View Full Version : 240 wiring: 10/3 wg?
01-12-2001, 06:49 PM
My questions are many here. I was going to run an extension cord from the dryer outlet (240 non-metallic 10/3 wg). I decided to run a new 30 amp circuit closer to my table saw. I am fine with the 30 amp double pole breaker, 10 guage/3 wire w/groung wire. I get to the wall mounted receptacle, and my choices are Levitron 30 amp dryer wall mounted receptacle WITHOUT GROUND. This alerted me. This is what my dryer was wired for just 6 months ago by a reputable electrician. Is this wrong? Should I get a differt receptacle? They have different plug configurations, as far as the shape and angles of the prongs. Eagle makes a 3 prong plug that is grounded but the prongs are shaped differently.
I also want to make an extension cord. The HD near me sells rubberized 10/3 wire. 3 strands of black, white, and green. Do I wire the green hot since I need 3 wires running to the plug? A reply to my previous post said he used 10/2 wiring. Is this not safe without a ground?
01-12-2001, 07:42 PM
Wire nomenclature can sometimes be misleading. The ground is considered a given and the denominator is the number of current carrying conductors - remember that the ground DOES NOT CARRY CURRENT!
So, 10/2 should have *three* wires in it, black, white, & green. 10/3 should have *four* wires in it, black, red, white, & green.
*DO NOT UNDER ANY CIRCUMSTANCES USE THE GREEN WIRE AS A HOT*
Buy the correct wire.
Read the data plate on your saw and post it here. Saws generally don't need three phase wiring. Also, read the manual, what type of plug do they suggest? It'll be a NEMA number like 10-30R or 14-30R or somesuch.
I would expect you to need a NEMA 14-30R plug for your saw. This looks like a large version of a standard outlet with an extra L shaped prong opposite the ground lug. You can also get it in a twist lock configuration (L14-30R).
This wiring will have two hot conductors (black-X and red-Y) and a shared neutral (white-W) along with the safety ground (green-G). This is a pretty standard configuration. You have two 120V circuits which are out of phase with each other to give you 240V across the two hots. From either hot to neutral you'll only read 120V.
Your dryer may use the NEMA 10-30R connector with two angled blades and an L shaped neutral. This configuration does not have a seperate safety ground and is common on dryers. If you follow the connection to your electrical panel you'll find that the neutral bussbar is actually grounded (or at least should be) The neutral in this case does NOT carry any current as the flow is between the X and Y (the two hot phases) only.
Please look at all of the connectors and let us know the NEMA #s on them. The first group of the NEMA identified the connector type and the next is the current rating. The L prefix indicates Locking.
01-12-2001, 08:27 PM
What then, is the the difference between 10/2 and 10/3 when my 10/3 has just two hot wires and a green ground? Should that not be considered a 10/2 by that thinking? My saw data plate mentions nothing other than 18 amps, 230 volt, 2 pole, and 3 HP.
My booklet says to use a NEMA L6-30. Why use this plug over a NEMA 10-30? The prongs are different, but why?
Another question...Why does a 10/3 non-metallic cable (standard indoor 30 amp) have 4 wires (Black-hot, Red-hot, White-Neutral, and Green-ground) when all the plugs for this circuit can only except 3 wires? What do you wire the 4th wire to when hooking up a plug or wiring the wall receptacle?
01-12-2001, 10:46 PM
Looking at the wiring diagrams in my "Ugly's Electrical Reference" the difference is in the grounds. The 6-30 has two horizontal blades and an earth ground (G) This configuration is for 240V ONLY (no 120V taps). The 10-30 has the slanted blades and a shared neutral so it can provide two differently phased 120V circuits AND 240V. The 6-30 cannot be split into 120V pairs as there is no provision for a shared neutral.
It's a subtle but important difference. In a dryer you often have a light and other ancilliary electrical devices in addition to the massive 240V heating element. Because the 10- series allows for splitting off 120V taps, the dryer can use a regular 120V appliance bulb and 110V motors for the timer and drum. The 6- series can ONLY supply 240V and therefore cannot be used in that manner.
