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I have an old 220 (8/3 stranded with grnd) line off the main box from a stove that I don't use anymore (we have gas now). I want to use it to install a subpanel in my attic. The subpanel is 100amp, 6 space (12 circuit, main lug, indoor). The old stove line is coming off a 40amp double pole breaker in the main box. The main box is 200 amp. I have an existing 3" PVC tube up to the attic which is 30' away (coaxial, romex). The attic has vaulted ceilings well over 6.5' and is currently temp controlled. I plan to run an additional floor AC unit (900 watt, 8amp) and 3 - 1000watt tank heaters. I want to make any changes to what is above mentioned to make sure I don't burn up. Help!

Ideally, the plan is to have 5 - 15a circuits (3 in use regularly, 1000watt heaters) and 1 - 20a circuit (AC 900watt, 8amp. As needed for backup to main system) at the subpanel. The other 2 circuits not mentioned would be used for small electronics. How many watts is each circuit capable of handling?

Edit - Would using two smaller amp breakers in place of the 40 amp double pole reduce EVERYTHING in question, and enable me to use romex in the same tube with the rest? I would also switch to 1- 1000watt heater and 3- 300watt MAX. If so, would I not need the panel?

  • Did you just buy the subpanel? Take it back, they lied to you. The "circuit" number is useless, what matters is the spaces and 6 ain't near enough. – Harper May 17 '18 at 0:17
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    You can’t put 100amps on 8 gauge wire. You can’t mix line voltage and coax in your 3” pipe. – Tyson May 17 '18 at 0:22
  • Typically you are going to need #2 wire for 100 amps. There may be other factors involved that may require even larger wire. – user76730 May 17 '18 at 1:03
  • If I pull the coax, can the voltatge lines be in the same tube? The one in question as well as 6 romex lines to the attic/2nd story ceiling. – Jeff F. May 17 '18 at 8:10
  • Cord and plug connected, yes. They plug into standard outlets. – Jeff F. May 17 '18 at 8:37
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Take that subpanel back

I know the panel is marketed as "12 circuit (6 space)". This is bold-faced lying. This claim depends on using "double-stuff" breakers (2 breakers in 1 space) which cannot support GFCI or AFCI, required in almost every circuit today. Who cares, though? I need 6, I have 6. True, that's fine for today. But look how quickly you came up with this idea. Tomorrow you could come up with another idea that needs some more spaces. As R.M.E. discusses, you are electrically undersubscribed (especially if you go 300W for the heaters). Or you could later upsize the supply wire.

Swapping a panel is a nightmare job, and you pay for the panel twice. The right time to upsize the panel is when you buy the first one. I for one like to finish a job such 60% of the spaces are still unused. So here, a 16 or 18 space panel would be my minimum. Also, larger panels come with free "starter" breakers - and if a panel is $35 extra but comes with a $40 AFCI breaker, that's a win. Another win is when a panel comes with the mandatory ground bar(s) instead of chiseling you for another $8 for it.

Subpanel requirements

I smell the influence of big-box stores here. Buy cable staples there. Complex things like this, go straight to a proper electrical supply house. Their staff gives good advice, most are competitive on price, and they stock a wide variety so they are able to sell you exactly the right thing.

The panel must have a separate neutral and ground bar. The neutral bar will be tied to ground/chassis, and you'll need to remove that tie.

The panel needs to be at least 40A obviously; larger is fine. A 200A panel can handle 40A. Since you're in the same building, main-lug is fine, main breaker is also OK, and the main breaker size doesn't matter. (>=40A obviously).

AFCI/GFCI

Attics are ambiguous so you'll need to have a conversation with your AHJ (that's Authority Having Jurisdiction, or your local electrical inspector) about whether you need AFCI, GFCI or both on these receptacles. Since I doubt you're gingerly stepping on open joists at peril of putting your foot through the ceiling drywall, I rather expect this attic is at least partially finished. If it's finished enough to be livable space, then it definitely needs AFCI and that needs to be in the panel (or right next to it in an awkward deadfront configuration) since AFCI's main job is protect the wiring after the panel, and as such, stop house fires.

Given the presence of water and a heater immersed in water, GFCI also seems like a pretty good idea, but that can happen at the receptacle.

Cable

The 8/3 cable (also with ground, meaning 4 wires in total) will suffice for 40A, the breaker you already have there. If you need to extend the cable, remember you must make splices inside a junction box! And the junction box cover must remain accessible without tools (pulling an oven out is OK).

Also beware the building codes: Some Codes require that you provision a 40A service to the oven location even if you aren't using it right now. You are still OK if it is legal for you to remove the subpanel and its extra circuits and doing so won't violate any other building or electrical codes (e.g. location of receptacles).

That "conduit"

You're not using it as conduit. You're using it as a protective shield to protect the cable from damage. That means you do not need to foll the rules of the "conduit wiring method", but do need to follow the rules of the "non-metallic cable wiring method".

You simply cannot have data cables running with power cable. You also can't have low voltage (thermostat, doorbell) cables run with power cables. If you are installing raceway and anticipate sending both power and data up it, install 2 raceways at least 4" apart.

