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a 200 amp panel is really 400 amps of power ,200 amps on each phase a and b but 80% of 200 amps is 160 amps so this is really the max on each leg as far as turing you your 240 volt outlet into subpanel as long as it has 4 wires should be no problem turning it into 2 20 amp recepticals dryer recs are usally 10 gage wire or 30 amps 12 gage is good for 20 amps 14 gage is good for 15 amps and 8 gage wire is good for 40 amps

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You can wire a sub panel using the 240 volt wiring for a dryer, if it has 2 blacks a white and a green or bare ground. Do NOT try and wire it up as two 120 volt plugs using a black on each and sharing the white (neutral). The neutral carries the same amps (load) as the hot. Circuit breakers and fuses limit the current the hot can carry, but nothing protects the neutral from carrying twice its rated load.

Your best bet is to get a qualified electrician and let him look, you would be surprised what we can do sometimes!

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Analogvideo wrote:

If in doubt have a certified electrician check your work. Not every electrician is certified - many journeymen (junior electricians) rely on their boss' authority to sign off on their own work.

A Journeyman electrician is one who has completed 5 years af apprentice training, a "junior" electrician is called an apprentice and he must work under a journeyman. In any craft the journeyman is the top of the craft.

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Mike Bluford wrote:

Analogvideo wrote:A Journeyman electrician is one who has completed 5 years af apprentice training, a "junior" electrician is called an apprentice and he must work under a journeyman. In any craft the journeyman is the top of the craft

Some states have a master electrician class. Had to get that when I worked in a couple of them.

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Mike Bluford wrote:

You can wire a sub panel using the 240 volt wiring for a dryer, if it has 2 blacks a white and a green or bare ground. Do NOT try and wire it up as two 120 volt plugs using a black on each and sharing the white (neutral). The neutral carries the same amps (load) as the hot. Circuit breakers and fuses limit the current the hot can carry, but nothing protects the neutral from carrying twice its rated load.

Your best bet is to get a qualified electrician and let him look, you would be surprised what we can do sometimes!

Mike,

Could you clarify this? Why do you think that the neutral leg carries the same current as the hot? Where does this current in the neutral come from if both of the hots are protected by breakers?

The standard residential powerarrangement in the USA (and Canada) is a "split-phase system" - the hot conductors are 180 degrees out of phase and have a 240V potential difference. If both are loaded equally then the neutral carries no current. If loaded uneqally the neutral carries the difference in current between the two hots. The worse case is one hot unloaded (or open) and the other loaded to the maximum - then current flows back to the source only through the neutral.

Comments Welcome!

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The standard residential powerarrangement in the USA (and Canada) is a "split-phase system" - the hot conductors are 180 degrees out of phase and have a 240V potential difference. If both are loaded equally then the neutral carries no current. If loaded uneqally the neutral carries the difference in current between the two hots. The worse case is one hot unloaded (or open) and the other loaded to the maximum - then current flows back to the source only through the neutral.

I couldn't of said this better that is exactlly right if you run a three wire circuit and share a neutral it only carries the unbalanced load that is why when i take amp readings i try to keep it as balanced as i can

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Since my earlier post, I have tempered my aversion to shared neutrals. I still wouldn't wire up a house that way. It is true, that if on opposite poles, the neutral will not be overloaded," The worse case is one hot unloaded (or open) and the other loaded to the maximum - then current flows back to the source only through the neutral". Remember we are talking about house wiring for Christmas lights, this could happen, but probably with not much problem. On the other hand, if I flip the breaker to work on a circuit, I want both the hot and neutral to be dead, not me if I disconnect the neutral. You can share a neutral with 2 loads if the loads are connected to different poles. When connected this way the neutral will carry the difference between the loads..not the sum of the loads. If you were to get the 2 breakers on the same pole however then the neutral would indeed carry the sum of both loads. Just measure the voltage between the 2 hot wires..if it's 240ish then you're on different poles...if it's zeroish then you're on the same pole and that's not good. Not suggesting Erin shares neutrals, just letting him know it's possible if done correctly. Shared neutrals are fairly common in commercial work because everything is labeled, there are prints and pretty much only electricians work on the electric. Houses however...yikes.

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Erin,

I also have a 220 outlet in my garage that I take advantage of. I have a 40 amp outlet that is connected to a 30 amp disconnect switch then to five 15 amp breakers to five outlets. Here is a picture to give you a better idea. I know that my wiring to the outlets is not up to code but I know what I'm doing and it is temporary. Aslo, by having a 30 amp disconnect switch it keeps me from using my breakers as switches and I can turn on a large portion of my display at once.

~ Jim

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East wrote:

Erin,

I also have a 220 outlet in my garage that I take advantage of. I have a 40 amp outlet that is connected to a 30 amp disconnect switch then to five 15 amp breakers to five outlets. Here is a picture to give you a better idea. I know that my wiring to the outlets is not up to code but I know what I'm doing and it is temporary. Aslo, by having a 30 amp disconnect switch it keeps me from using my breakers as switches and I can turn on a large portion of my display at once.

~ Jim

Unless I'm missing something, that's not 100% safe.

You have a 40A outlet at 240V. From that you have 75A of 120V circuits, which is fine, since you could pull a max of 80A @ 120V (40A from each of the 2 hots on the 240V outlet). Since 3 of those circuits are on one leg (45A) the 40A breaker could trip if you actually loaded those 3 circuits to a combined total > 40A, but you're still protected and that shouldn't be a problem.

What troubles me is the 30A disconnect switch, which is rated for 30A @ 240V. You should find one that's rated for 40A @ 240V to be safe. The 30A one could overheat or be damaged, particularly when opening and closing.

-Tim

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Tim,

Your very observant... I've used a meter and I am only pulling 25 amps on one leg and 27 amps on the other. Next year I plan to abandon this set up and start over with a new 60 amp outlet feeding a new sub panel.

Also, I purposely placed a 30 amp fused disconnect switch inline to make sure I would blow there first before going into the house. I wanted my weak link to be outside of the house.

~ Jim

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The part that looks unsafe to me is the plug on the cord. It only looks to be a 10-30p, or a 10-50p which would only be a 3 wire plug. But its hard to tell from the angle of the picture, and the lack of GFCIs.

Temporary installations is covered in the code, Article 590. And it does apply to holiday decorative lighting.

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