Wire gauge for 80 amp continues load

diyguy41 · 11-27-2012, 01:37 AM

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, 07:12 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, 11:23 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, 12:21 PM

What is the load?

Glennsparky · 11-27-2012, 02:18 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, 03:25 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, 04:03 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, 04:10 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, 04:43 PM

Quote:

Originally Posted by kbc1

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, 04:43 PM

Quote:

Originally Posted by kbc1

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, 04:49 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, 04:51 PM

Quote:

Originally Posted by kbc1

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, 04:57 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, 05:09 PM

Quote:

Originally Posted by mpoulton

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, 06:11 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!