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BillsBayou 10-01-2012 11:32 AM

Multi-Phase Circuit Breaker? Maximizing my 8-guage wire
 
How can I max out the capacity of 70 feet of 8-3 direct-bury wire to my greenhouse?

I have a 12'x12' greenhouse. I'm installing an exhaust fan, vent motors, circulating fan, heater, lights. And I'm going to daisy-chain a utility shed on the same line.

I calculated my need as 8 gauge for the amp load.

I ordered 8-2 direct bury wire from a friend who works at a electrical supply warehouse. He delivered 8-3. Hmmm... That's one more useful wire!

I was planning on running one 30-amp circuit breaker from the main panel on the house with a sub-panel at the greenhouse. The sub-panel would have 2 15-amp breakers.

Now, with the addition of the third wire, I'm thinking I can run 2 20-amp breakers at the sub-panel if I run them out of phase. HOW DO I DO THAT?

I cannot run two 20-amp hot with one neutral because the load on the neutral would be 40-amps. Too much, right?

AllanJ 10-01-2012 11:54 AM

When the two hot wires are on opposite sides of the 120/240 volt service then the neutral carries the difference, not the sum. So with approximately 20 amps drawn on each hot wire simultaneously the current on the neutral is approximately zero. The wiring is correct when you measure 240 volts between the two hot wires.

Perfect operation of two hots and one neutral, an example of a multiwire branch circuit. Also the most efficient way to feed power between two buildings.

Because you are typically not using the maximum on each breaker all at the same time, the sum of the breaker ratings in the subpanel is not limited to the available amperes i.e. the breaker rating for the feed in the main panel.

For 8 gauge wires you are permitted to feed with 40 amp breakers (40 amp double wide double breaker with handles tied together in the main panel; things will not burn up with 40 amps on any one conductor) although for a long run it is advisable to use a smaller breaker that would trip before the voltage drop got too severe.

Special bonus: If you compute a maximum of 20 available amps given the distance and voltage drop using just one hot and neutral for 120 volts, you actually get to draw more than 20 amps on each leg when you run the MWBC and the load is somewhat balanced between the two legs, before voltage drop due to the distance gets too severe. The calculations to prove this are a little too complex to include here.

Don't forget that the cable for the above operation has to have a ground wire as a 4'th conductor, namely be "8-3 with ground".

BillsBayou 10-01-2012 12:14 PM

So I should be ok with the following?

At the main breaker at the house:
Black - 30-amp breaker
Red - 20-amp breaker
White - Neutral shared by both circuits?

At the greenhouse sub-panel:
Black wire feeds 2 15-amp breakers for the greenhouse
Red wire feeds 1 20-amp breaker for the utility shed
White is shared Neutral

jbfan 10-01-2012 12:18 PM

Quote:

Originally Posted by BillsBayou (Post 1021138)
So I should be ok with the following?

At the main breaker at the house:
Black - 30-amp breaker
Red - 20-amp breaker
White - Neutral shared by both circuits?

At the greenhouse sub-panel:
Black wire feeds 2 15-amp breakers for the greenhouse
Red wire feeds 1 20-amp breaker for the utility shed
White is shared Neutral

Why cheat yourself?
Connect the black and red to a 40 amp 2 pole breaker

BillsBayou 10-01-2012 12:35 PM

Quote:

Originally Posted by jbfan (Post 1021145)
Why cheat yourself?
Connect the black and red to a 40 amp 2 pole breaker

If they're both coming off the same leg in the main panel, aren't they in phase? Thus, they'll overload the neutral.

I'm assuming that you have me feeding 40-amps to the sub-panel at the greenhouse where I break this down to two 20-amp breakers.

One additional note: This is NOT a grounded cable. I'm driving an 8-foot copper grounding pole at the sub-panel at the greenhouse.

BillsBayou 10-01-2012 12:48 PM

Ok. I'm getting it. I think. I'm going to do this with 30-amp breakers on each hot wire.

