Hey, everybody! Hope you’re all well, and everyone’s gardens are running smoothly.
We’re between runs here, and getting some other chores done in the meanwhile, including replacing a bathroom exhaust fan. Long story short, that project met an impasse when we found some weird wiring installed before we owned the place. Gonna have to call in an electrician to figure that out, but it’s all got me thinking about the wiring in the garden.
As a disclaimer, I am not an electrician, and may have some things about load capacity and the like backwards in my head.
We grow in a spare bedroom, which is on its own 15 amp breaker. That’s split between 4 standard grounded American 115V outlets and one switch for the exhaust fan. With all our equipment running at full-bore, which never happens, we have calculated our Wattage at about 756W and Amperage at something along the lines of 7.09A.
My main question to anyone with a better understanding than me would be if that load seems safe for the size of the circuit? Even if it is, is there any recommendation to shore up the supply for that room? We do have extra space on the house’s panel for expansion. Would putting that room on a 20 amp breaker be a safer idea? Would it be better to split the outlets to two seperate circuits? How do I even calculate the load to check if I’m at the recommended 80% or lower to the max capacity?
We’ve never blown that breaker, or noticed any strange heat sources, etc. but we’re kind of in the dark on this, no pun intended, unsure of what to even ask a pro for possible help with. (We’re in a legal state and muni, so no concern of the contractors outside of general security)
Thanks in advance for any tips or advice.
Stay up!
Coffin_Dodger
Spanish here, 230V and normative/laws for installations sure are different. But …
The breaker is for the cables/wires be safe. If you put a 20Amp breaker but the wires are sized for 15Amps, that is not good and could cause fires.
You have to see the section of the wires (I think that there it is called AWG) to check if the wires already installed support 20A.
Anyway: P= U*I. If your breaker is 15A and working 115V. The max for that would be 1725watts…and the 80% of that is 1380W.
Here the installations are designed to could handle 100%. The safe side is done with the design/calc of the lines to support a little more than that. So if here we have a breaker of 15Amps, probably the line/wires installed could handle around 18-20 Amps. (Telling from memory of a stoner brain…take that in consideration)
Okay, so I’ve seen that 1380-1400W number used in calculations of max loads here, so that makes sense. Our wattage draw being actually 756W (or less) puts us well under that, right?
So I’m probably overthinking it unless we add something heavy like a dehumidifier or heater?
Thanks for the reply!
Stay up!
Coffin_Dodger
I’m not and electrician either @Coffin_Dodger, but I have some background in the field and I’ll suggest that you’ll be much happier in the long run if you use your extra breaker box slot to add a 20 amp GFI breaker, running to your grow room via 12 gauge grounded wire for a dedicated outlet in your room.
Electrical demand in the grow room has a way of growing over time and drawing from two different circuits will keep you safe and happy over the long run.
-Grouchy
PS, I went through the same dilemma, paid the bill and enjoyed the peace of the new 20A circuit. I also traded the contractor an oz of heinous bud for a discount!
I think this is a much more experienced description than I could give of the basic idea I was thinking of. And I definitely hear you about grows having a way of growing equipment alongside the plants! hahah
Got a better idea of how to pitch it to the guys who come for estimates, anyway.
Thanks!
Stay up!
Coffin_Dodger
just be aware that a 12ga wire run is only good for about 50’ (20amps)after 50’ you need heavier gauge wire. 10ga at least.
first calculator i found
http://www.paigewire.com/pumpWireCalc.aspx?AspxAutoDetectCookieSupport=1
If you are getting rewired anyway you might as well put in more power too.
But that load looks fine to me. I run about 450w on a 15 amp circuit. It just means there isn’t room for much else.
Gaming computers, whatever. Depending on where you live there no it may not be a reason to lie. Up here it’s normal to say “I’m putting in a weed room.”
Electrician here! I work in Canada to the rules might be different depending on where you live.