Now your saw, being the type of beast that it is, doesn't have any aux motors on it. It has the one load and that's it (unless you have a light or something else wired to it). So it needs *pure* 240V and thus calls for the 6- series. Now I believe that you could run it just as well off the 10- series but there is the minor matter of no safety ground.
The exception to the naming convention on the cabling comes when you move from SOLID wire to STRANDED. So 14/2 w/ground in a solid wire (for in wall wiring) has two #14 conductors and a separate bare ground. But in an extension cord you would call it 14/3 (no w/ground modifier) and you'll have *three* #14 conductors, one black, one white, and one *insulated* green. The denominator can therefore be thought of the number of insulated wires. So 14/2 w/ground and 14/3 both have three conductors in them, the third is insulated in the 14/3 but not in the 14/2 w/ground. The 14/3 cable is made of stranded wires where there are multiple smaller wires laid into a single larger strand instead of a single solid wire. This is for flexibility as the stranded will bend a *LOT* easier than the solid will. The stranded will not break from flexing fatigue as readily as the solid will. Confusing? You betcha! (Now you know why it sometimes pays to hire an electrician!)
Now when you use 10/3 w/ground in a FIXED (read solid wire) configuration, you have black, red, white, bare and the connectors don't matter as your wiring to stationary equipment and the bare wire goes to a grounding lug ON THE BOX and not on the outlet itself.
Since your saw is specing the 6- series connector you should use 10/3 with the white banded w/black to indicate that it's a 2ndary hot phase. You can tap off a two hot 240V connection and run the third wire (green) to a hard ground or the metal box. If you wire the saw to the 10- series connector it should work just fine. This would allow you to unplug the dryer and plug in the saw so you could only have one load running at a time. Since the saw doesn't need any 120V power this will work, but the reverse of using a 6- connector on the *dryer* will NOT.
Clear as mud?
01-13-2001, 04:22 AM
Marks answers are correct, but I even got a little confused with the back and forth answer. It is not a straight forward subject.
The simple answer is your saw needs three connections two hots (one wire from each connection to the breaker) and a ground. You don't need a neutral wire.
The way 220 works is that you have 110 from one hot to ground (or neutral) and 110 from the other hot to ground (or neutral). Because of phase relationships you end up with 220 between the two hot wires. You always want to have a good ground to any tool. The Ground is your safty line. In case anything goes wrong in the motor, having it grounded will cause the breaker to go. If it is not grounded then you (your body) might provide the path to ground and in this case you blow and not the breaker.
Now you have run wire to a new box. If you take a neutral you could have 110 circuits taped between one hot and the neutral and you do have about a 12 amp margin in you breaker and wirering, but if you don't know what is going to be loaded on the plugs you might cause an load imballance between the two hot lines and that might cause a problem. With 220 tools I run a dedicated line to each tool and run a differnt off of a differnt breaker to handle 110 stuff in the same area.
I don't know if this helps or makes it more confusing, but email me and maybe I will be able to draw up a wireing diagram for you.
01-13-2001, 09:53 AM
Thanks a lot you guys. After calling several electricians and visiting two home centers, no one could answer my questions except to say my saw should have been wired with 4 wires as per 1993 code. I wasn't sure that useing the two hots then the third wire as a ground, would cause a problem with no neutral.
So, I won't use a neutral and it will still be safely grounded. For many years, neutral and ground were run on the same wire. This was thought to be unsafe in certain situations? Still a little confused, but I know what I have to do in my situation.
Thanks again for your help.
01-13-2001, 10:03 AM
When you have a neutral and a ground in one then you can't have GFI protection. The GFI monitors the current flowing out and the current flowing back and if it senses an imbalance (current "leaking" out - most likely thru *YOU*) then it shuts down the flow even if you're only drawing a fraction of the full rated load.