6/2 romex vs 6/3 for 50A 240v circuit

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Old 06-06-09, 02:30 PM
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6/2 romex vs 6/3 for 50A 240v circuit

We are having a central a/c system wired up soon. Someone told me you can use 6/2 romex vs 6/3. I always thought for 240v, you needed red and black (2 hots). With 6/2, would you be using the white for the second hot? Is that acceptable? Where would the neutral be in that case?

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Old 06-06-09, 03:15 PM
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6-3 is used for a 120/240 circuit but an AC generally uses straight 240v so no neutral needed. Yes, in the USA you would remark the white wire black. or red.

Where the confusion comes in for novices is they are familiar with stoves and dryer that have some parts that run on 120 volts and some parts that run on 240v. Because some parts are 120v the neutral is needed.
[Where would the neutral be in that case
There would be no neutral. A 240v circuit does not need a neutral. Electric is provided by a step down transformer with a center tapped secondary. Between the ends of the secondary winding you have 240v. From the center tap to either end you have half the number of turns of wire and half the voltage, 120v therefore. It is that center tap that provided the neutral.
 

Last edited by ray2047; 06-06-09 at 03:32 PM.
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Old 06-06-09, 03:37 PM
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Learn something every day I guess. So the only path back is the ground?

Is the same true for the air handler?
 

Last edited by steveg_nh; 06-06-09 at 03:55 PM.
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Old 06-06-09, 05:40 PM
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Normally the outdoor compressor unit useally are stright 240 volts very rare they need netural for it.

Make sure you double check the nameplate for rating like max OCPD it will say " fuse/circuit breaker XX amps " or " Fuse max XX amp " if you get the latter part say fuse only then you must put a fuse disconnection switch but if say both fuse and breaker then the breaker inside load centre will act like master breaker for outdoor unit however you still need a disconnection switch outside near the unit.

As far for indoor airhandling unit it will depend on nameplate due some are wired for 120 volts and some wired for 240 volts so that something you have to watch out on that part.

Most case 6-2 useally work very well with large outdoor unit just remark the white conductor to black or red or bleu but for indoor airhandler unit it will depending on the set up they can run anywhere from 14-2(2.5mm˛) all the way to 6-2(16mm˛) or larger if the indoor unit have back up electric heat in there that will useally get them if not carefull with conductor size.

Merci,Marc
 
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Old 06-06-09, 05:57 PM
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Originally Posted by steveg_nh View Post
Learn something every day I guess. So the only path back is the ground?

Is the same true for the air handler?
what path back?

If you are referring to the electricity, the path back is the second conductor. The only thing the ground is for is a path for fault current which, under normal circumstances, it not present.

240 volt systems only require 2 hot. the ground is for safety.

with a 120/240 system, you have two hot legs and a neutral. You have 240 volts between the two hot legs and 120 from the neutral to either hot leg.

if your appliance is listed as 240 volt, you only need 2 conductor (plus the ground). If your appliance is listed as 120/240 volt, you need 3 conductor (plus the ground).
 
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Old 06-06-09, 08:46 PM
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The current flows between the two hot conductors. In single phase AC, which most residential is, when one hot conductor is on the positive of the sine wave the other is on the negative. Thus, current will flow between them. Just like on a 120 volt circuit the neutral is always at zero (not pos or neg) and the hot conductor will be either positive or negative. All this is changing at 120 times a second. Or 60Hz (in the US)
 
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Old 06-06-09, 08:55 PM
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It's funny, when I started out I asked a similar question. I asked "where is the neutral return for this 3 phase motor, how can you connect 3 hots but no neutral).

I had a 20 year veteran electrician tell me that neutral was derived from the ground wire He tried to explain it by making things up on the spot.

I guess that just goes to show how "electricians" are becoming simple "installers".

I know this is off topic, but your question reminded me of this.
 
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Old 06-07-09, 04:52 PM
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I guess I just didn't think of a difference between 120v and 240v devices. Interesting.

The condenser says it's voltage/Hz/Phase is 208/240-60-1. Its minimum circuit ampacity is 33A. The maximum overcurrent protection, using either time delay fuses or HACR circuit breakers, is 50A. So we're using 6/2 and a 50A breaker.

The air handler says it's voltage is also 208/240. Its minimum circuit capacity is 7.8A/7.8A. The maximum overcurrent device/protection, is 15A/15A. It doesn't say anything about fuses or breakers. So we're using 14 gauge wire and a 15A breaker.

So in both cases, these are 240V only devices right?

Thanks for all the tips and the education!
 
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Old 06-07-09, 05:00 PM
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Originally Posted by steveg_nh View Post
I guess I just didn't think of a difference between 120v and 240v devices. Interesting.

The condenser says it's voltage/Hz/Phase is 208/240-60-1. Its minimum circuit ampacity is 33A. The maximum overcurrent protection, using either time delay fuses or HACR circuit breakers, is 50A. So we're using 6/2 and a 50A breaker.
Yep that is correct you are good to go on that part.


Originally Posted by steveg_nh View Post
The air handler says it's voltage is also 208/240. Its minimum circuit capacity is 7.8A/7.8A. The maximum overcurrent device/protection, is 15A/15A. It doesn't say anything about fuses or breakers. So we're using 14 gauge wire and a 15A breaker.

So in both cases, these are 240V only devices right?

Thanks for all the tips and the education!
Again that is proper setup both are straght 240 volt devices no netural conductor need at all.

Just remember to remark the white conductor to black or red or bleu { this only applied to the cables not the indivual conductors } then you are good to go and set up a nice penquin play pen in the room

Merci,Marc
 
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Old 06-07-09, 08:47 PM
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Since the unit only requires 33 amp circuit capacity you could use 8-2 copper.
 
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Old 06-07-09, 08:59 PM
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Thanks PCboss for catching that one I need extra >>

Merci,Marc
 
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Old 06-08-09, 07:43 AM
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I would imagine since he's using a 50A breaker, that's why 6/2 is being called for.
 
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Old 06-08-09, 08:02 AM
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The breaker size is spec'd for the startup inrush.

The wire is sized to the running load which was stated as a 33 amp minimum. Number 8 copper would be fine for this load.
 
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Old 06-10-09, 08:24 AM
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Thanks for all the help guys. Sounds like we're good.
 
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