which gauge wire do i need? (and 208/240?)

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Old 03-18-10, 03:37 PM
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which gauge wire do i need? (and 208/240?)

hi there wonderful world of knowledgeable and helpful masterminds of the sparky arts,

i have a double wall oven going in a kitchen i'm doing, all i can see on the spec sheet is a diafraggm that looks like this =

KW Rating
------------------
240V | 6.8 |
------------------
208V | 5.1 |
------------------
Breaker Size |
------------------
240V | 30 Amps |
------------------
208V | 30 Amps |
------------------

well, ok, it's not a diaphragm. :P *diagram

so the question(s) is(are)...

what gauge wire? i'm going about 40ft from the breaker box. (I assume i just get a double wide lookin 30 Amp breaker and wire up a black and a red to either one and a green/bare to the ground. correct me if i'm wrong. maybe 10 awg?)

also, what's this 208/240 weirdness? do i just do 240 and it's fine? it seems to be telling me either/or, and i've seen some people on here refer to a 240/208 either/or type of appliance, any elaboration on this point would be vuuunderbar.

also if someone could explain single phase/ 3 phase whatever the hell that means... i'm a wood guy. i'm careful, conscientious, and i don't just do stuff without making sure first, but at the end of the day, i'm a wood guy.

thanks in advance you beautiful wonderful wealths of knowledge yous!

-ben huff
 
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Old 03-18-10, 03:59 PM
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Number 10, residential electric is 240
 
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Old 03-18-10, 04:05 PM
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Forget all the other numbers, you need 240/30A for that oven. That would normally translate to #10-3/w-gnd, but for 40' I would suggest #8. That will guarantee no voltage drop if you have the thing turned up full blast.

No, or VERY few residential houses have 3 phase(208). Nearly every house has 240 single phase. 3 wires from the transformer, 240VAC between the hot conductors, with ground wire(neutral). From one side of each wire to neutral you get 120VAC, for most of the house wiring. 240 is for large appliances. For wiring purposes, voltage does not matter. All wiring is rated at 600V. Current is what determines the wire size. 15A requires #14, 20A requires #12, 30A normally requires #10, but I like to design for overkill.
 
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Old 03-18-10, 07:33 PM
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There are some apartment and/or condo complexes that provide single phase service to each unit that is derived from 120/208 volt, 3 phase 4 wire service feeding the building's main electric service. In these cases, each unit gets 120/208 volt single phase service. Don't worry about it.
 
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Old 03-18-10, 08:20 PM
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thanks a bunch!

hey thanks everyone!

so now we just have a disagreement as to whether i should really go with 8 gauge warr.

so i wanna see you guys tear each other apart fighting this one out!

no

but seriously, i'm always for a little overkill, but am i overoverkilling if i use 8, you promoters of the 10 gauge?

and bill, sell me on the 8 gauge. why's it better?
 
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Old 03-18-10, 09:57 PM
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imo. 30 Amps seems very little for a double wall oven. All wall ovens ive done in my expeirence have been 40+ amps. Did anyone notice that the diagram in the OP has the KW rating for 240v being higher than that for 208v. I know ive been outta work since november, but i though the actually draw decreases when more voltage is applied. Hence the benifit of running motors at 240 as opposed to 120, or lighting at 277 as opposed to 120. Correct me if im wrong. I agree with bill though, i tend to over design. i would run 8-3 with ground, to compensate for the voltage drop(which would be minimal at best) but still, just to protect yourself for future upgrades, and becuase i think 30 amps isnt enough for a double wall oven.

Just a note to others, 208 dosent mean that its 3 phase. CasualJoe is corect, many condo complexs use 120/208v single phase service, especially in new jersey.
 
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Old 03-18-10, 11:10 PM
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The nameplate ratings indicate that the oven can be used either on 208/240v supply voltage. If your power supply is 240v, then the 240v ratings will be followed. For me when I size up my wire based on KW ratings or current(ampere) I add a safety factor to it considering the voltage drop. For me #10 is just enough and I agree with Joe. 6.8kw at 240v will be around 28amps adding a safety factor of 25% will be 35 amps. In this case you have to use #10/3 + grd, and a 40 amps 2pole circuit breaker. Hope this will be of help.
 
