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Update

Based on many inputs a much better solution is the heat pump. Big thanks to Harper and manassehkatz for the help, both answers are great and very helpful.

Original Post

Long story short, my dad rigged up a crude ~800 sq ft. greenhouse using some 8 mil greenhouse film, and with the weather dropping to about 10°F in a couple of days, I am now charged with somehow keeping our tropical plants alive, outside, through winter. I plan to install a space heater to keep the temperature to about 45°F.

The heater I am about to install is this 5000W unit, but for contingency, should I need to go bigger, let's assume I'll be installing a 7500W unit. At 240V that works out to 31-ish amps, ideally. So the plan is to install a new 240V 50A double pole breaker in the panel, runs wire to where I need the outlet to be, install a weatherproof outlet box, and make a cord from outlet to heater.

This conduit will run from the 200A breaker panel (based on the main breaker, so probably not accurate), into the wall, to the greenhouse, which is about 50ft down the same wall (The breaker panel and greenhouse are on different sides of the same wall). The outer wall is brick. There is no basement or crawlspace (I wished...).

My questions are:

  1. Does this have to be a GFCI circuit? Or is GFCI even an option?
  2. I planned to use 8/3 UF wire to be on the safe side, but can I just use normal indoor wire since most of it will be in the wall?
  3. Do I need any kind of conduit tubing for the wire inside the wall?
  4. Can I use the same 8/3 UF wire to make a cord (like this) from outlet to heater with some male connector? Is it better to buy one?
  5. The heater shows two live connection and a ground, so 3 wires total. Does that mean I could just ignore the neutral wire altogether?
  6. Do I have to worry about the total power of my breaker panel? I have a feeling the main breaker rating isn't what the house can provide.

Disclaimer: I did some indoor 120V wiring for outlets and lights/fan fixtures and replaced breakers and 240V dryers outlets, but this is on an entirely new level. As much as I wanted to learn to do it myself, please let me know if I am making some fatal mistakes and should just get an electrician.

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2 Answers 2

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First, you need to use a heater that is UL Listed (or CSA or ETL are acceptable). The heater will have instructions which are also approved by UL.

The instructions will specify the ampacity of breaker required, and whether or not you are allowed to connect it via cord-and-plug.

If it says 30A breaker and hardwire only, that's the last word. However, you are always allowed to use larger wire than is required.

Barring any other requirements, the breaker must be over 125% of the actual ampacity of the heater. Thus 7500W derates to 9400-ish watts, which nicely fits under the 9600W of a 40A circuit. Not every heater size is chosen so artfully; 4000W and 6000W heaters in particular "just miss" the breaker size and are forced to use larger circuits when the heater could have simply been downsized by 4%. Bad design.

Does this have to be a GFCI circuit? Or is GFCI even an option?

A GFCI breaker is an option. In NEC 2020 it is mandatory for cord-and-plug connections. As of January 1 2023, it is mandatory for hardwired connections too.

I planned to use 8/3 UF wire to be on the safe side, but can I just use normal indoor wire since most of it will be in the wall?

NM-B cable is not approved for wet locations.

If you are going to a disconnect switch or a receptacle, and you can find a 75C/aluminum rated receptacle, you can run 6 AWG aluminum wire such as 6-6-6-6 SER. This is much cheaper than #8 copper, and is perfectly safe when used on AL-rated terminals and torqued to spec with a torque driver.

Do I need any kind of conduit tubing for the wire inside the wall?

Whether it needs physical protection depends on where it goes. Some conduit is inadequate for physical protection. Schedule 40 PVC is one such, and worse it is glued, so it's very unforgiving to mistakes.

Nobody ever got fired for using EMT, and it comes apart like an erector set, so mistakes cost almost nothing.

Can I use the same 8/3 UF wire to make a cord (like this) from outlet to heater with some male connector? Is it better to buy one?

No. In-wall wiring is UF, NM, and other such things. Flexible cord is *cordage which is in the "S" family (SW, SOOW, SJOOW etc.) and is very different stuff. abuse of in-wall wiring as cordage will result in strand breakage, hotspots and fires.

The heater shows two live connection and a ground, so 3 wires total. Does that mean I could just ignore the neutral wire altogether?

Correct, many heaters do not require neutral.

Do I have to worry about the total power of my breaker panel? I have a feeling the main breaker rating isn't what the house can provide.

Yes, you sure as heck do! A NEC Article 220 Load Calculation should be done on the house's entire service, to determine whether the house can handle the loads already on it and to what extent it has headroom for additional loads.

If a service upgrade is required, a mini-split heat pump will be much cheaper, especially a DIY type like MrCool or Pioneer.

Most likely your electric bills will be crushing, and you will find the heat pump would pay for itself in a season. But I doubt you'll believe me until you try it for yourself, so give it a season.