Since you're in split-phase 120V/240V power, this is simple: generally you can't have more than 4 circuits running together without having to painfully derate their capacity. Exactly 4 circuits is OK if wire size is #8 or smaller. For larger wire, you have to get out the sharp pencil and run the numbers. (The derate rules actually apply to 2 circuits or more, but that's only an issue for heavy feeder.)

Subscribing your power

First, what you have (4-wire, 40A) is 240V x 40A = 9600 VA of available service. If you are using 120V, you have two 120V "legs" of 40A each, or 4800 VA per leg. Do everything you can to balance the circuit, overloading 1 leg will cause both legs to trip down at the main panel. Re-read my answer on "double-stuff" breakers above for how poles are laid out inside panels, for more info on balancing.

A "continuous load" (potentially on for 3 hours or more) must be derated by 125% (i.e. a 1000W load counts as 1250W). Only you know your setup, but I assume if a 1000W tank heater ran at full power for 3 hours solid, you'd have fish soup. So I'm not derating those, correct this if I'm wrong. For a resistive heater, watts = VA.

  • A/C: 8A x 120V = 960VA x 125% = 1200VA
  • Heaters: 1000W = 1000VA ..... x3 = 3000VA (on at the same time after a power outage; otherwise intermittent). Now if the heaters are continuous loads, they count as 1250VA each.

So your installed, known loads take 4200VA (derated) out of 9600VA available, leaving 5400VA for your two receptacle circuits. You're fine.

In fact, I would pencil in 500VA for each receptacle circuit, and then you're at 54% of circuit capacity. I'm worried you have too few spaces, because this can support more.

By the way, this sort of "oversubscribing" panels - having 95A of breakers in a panel being supplied with 80A - is totally normal at much higher ratios than this. Use known loads if you know them, but for the loads you can't know, you pretty much count on not every circuit being used to the max at the same time.

If, indeed, you do not need to derate the tank heaters, you may be able to get away with 2 of them (1000VA each, derated) on a 20A circuit (2400VA). This could save you some money on AFCI/GFCI. One tank plus the A/C (1200VA, derated) would total 2200VA and would also fit on a 20A circuit.

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Reading through everyone's comments and answers I think everyone has helped you line out what you're going to be doing. So I am just going to jump to the beginning of the question.

If you are connecting (1) 900W AC and (3) 1000W tank heaters. Then that comes to 3900W and on a perfectly balanced 240V panel that would be 16.25A per phase, and you say there will be (2) extra 15A breakers for misc. So for the sake of calculation let's add 1500W per circuit or 3000W total. That still only gives you 28.75A per phase on a perfectly balanced circuit. Now for argument we give this a 25% imbalance between the two phases. That would mean one phase would be carrying 35.94A and the other phase the difference.

All this means is that you can reuse the 40A feed you have right now and install the Panel and if what you have given us is true, you will not overload the circuit. There are three things that must happen.

  1. You must keep the circuit on the 40A breaker since the #8 is rated for that 40 A's.

  2. Just because the Panel says it is 100A it really doesn't mean anything in this scenario.

  3. Make sure the Subpanel, the additional circuits and connections are properly installed and meets all NEC requirements.

If you are not capable of fulfilling these requirements then you need to seek out a qualified contractor.

Hope this helps and good luck.

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Im a little busy today and cant take the time to do all these calculations right now but I saw no one had answered you yet. You can google copper /aluminum amp capacity charts that will tell you right away. Please be careful though. There is a lot of other stuff involved, Like for instance balancing the load and why that is very important. Making sure why everything is done to code and why that is important. Calculating total load and demand factors. Installing neutrals correctly, It kind of goes on and on.

I think the main thing is, electrical stuff is so dangerous You have to make sure you dont miss anything or fallow bad advice you or your family or your home could pay dearly. If there is anything you dont know for sure dont do it until you find out from someone you KNOW is qualified. If you guess at anything or miss anything it could be a bad situation. I dont want to get in trouble right away on this site but How do you know how qualified anyone who answers you really is. Please just be careful safety is so important. I can tell by the type of questions your asking, you might want to get some professional help. lt took me a degree and twenty years for me to learn what I know and because of advances and changes im still studying things on a constant basis.

If there is ever a problem and insurance companies find out you did this yourself they very well may not pay on the claim. I know this isnt exactly what you were looking for but if I saved one life or dangerous situation today it was worth it. Take caree and please be careful. Respectively Jim

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    Welcome to Stack Exchange. Just so you know, going straight into "Hire a Professional" is not our format here. The scare tactics are uncalled for in the vast majority of cases (OP said "stranded" didja notice) and if it's discomfiting to you to advise people of unknown skill, there are about 100 other stacks. So take the tour and see if this place feels right. – Harper May 18 '18 at 16:21
  • If you don't have time don't answer (btw #2 copper was answered in the comments). I fully agree with @Harper I know there are several of us that support the questions on this site that are / were professional electricians if there were bad advice there would be multiple flames. – Ed Beal May 18 '18 at 18:49

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