Here is how the math breaks down. In Canada, when calculating circuits allowable ampacity, as per code, you can load each breaker to 80% of its rated load. 15A * .80 = 12A (each 15A circuit in Canada is allowed 12 plugs rated for 1A each. 12A * 120V (north American standard voltage) = 1440W - this is how much you can safely load a 15A circuit in Canada.
Edit - Just read through some of the other comments and if you are getting a new, dedicated circuit run ask the electrician to run a 12/3 to the grow room. its called an edison 3- wire circuit and it allows you to have 2 circuits that share the same neutral back to the panel. try to split the loads evenly on both circuits for best practices.
Also if your in a legal state, just ask the electrician if he will work for weed, most of us will or dont give a shit about what your growing 
I had to add a few new circuits when I started growing in the basement. I told the electricians it was for crypto mining; a few of them shook their heads, presumably because they thought it’s a silly fad or because home mining can’t possibly make any real money, but it’s not illegal yet. And yeah, with the numbers I’m seeing you don’t really need more power, but if you’d rather be extra safe and have room to expand later it won’t hurt to add a bit more. Worst case, you might find yourself needing a new tent eventually. 
This is what I understand to be correct as well. Only use 80% of the breakers capacity.
this applies to new installations when determining allowable ampacity on a circuit. in reality its over kill and I would be confortable loading breakers up closer to 15A but its a rule for a reason.
Gotcha. So still sounds like the numbers I’m running should be just fine, unless things get much bigger, like @GrouchyOldMan mentioned.
That’s where I’m pretty much at, depending on how crazy the estimate gets.
Oh, for sure. We’re legal, but urban and residential- security culture is still some concern, but no big worry. Mostly meant having the language to describe what I might need to the contractors, like.
Thanks again, guys! Already feel more comfortable about what I’ve been running, but might just have to invest in a little preemption.
Stay up!
Coffin_Dodger
other things to consider are the type of loads you are running. things like fans, heaters and dehumidifyers have motors that can have inrush cureent when starting and stopping constantly. this can lead to nusisance tripping if the circuit is heavely loaded already. If you are getting new circuit run into the room, and its hard to do, oversize everything. its always better to future proof.
A friend of mine is an electrician and he told me a story about how he went to some ladies house did a tiny little thing and was done in 10 minutes.
He said “I feel bad because I have to charge you for at least 3 hours. Is there anything else you need? Run to Home Depot but a new light fixture and a new ceiling fan or something and I’ll put them in for you. I can wait a little.”
He’s a nice guy though.
So the only trouble with this is if your load is ballast lighting the current on the neutral can be greater than the hot wires due to 3rd harmonics. Not sure how led and the newer drivers do with this and it is not a factor unless the load on each leg is at or near the max for the circuit and lighting is most or all of the load on both. In these cases it has been determined the neutral should be one size larger than the power wires.
Also you are not likely to run into this in a residential setting.
This made me laugh, my chimney sweep was just here and he wanted to check the chimney vent for the furnace, located in my (currently empty) growroom. I took him down there and after a long look at my OverGrow poster on the wall, he started laughing and asked if I ever traded weed for work! A new “Best Friend” 
Haven’t talked about harmonics in ages! While what you are saying is technically true, I would argue it is very unlikely. Not that I’ve seen it all, but I have literally never heard of a harmonics issue with any residential installations ever. Harmonics can be created by any non-linear load, so sharing any neutral could create them, an example is the blender in my kitchen shares a neutral. My experience with harmonics has been with very large imbalances in non-linears loads in large 3 phase installations. But I guess if you were factoring in all possibilities its something to consider. I would need a list of all his equipment and their specs to really decide, also need to see the space as there could be lots of easy options.
The one instance I saw this was 277 lighting in a hospital with the lights all the load on the circuits. As I said not likely to see this in residential. I also consider there are others reading these threads and I venture another reader coming behind and doing something commercial with fully loaded lighting circuits.
Yeah its really cool when that stuff comes up. I’ve only seen it twice!
I really hope, that if you are doing a something commercial with fully loaded lighting circuits, you are not getting your plan from the comments sections of OG