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Old 03-18-10, 11:49 PM
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Did anyone notice that the diagram in the OP has the KW rating for 240v being higher than that for 208v. I know ive been outta work since november, but i though the actually draw decreases when more voltage is applied. Hence the benifit of running motors at 240 as opposed to 120, or lighting at 277 as opposed to 120. Correct me if im wrong.
The numbers are correct. Remember that the oven is a heating device and therefore you need to take into account the fixed resistance of the heating elements.

In the original post it states 6.8 kW at 240 volts. Dividing 6800 watts by 240 volts gives an answer of 8.47 ohms resistance. Using Ohms law you would divide the applied voltage (208) by the resistance and get 24.56 amperes and then multiplying the amperage by the voltage (24.56 x 208) gives you 5,107.56 watts or 5.1 kW.

With a motor the windings are specifically designed for the applied voltage so the calculation is simpler for apparent power but in reality you have to apply a correction factor for power factor which will change according to the mechanical load on the motor.
 
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Old 03-19-10, 03:58 AM
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what could it hurt?

ok so i don't WANT to buy 8 gauge wire if it's not going to amount to a hill of beans. IF however, a hill of beans will infact be amounted to in the end, OF COURSE i would want #8 wire!

so again, what are the odds that it will make a difference, and like the other fellow said, someone may upgrade later, needing a thicker conductor ey?

so tell me oh gurus of the gauges, i'm not convinced either way, and what could it hurt if i did over gauge it anyway? will it cause me some other not heretofore seen problem i'm not aware of? is there an "overgauge" issue? is it going to cost $20 more? maybe $30? i don't even know what to expect. and how hard is it to twist #8 wire and wire nut it, or is there a better way to connect the ends...

and i thought i only had ONE question. ha.
 
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Old 03-19-10, 06:07 AM
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Except for the additional cost there really is little downside for upsizing the wire.

Connecting the #8 will be about the same difficulty as connecting #10s. Just be sure to use a properly sized wire nut for the sizes of wires used.
 
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Old 03-19-10, 06:33 AM
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ok

so i think i'm gonna use #8 wire with a 40 amp breaker. wanted to make sure i wasn't gonna wreck something by doing too much.

to quote my good friend's father in law "if that don't hold it, more won't." - there's awful lot of truth in that.

thanks everyone, and keep posting if you have anything else to say, i like learning new stuff!
 
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Old 03-19-10, 07:54 AM
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8 ga wont hurt a thing but its only using 6.8 when both units are wide open. 98% of the time while its operating it probably wouldn't matter much and when its off or only one unit is used it would be irrelevant.
I was happy to see furd kick in, splain it better than I could. The ratings sound right, I put a single in a while back, 3.4kw. Due to some changes it needed 3 conductor and I was getting rid of all the bonded appliances. It had 50A service shared with cook top so I toss that wire, run a new one for the range, steal the old dryer wire for the cook top (only required 2 conductor) and pull new 3 conductor for the dryer.
 
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Old 03-19-10, 10:19 AM
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Originally Posted by Huff RnR View Post
so i think i'm gonna use #8 wire with a 40 amp breaker.
You have to use a 30A breaker, no matter how big the wire is. Max breaker size is listed at 30A so you must follow that. Double check the wiring requirements to see if a neutral wire is required. This will determine if you buy xx/2g cable or xx/3g cable. I would probably install the 8/3g anyway so you have flexibility to replace this appliance in the future without having to re-redo the wiring.
 
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Old 03-22-10, 06:17 AM
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Originally Posted by ibpooks View Post
You have to use a 30A breaker, no matter how big the wire is. Max breaker size is listed at 30A so you must follow that.
max breaker for what the appliance? the appliance calls for a 30 amp breaker. the wire can be used with a 40 i believe. you think i'm gonna break my appliance if i use a slightly larger breaker than they reccomend?

also, i installed 2 conducter wire, and it cost like 70 bucks. now it soounds like 3 conducter is needed sometimes. the spec sheet doesn't mention it. so am i supposed to tear out and buy wire with my own money in case someone needs it in the future? i would've done that originally if someone had mentioned it to me. i'd rather do it now than later...
 
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Old 03-22-10, 06:24 AM
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yeah i think it's min breaker size, not max. i mean, maximum implies that if i put a 20 on there, i might be ok. it certainly cannot mean that. and #8 wire is listed for 40 amp circuits.

i'm sure i do not know what you are trying to say.
 