At the main panel at the house:
Black - 30-amp breaker on the right side of the box
Red - 30-amp breaker on the left side of the box
White - Neutral

I have two significant power needs in the greenhouse. First is the exhaust fan. The other is a heater. My plan was to feed two circuits off of one sub-panel. I was originally going to do this with one 30-amp breaker at the house feeding two 15-amp breakers at the greenhouse. Now I can run a larger panel at the greenhouse. One circuit for the exhaust fan circuit, one for the heater circuit, and one for the utility shed. Each building (they're immediate neighbors) will draw from their own 30-amp feed.

petey_c 10-01-2012 12:55 PM

BillsBayou, As AllanJ stated, "When the two hot wires are on opposite sides of the 120/240 volt service then the neutral carries the difference, not the sum. So with approximately 20 amps drawn on each hot wire simultaneously the current on the neutral is approximately zero. The wiring is correct when you measure 240 volts between the two hot wires." If the circuits are not of equal current the level will be the difference between the two.
If you're sharing a neutral the breaker that controls the two circuits has to be a two pole breaker. This way when one leg shuts down, so does the other. No surprises if you're working on the other circuit (current is still present on the neutral).
Pete

danpik 10-01-2012 01:10 PM

Quote:

Originally Posted by BillsBayou (Post 1021173)
Ok. I'm getting it. I think. I'm going to do this with 30-amp breakers on each hot wire.

At the main panel at the house:
Black - 30-amp breaker on the right side of the box
Red - 30-amp breaker on the left side of the box
White - Neutral

.

You need to use a double pole breaker. This will put the load on both "phases" of the service. The opposite sides in a service alternate every-other-one down each side in a panel not side to side. To run a Multi-wire branch circuit you will need a double pole breaker, two hot wires, a neutral and a ground wire as well as a ground rod at the unattached building if this will serve a sub panel

BillsBayou 10-01-2012 01:15 PM

GOT IT! Thanks to everyone.

BillsBayou 10-01-2012 01:17 PM

Last question, then:

This is 8-3 wire, it is not grounded. I'm driving an 8-foot copper grounding pole at the greenhouse's sub-panel. Any problem with this?

AllanJ 10-01-2012 01:33 PM

You need two 8' ground rods at least 6 feet apart. Use a #6 bare copper wire to connect them to the panel.

The ungrounded cable is not going to pass inspection unless it is in a metal conduit to be the equipment grounding conductor. Presently I don't know of a good workaround for this. Unfortunately, buying the wrong material is one of the pitfalls of not seeking help first from professionals.

BillsBayou 10-01-2012 01:46 PM

Quote:

Originally Posted by AllanJ (Post 1021210)
You need two 8' ground rods at least 6 feet apart. Use a #6 bare copper wire to connect them to the panel.

The ungrounded cable is not going to pass inspection unless it is in a metal conduit to be the equipment grounding conductor. Presently I don't know of a good workaround for this. Unfortunately, buying the wrong material is one of the pitfalls of not seeking help first from professionals.

Even though this is rated for direct burial?

AllanJ 10-01-2012 02:13 PM

Here the conduit is not so much needed to protect the cable from water but rather is needed for ground bonding back to the the house. Running a separate ground wire in the trench, although it works functionally, does not meet code.

mpoulton 10-01-2012 02:28 PM

You cannot use an ungrounded cable. How did you find 8-3 without ground, anyway? Exactly what kind of cable is this? 8-3 ungrounded UF cable probably hasn't been manufactured for decades, which leads me to think you have SE cable or something, but that's not usually found in sizes that small. Regardless, you need different cable, or metallic conduit (which is not a good solution here). Ground rods at the greenhouse are required, but they exist only to help limit the voltage difference between the earth and the grounding system at the outbuilding. They cannot take the place of a grounding conductor going back to the main panel (where neutral and ground are bonded together). Ground rods have very little to do with the operation of the grounding and bonding system.

BillsBayou 10-01-2012 02:34 PM

Lesson learned. I'm going to have to go back to my source and get the correct wire.

This is new, cut to order wire, so I don't think it's old.

Well, the cost of doing it over and doing it right is always going to be less than doing it wrong from the get go. ESPECIALLY with electricity. So to hell with the cost I've already shelled out. Time to take it to the recycle yard.


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