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-   -   Wire gauge for 80 amp continues load (http://www.diychatroom.com/f18/wire-gauge-80-amp-continues-load-164838/)

diyguy41 11-27-2012 12:37 AM

Wire gauge for 80 amp continues load
 
Hey there guys,
I am new to the forum but certainly not new to the DIY world. My back can attest to that:)

At any rate, I am running some serious electrical current for a project I am working on and as these things can be serious, I wanted to double check with those in the know before I go out and purchase anything.

So basically I am going to need to pull 80 amps of 240v continous load off of the 125 Amp panel in one side of the gargage and wire it to a service panel on the other side of the panel.

I plan to install a 100amp breaker in the main panel and then run 3gauge copper THHN for the two hots. I know that I have read somewhere that the ground can be a smaller wire?

Also, there seems to be some debate on wheather or not I need to run a neutral wire. EVerything off the service panel is going to run off of 240. The service panel will have 5 20amp circuits in it, each of them drawing about 19.4 amps. I have read people claiming that you HAVE to run a neutral wire to a service panel, and at the same time, a hot tub disconnect or other type of disconnect you don't need to run a neutral wire.

Any help would be appreciated.
Thanks!!

kbc1 11-27-2012 06:12 AM

for a sub panel you need to run two #3 hots, a #3 nuetral, and a #6 ground would be fine, keeping them isolated in the sub panel with a separate ground bar, as for using 19 amps of a 20 amp circuit, it is usuallt good practice to keep the load down to 80% , so i would consider splitting some of those circuits up, will the sub panel be in a attached or detatched garage? what size conduit will you be using?

mpoulton 11-27-2012 10:23 AM

You say that the load is "continuous". If it is actually "continuous" per the NEC's definition, then you must not load any circuit above 80% of its rated capacity.

J. V. 11-27-2012 11:21 AM

What is the load?

Glennsparky 11-27-2012 01:18 PM

No neutral is necessary. If you put one in, it can be no smaller than the ground wire. I don't have my codebook with me. I believe #8 is the smallest size ground or neutral for 100A. Considering the cost of the wire vs. the amount of work, I often put in the neutral for future projects. A 19.4A continuous load will need a 25A breaker. 12 wire may or may not pass code depending on the type of load and wiring method.

diyguy41 11-27-2012 02:25 PM

Thanks a bunch for the info.
The load on that circuit consists of green house lighting for ornamental plants. Just purchased an older greenhouse and we are revamping it to modern standards.

Each individual breaker will be 10/2 wired to a relay with a timed trigger. 2 of the 4 relay triggers will be wired to a 15 second delay relay to avoid overloading the circuit when the bulbs are fired up. Additionally, 3 of the four relay triggers are wired to a thermostatic controller that will shut 3/4 of the lights off if the greenhouse exceeds 94 degrees. The 10/2 from the relays is wired to 3 double 6-15r receptacles. Wires coming in and out of the receptacle are installed in the same side and clamp of the receptacle to avoid passing current through the receptacles in a daisy chain type set up. Didn't want to include all that info and confuse people.

I have a schematic of that controller if anyone is interested.

mpoulton 11-27-2012 03:03 PM

You shouldn't need #10 wire for 20A circuits. These runs aren't likely to be long enough for voltage drop to require up-sizing the conductors, so #12 is fine. You also shouldn't need delays between lamp startups, since the 80% load factor provides plenty of overhead and most lighting doesn't have a significant startup surge. If the greenhouse is not attached to the garage, then you need to put the new panel in the greenhouse, not in the garage. If it's attached to the garage then having the panel and other equipment in the garage instead of the greenhouse is probably better since it spares the equipment from humidity. Greenhouses are very hard on electrical equipment. But If the greenhouse is IN the garage, then the plants probably aren't "ornamental"...

kbc1 11-27-2012 03:10 PM

Quote:

Originally Posted by Glennsparky
No neutral is necessary. If you put one in, it can be no smaller than the ground wire. I don't have my codebook with me. I believe #8 is the smallest size ground or neutral for 100A. Considering the cost of the wire vs. the amount of work, I often put in the neutral for future projects. A 19.4A continuous load will need a 25A breaker. 12 wire may or may not pass code depending on the type of load and wiring method.

how do you not need a nuetral, how would he get 120 volts without one?
when installing a sub panel for 100 amps you use two #3 thhn for the two hots, one #3 thhn for nuetral and a #8 or 6 for ground to keep them isolated from each other

Dave632 11-27-2012 03:43 PM

Quote:

Originally Posted by kbc1 (Post 1061793)
how do you not need a nuetral, how would he get 120 volts without one?
when installing a sub panel for 100 amps you use two #3 thhn for the two hots, one #3 thhn for nuetral and a #8 or 6 for ground to keep them isolated from each other

From the OP: EVerything off the service panel is going to run off of 240.

If there's no need for 120, then there's no need for neutral in this situation, it's a 240V system exclusively.

stickboy1375 11-27-2012 03:43 PM

Quote:

Originally Posted by kbc1 (Post 1061793)
how do you not need a nuetral, how would he get 120 volts without one?
when installing a sub panel for 100 amps you use two #3 thhn for the two hots, one #3 thhn for nuetral and a #8 or 6 for ground to keep them isolated from each other

Because all the OP's loads are 240volts, you are not required to run a grounded conductor (neutral) to a sub-panel.

kbc1 11-27-2012 03:49 PM

Quote:

Originally Posted by Dave632
From the OP: EVerything off the service panel is going to run off of 240.

If there's no need for 120, then there's no need for neutral in this situation, it's a 240V system exclusively.

ah didnt read that part now that makes sense, i didnt know there were no 120 volt loads

stickboy1375 11-27-2012 03:51 PM

Quote:

Originally Posted by kbc1 (Post 1061822)
ah didnt read that part now that makes sense, i didnt know there were no 120 volt loads

The last paragraph was pretty clear in the OP's first post. Kind of helps to read the entire post before posting advice, it just adds to clutter and confusion.

kbc1 11-27-2012 03:57 PM

Quote:

Originally Posted by stickboy1375

The last paragraph was pretty clear in the OP's first post. Kind of helps to read the entire post before posting advice, it just adds to clutter and confusion.

good point i was just answering the questions

Glennsparky 11-27-2012 04:09 PM

Quote:

Originally Posted by mpoulton (Post 1061789)
You shouldn't need #10 wire for 20A circuits. These runs aren't likely to be long enough for voltage drop to require up-sizing the conductors, so #12 is fine.

Maximum breaker for #12 NM (NM-B/Romex) is 20A. Since he needs a 25A breaker. It's either upsize the NM or use a different wiring method.

And, does somebody have a code book? What's the de-rating at 94 degrees?

diyguy41 11-27-2012 05:11 PM

That is a good point about the 12/2 vs 10/2 in the receptacles I could route things to run 4,200 watts per circuit which is around 18 amps I was just concerned about bumping up to the 85% rule. On the flip side, that has to be a little on the conservative side, no?

One last question, what size PVC conduit am I looking at in order to run two #3 and a #6 wire. Is that 2"? It is a 24 foot run with a 90 degree bend on each end. Not going to be terribly fun I imagine :(

More fun that cutting an egress window in my 110yo foundation. Did that last year. Not messy at all LOL

Thanks again guys!


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