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Old 03-22-10, 07:54 AM
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This isn't like a motor load where there may be options for breaker sizes unless is specifies it in the manual. Like a water heater they list 30 and thats the deal. In some sense it will have the wire listed in the directions, in the drawing it will show 3 or 4 wires, an oven I would guess 3 conductor + ground, there is likely electronics, clocks. Some cook tops are still 2 conductor.
 
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Old 03-22-10, 09:15 AM
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Originally Posted by Huff RnR View Post
max breaker for what the appliance? the appliance calls for a 30 amp breaker.
Yes. That means you need to install a 30A breaker and a wire that can handle at least 30A, although bigger wire is okay and usually better.

you think i'm gonna break my appliance if i use a slightly larger breaker than they reccomend?
Yes it's possible. The appliance passes safety testing with a 30A breaker, but could catch on fire during a malfunction with a larger breaker.

also, i installed 2 conducter wire, and it cost like 70 bucks. now it soounds like 3 conducter is needed sometimes.
Sometimes three conductor is needed for cooking appliances. The install sheet should tell you if the neutral is required or not. It's almost always needed with a freestanding range, but cooktops and ovens often only need two conductor.

so am i supposed to tear out and buy wire with my own money in case someone needs it in the future?
If the install instructions only require two conductor then that is all you need to do. If you haven't bought the wire yet, I would go three conductor; but if you already bought it then don't bother changing it if your oven will work with two conductor.
 
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Old 03-24-10, 08:07 AM
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thanks again you guys really are impressive...

Originally Posted by ibpooks View Post
The install sheet should tell you if the neutral is required or not. It's almost always needed with a freestanding range, but cooktops and ovens often only need two conductor.

If the install instructions only require two conductor then that is all you need to do. If you haven't bought the wire yet, I would go three conductor; but if you already bought it then don't bother changing it if your oven will work with two conductor.
yeah, the sheet i have doesn't mention anything about neutral. craziness.

i guess i'm not finishing my drywall this week. i'll have to call them, or just get 3 wire in case crap. WELL.

but seriously, thank you. i was not aware of the factors involved, which is of course why i posted this thread. you guys rock.
and like i said before, the more explanation, the better, i really like to learn new things and understand a little bit more how this world works.

peas.
 
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Old 03-24-10, 09:47 AM
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Do you have a make/model for the oven? Sometimes the electrical specs are on the manufacturer website so you can get a firm answer sooner.
 
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Old 03-24-10, 04:08 PM
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It should mention it in the instructions or show a drawing of the connections.
 
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Old 03-24-10, 05:41 PM
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If the voltage is 240 or 208, you don't need a neutral conductor. If the voltage is listed as 120/240 volt, you do need a neutral conductor.
 
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Old 03-24-10, 07:01 PM
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That is some pretty good deductive reasoning. Its obvious why they pay you the big bucks.
 
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Old 03-24-10, 09:48 PM
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Originally Posted by sberry27 View Post
That is some pretty good deductive reasoning. Its obvious why they pay you the big bucks.
It's actually pretty simple. Pay attention and you might learn something.

KW Rating
------------------
240V | 6.8 |
------------------
208V | 5.1 |
------------------
Breaker Size |
------------------
240V | 30 Amps |
------------------
208V | 30 Amps |
------------------
The OP clearly stated the info from the spec sheet.

It should mention it in the instructions or show a drawing of the connections.
No further instructions or drawings are necessary.
 
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Old 03-25-10, 10:44 PM
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ok boys, fight nice....




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Old 03-26-10, 08:04 AM
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I meant no disrespect, really it was good simple reasoning that went over some of our heads, one of the natures of forums.
 
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Old 03-26-10, 06:48 PM
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I like simple! Beer 4U2
 
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Old 06-07-10, 01:12 AM
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alright. NOW what?

so all the wire for this thing is in strands of some sort. the "solid conductor" 8 g is in little like 16 g strands, which i guess is normal for that guage, if it was really solid solid i'd think it'd be impossible to work with. but on the oven side the wire is stranded like little strands, normal stranded fixture type stuff. do i have to solder this or something?

i don 't wanna close up the box and have a problem waiting to happen, i twisted and wire nutted it, but i haven't shoved it in the box yet. tell me more, tell me more...
 
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Old 06-07-10, 05:57 AM
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I just twist each conductor clockwise separately to tighten up the strands. Then I twist the two together and install a twist-on connector.
 
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