electrical question

[gerard]

Wasn't quite sure where to post but I have an electrical wiring question for those of you better versed in wiring than I. Here's the situation. I have a 60 amp breaker sub panel in my house that feeds my barn. The wire looks to be adequate for that service, (looks about the same size as that used for an electric range). This breaker feeds a 100 amp sub box in the barn that has space for 6 breakers. 5 breakers are in use with five circuits. three 15 amp breakers and two 20 amp breakers, one vacant spot. We added a three stall additiion that I now need to power. My plan is to power it with a new 20 amp breaker using 12/3 for about a 150 foot run. Using three wire would allow me to put some outlets on one hot wire and the lighting on the other. My friend says that would be overloading the box because the total amp rating (adding up all the breakers) exceeds 60 amps. He says you would burn up wires because you could draw too much current. My understanding of electric is IF you maxed out every available circuit (which would not happen anyway), the 60 amp breaker would trip. Am I missing something here? I have 150 amp service coming in to the house and if I add up all the circuits there it's way over 150 amps. To me all that means is I couldn't run every circuit to the max at the same time without tripping the main breaker. Any thoughts or is my logic sound??

 

[Inspector507]

gerard,
You're good to go. Now tell your friend that has no idea about electric to play on his tractor /forums/images/graemlins/grin.gif

 

[gerard]

thanx for the quick reply - I guess I was just looking for some validation of my 42 odd years of practical wiring experience and common sense............ /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif

 

[DavidJ]

I-507 is right, you're fine with that setup. You could put 1000 amps worth of breakers in there and as soon as you exceeded 60 amps you be in the dark /forums/images/graemlins/smile.gif. One thing you should do is double check the wire size. It needs to be at least #6 awg, that's good for 70 amps. You said <font color="blue"> (looks about the same size as that used for an electric range). </font> The wire to each of my units is #8 awg, which is rated for 50 amps.

 

[BB_TX]

<font color="blue"> Using three wire would allow me to put some outlets on one hot wire and the lighting on the other. </font>
Is this just for convenience of connecting the devices? If you are thinking of reducing the current thru each wire remember that the neutral wire will be carrying the full current for both.

 

[gerard]

Partly for convenience but also do to the fairly long run, I figured this would almost be like upsizing the wire to reduce voltage drop and reduce the current being carried by one wire since in effect the max 20 amp draw would be split among 2 12 guage wires instead of 1. I realize there's only 1 neutral but I figured the current that the neutral carries is reduced by the amount of current that is "used up" by the light/fan or whatever load is on the circuit. Is that accurate? Guess I never really thought too much about the current that the neutral carries......... Maybe my using two hots won't make much of a difference?

 

[Inspector507]

gerard...........I'm confused a little. You said you only have ONE vacant spot in the subpanel. But you're going to connect TWO circuits.
If you use a 12/3, and connect them to opposite "legs" in the subpanel, the neutral will not carry much current, if you have the loads balanced. If you connect them to the same "leg" the neutral will carry the combined load and will overheat.

 

[WVBill]

</font><font color="blue" class="small">( I figured the current that the neutral carries is reduced by the amount of current that is "used up" by the light/fan or whatever load is on the circuit. )</font>

The load doesn't "use up" any current. The amount of current flowing in the hot wire(s) is the same as the current flowing in the neutral wire.

This is why a GFCI works. If the current in hot and neutral is NOT the same, the GFCI says "Hmm... the missing current must be going SOME where BAD." (like to ground through a body) and trips.

 

[gerard]

Think I was probably thinking about this the wrong way and wasn't thinking about using two separate legs. I guess I have to check my subpanel and make sure I have 220 coming in there, then use a double pole breaker to run two circuits? Or should I just use 10 or 12 2 wire for the whole thing and just use a single 20 amp breaker? I don't plan on a large load out there other than some lighting and three, 130 watt heated water buckets. (I'd rather oversize than undersize but don't want to "go crazy"

 

[BB_TX]

A basic principle of an electric circuit is that it must be a closed loop. That is, all the current going out of the source to the load must be able to return to the source thru another path. Therefore the current in the return wire (neutral) is the same as the current in the source wire (hot). You may have heard comments about "losses" in an electrical circuit. But that is losses of power (or the ability to do work) and is generally in the form of heat. The heat you feel on an electric motor for example is losses due to the inefficiency of the motor itself.