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Old 04-08-2009, 07:31 PM #1
Phillthy
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Growroom Electricity and Wiring

Since there are many, many threads with people asking how to wire this and what wire to use for that, I thought I'd make a thread to have those with some knowledge share it with those that may not have as much. Hopefully we can get this sticky so it can help people in the future.

I will start off with a couple of simple diagrams and charts (not mine ) that may help explain the very basics and hope that others will add diagrams or photos to explain things that weren't covered. I welcome anyone's input so lets get going...

Basic wire color/wire gauge chart:


Wiring size/amperage/voltage chart:


Wiring up a basic outlet: If only one outlet per circuit follow picture to the right.


Wiring up a GFCI outlet:This shows how to wire a GFCI outlet that is NOT protecting the outlet down stream. To protect that one as well, wires going out to the next outlet would use the "LOAD" terminals of the GFCI outlet.


The green grounding screws are located within the metal outlet housing.

I hope this gets the ball rolling. Let's share what we know
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Old 04-08-2009, 09:29 PM #2
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Old 04-09-2009, 05:44 AM #3
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That's great Phillthy. Here are some more basics for the noobs:

Watts = Volts (usually 120/240 in the States) x Amps. If a wire ever feels warm to the touch, replace it with one of higher gauge. 120 volt wiring has a hot (black), neutral (white) and ground (green/bare). 240 volt wiring has NO neutral, two hots (red/black) and a ground.

A 240 volt breaker simply takes two slots in your breakerbox, to supply two hot lines. You can install a GFCI circuit breaker if you do not wish to use all GFCI outlets.
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Old 04-09-2009, 08:03 AM #4
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Yes. Great idea. Can't believe there is no sticky already on the basics of wiring.

Also, the ampacity ratings on the above charts are already calculated for continuous loads (equipment runs 3 hours or more). You can actually draw up to 1800w on a #14 AWG 15 amp circuit. But for the purpose of grow rooms, you should consider everything to be a continuous load and size accordingly.

Some nifty equations:

P = I x E
E = P / I
I = P / E
E = I x R
R = E / I
I = E / R

P = Watts
E = Voltage
I = Amperage
R = Resistance
d = Drop
K = 12.9 for copper & 21.2 for aluminum
L = Length

CMA values (Circular Mil Area or diameter of individual wire):
Note: I'm not listing 16 or 18 gauge wire because you shouldn't use it period for anything. (unless its the cord on a small fluorescent fixture)

#14 AWG = 4110
#12 AWG = 6530
#10 AWG = 10380
#8 AWG = 16510
#6 AWG = 26240
#4 AWG = 41740
#3 AWG = 52620
#2 AWG = 66360
#1 AWG = 83690
1/0 AWG = 105600
2/0 AWG = 133100
3/0 AWG = 167800
4/0 AWG = 211600

Voltage Drop should also be considered when connecting loads to an existing circuit or running a new circuit.

For Single Phase Residential Service Only:
NOTE - The "2" is to account for both ways. The power goes out to the load and then returns back to the panel. For commercial/industrial 3 phase loads, you would use 1.73 (the square root of 3).

Ed = K x I x L x 2 / CMA

CMA = K x I x L x 2 / Maximum Ed you want

L = CMA x Maximum Ed you want / 2 x K x I

Maximum Ed should never exceed 5% at the farthest/last receptacle. So:
120v circuit would be 120 x .05 = 6v
240v circuit would be 240 x .05 = 12v

Example:

Joe needs to run a new circuit to his bloom room in the attic. He has 1 x 1000w HPS ballast and 1 x 600w ballast @ 120v that will be connected to this circuit. From panel to receptacle will be 250 feet "as the wire travels". (It's a really big house). What is the minimum wire gauge Joe can use and what size breaker should it be connected to?

I = P / E
1600w / 120v = 13.33 amp draw

We know #14 AWG is already off the table by looking at the illustrated wire charts from the first post.

Easier just to use the "CMA =" equation since we are looking for wire size. All standard residential cable is CU (copper). Don't use AL (aluminum) for anything but feeders to a panel.

CMA = K x I x L x 2 / Maximum Ed you want
CMA = 12.9 x 13.33 x 250 x 2 / 6
CMA = 14330

Joe would need #8 AWG to stay under 5% voltage drop!!!! That's some big bucks in cable for 250 foot. Joe would also want to use a 20 amp single pole breaker in the main panel because the total wattage exceeds what is listed in the first post for a 15 amp circuit. You want the wire and the breaker to stay cool. You can't use a 30 amp breaker unless the receptacle is rated for 30 amp (even tho the wire is rated for more than 30 amp).

Pretty sure you also can't fit #8 AWG on a receptacle screw :wink:, so you would want to transition down to #10 or even #12 to attach to the receptacle, but not #14 AWG because it's not rated for that amperage. If you transition down to #12 for the connections, then you can not exceed a 20 amp breaker.

Your whole electrical run is only rated for the weakest link.

Last edited by madpenguin; 04-16-2009 at 12:10 AM.. Reason: Added cmil for larger wire sizes
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Old 04-09-2009, 08:09 AM #5
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Also, always run a ground to everything. Most ballasts will fail to ignite if they are not properly grounded.

Case in point, I had a fluorescent fixture in my basement that was wired with 2 conductor lamp cord (no ground). I had to grab a copper water pipe and then touch the fixture before it would came on... Re-wired it with 3 conductor cord and now it's not an issue. You should always have a ground for safety reasons anyway.

Also, if ever wiring a 240v circuit, just buy 12/2 since you don't need a neutral. But make sure you re-identify the white wire with black or red electrical tape ( at all accessible points) to warn that the white conductor is now a hot.
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Old 04-09-2009, 08:31 AM #6
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Oh... This is a big one.

NEVER, EVER "backstab" a receptacle. Those are the little tiny holes that you jam your wire into. I've had more service calls than I can count because of this.

The mechanism that holds the wire in place is spring loaded. Over time, the heating of the wire will wear out those springs and then the connection becomes weak. Once that happens your resistance shoots thru the roof and as resistance increases so does heat. Build up enough of it and you'll completely melt the receptacle and start a fire in the outlet box.

Got some pretty neat/scary pictures of a few of them. If I can find them, I'll post em'.

Your best bet is to buy the "spec grade" receptacles. They have the screws on the side, but slots that you slide the wire into. Then you screw down the terminal and a brass plate sandwiches the wire. They are made way better (and cost a little more too).
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Old 04-09-2009, 02:26 PM #7
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Ohms law is good to know but not for sizing circuits using the wattage of an appliance.Always use nameplate amperage rating when sizing circuits.Anything over a 100 ft run could cause voltage drop issues,upsize 1 wiresize per 100ft of circuit.
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Old 04-09-2009, 05:30 PM #8
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Yes, good comment. Wattage of bulbs doesn't always jive with what the ballast actually draws. Always go off the nameplate.

That's a pretty decent rule of thumb for wire length/size too. But seeing as how the voltage drop equations are so easy and quick, you should probably figure it out manually before hand just so you know.
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Old 04-09-2009, 06:34 PM #9
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Old 04-10-2009, 03:34 AM #10
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Thank you for your additions! Keep them coming!
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