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    Wow! That is a lot of useful and insightful information! Thanks for both answering my questions and also offered alternatives. I will look into those. And I actually do believe you, as I do NOT want to go through all of that just to realize there was a better option. Dec 19, 2022 at 3:52
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Before I get to your stated questions:

  • Be extremely careful buying a heater on Amazon, seriously. Amazon is wonderful for a lot of things. It does not do as well with heavy things (because shipping costs, even if you don't pay it directly, so it gets rolled into the price) and it is generally not advisable for electrical things (i.e., running on 120V or 240V) unless UL, ETL or similarly listed. These particular heaters do say they are ETL listed, and if they actually are then that is fantastic - but if you get one of them, double-check to be sure. With a quick search, I found similar heaters, for similar prices, at Home Depot, and I am sure there are plenty of other bricks 'n mortar sources.
  • The size of these heaters is not as random or "round number" as you might think. 7,500W/240V = 31.25A. Heaters require a derate for continuous use. So a 32A (31.25A is just barely under that) requires a 40A breaker. That is a deliberate choice. Similarly, 5000W/240V = 20.8A, so it uses a 30A breaker. Don't oversize the breakers more than that. If you do oversize the breakers (in some cases that is OK relative to the connected appliances) you must oversize the wire to match, which gets expensive very quickly if it is more than a couple of feet. So 30A for 5000W, 40A for 7500W.
  • Wire sizes are pretty standard. See this Cerrowire chart for some details. For various reasons, you are generally going to be limited to the 60C or 75C columns, and for other reasons you will likely need to use copper. Aluminum is perfectly safe when installed correctly, but everything involved needs to support it - breaker (no problem), device (often very big problem), junctions in between if needed (can be done but you need to get the right stuff). So stick with copper for this project. 10 AWG for 30A/5000W, 8 AWG for 40A/7500W.
  • A heat pump will be far more energy efficient. But initial cost will be quite a bit higher. Whether that is worth it or not depends on your electric rates.
  • A gas heater, as suggested in another answer, is often a better solution. It generally costs less to run than a giant toaster but still more than a high-efficiency heat pump. However, a fairly well-sealed greenhouse is probably not a good place to run a gas heater, because the combustion air requirements will either result in a dangerous environment (for you...the plants like CO2) or suck so much cold air in from every crack and crevice as to cause more energy loss through cold air than you save through more efficient heating.

And now to the stated questions:

  1. Does this have to be a GFCI circuit? Or is GFCI even an option?

That depends. In the latest NEC (2020), which is gradually being adopted around the country, receptacles on 240V circuits generally do require GFCI. Which can get very expensive for a 30A or 40A breaker. And for some panels you may not even be able to get that breaker. Many states (and parts of states) have not yet adopted 2020. And many places that have adopted 2020 have put in exceptions to this specific requirement. On the other hand, hardwired 240V circuits generally do not require GFCI. Whether or not it saves you from a GFCI requirement, I would definitely hardwire a heater of this type as a plug/receptacle just adds another point of failure and you aren't going to plug anything else into the receptacle.

  1. I planned to use 8/3 UF wire to be on the safe side, but can I just use normal indoor wire since most of it will be in the wall?

If the wire will literally go from the panel into the wall and from the wall into the heater without going outside then you probably don't need UF and can use NM instead. But that is somewhat up to the inspector. For that short a distance it makes little difference anyway. However, you may want to consider metal conduit and individual wires instead. You also may want to mount the heater towards the middle of the room for better circulation, and in that case you definitely need either UF or conduit + wires.

  1. Do I need any kind of conduit tubing for the wire inside the wall?

The wall provides protection, except that you need nail plates or similar protection if the cable goes through holes in studs. That being said, conduit is always an option, and if you use conduit then use individual wires instead of cable.

  1. Can I use the same 8/3 UF wire to make a cord (like this) from outlet to heater with some male connector? Is it better to buy one?

No. UF and NM are supposed to be fixed wires. When a wire is exposed like that it is considered cordage which uses a different type of cable. You are probably better off with flexible metal conduit. It provides protection, so you can use individual wires instead of cable, but it is flexible so you can use it for mounting a heater without having the heater right up against the wall.

  1. The heater shows two live connection and a ground, so 3 wires total. Does that mean I could just ignore the neutral wire altogether?

Correct. Note that if you use cable with 2 regular wires + ground (so 3 physical wires, but commonly referred to as 2-wire or /2 cable) then the regular conductors will be black and white. White, in this situation, can be used as a hot wire instead of as a neutral wire, but you need to mark both ends with black or red (or other color, not green) tape to indicate it is hot. If you use individual wires, use either two blacks or black and red.

  1. Do I have to worry about the total power of my breaker panel? I have a feeling the main breaker rating isn't what the house can provide.

Yes! This is a potentially big issue. Your 200A panel may be matched up with a 200A service or your service may be smaller. But either way, you need to do a load calculation and compare that to your service feed (from the utility) to determine how much room you have to spare. If you have 200A service and 170A load calculation then you have a problem here and would have to limit to a 5000W heater. If you have 200A service and 180A load calculation, don't add anything. If you have 200A service and 160A load calculation (or lower) then you can add the 7500W heater without a problem. Going over the load calculation risks tripping the main breaker (at best) or causing other problems (possible worse).

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    Thank you for your input and suggestions. It seems like a heat pump is a much more reasonable approach, and despite the initial upfront cost, it's far, FAR better than doing all this (rather questionable) electrical work to just rip it out for a better solution. Dec 19, 2022 at 3:57

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