Practically speaking, unless it is incredibly hard to run new wires for additional heaters, there is no way to run more than 3,840W of heaters on a single 240V 20A circuit (240V x 20A x 0.80 derate = 3,840W). Setting a heater to low or medium will do one of two things:
- Turn on fewer heating elements. If it does this then it will use less power and be OK in this scenario. However, that requires a way to make sure that the high setting is never used - e.g., physically disconnecting the "high" element. Which in turn gets into issues of (a) modification of equipment in an unauthorized manner (invalidates warranty, which is usually no big deal, but if there is ever a problem (such as a fire) it could be blamed on unauthorized user modification of equipment) and (b) when the next owner replaces the heater because "high" doesn't work properly then the problem comes back again.
- Cycle on less of the time using an internal thermostat. The problem is that you can't control what time each heater will be "on". And on a cold day they will almost certainly all be "on" at the same time.
So the proper, and likely most cost-effective, answer is to run an additional circuit. But there is one interesting alternative: water heater mode.
A typical US tank electric water heater is a very simply, but also very clever device. I learned about this from this Technology Connections video. Essentially there are two elements wired in such a way that only one at a time is actually on, but which one is on varies depending on the temperature of water in the top vs. bottom of the tank. This has the effective result of providing hot water more effectively under many circumstances than a single element would provide, without actually running both elements at the same time (which would require a larger circuit). A typical electric water heater is installed on a 30A circuit and has two 4,500W or 5,500W elements - a total of at least 37.5A - but since only one element is on at a time it works fine (and is safe) on a 30A circuit.
Can you wire up some relays/contactors and stuff to make a pair of your heaters work this way? Sure. Is it worth it? Probably not.
And on to what seems to be the real issue based on comments:
what I'm trying to figure out is these 4 cadet heaters each use 4.2amp they are 240 on a double pole 20 amp breaker which I believe the max is 3840Watts or 16amp breaker I'm at 4000watts and 16.8amps on the breaker currently. is there room on this electrical system to handle the extra load? It has not tripped the breaker but I live in cold below freezing temp state where these heaters are on continuously most of the time for 5-7 months out of the year
Emphasis added. There are really two separate issues when it comes to "handle the extra load": circuit size and service size. And a related issue of economics.
As already explained above, 12 AWG wire means a 20A circuit. A 20A circuit when used for heating which can (and as noted, does) run continuously, is derated. So you only get to use 16A. These heaters add up to 16.8A. That's technically too high. But that derating is a safety margin. It is important, but going over it won't trip the breaker. It is on the honor system.
In fact, if you added another header and ran 5000W = 20.8A, the breaker still won't trip in any guaranteed measurable amount of time, thanks to the magic (or fudge factor) of a trip curve. That's in some ways a good thing - you don't want to have nuisance trips. But it means that if someone really pushes the limits (violating code along the way) they can end up in some potentially dangerous situations.
So the short answer for the circuit is: not big enough, replace one of the heaters with a smaller one or split this into two circuits.
But then there is the total load issue. As in "load of the entire house". That is where a Load Calculation comes in. Impossible to guess how close to (or even over) your service feed you are relative to a Load Calculation without knowing a lot more details:
- Size of house
- Utility feed size (100A? 200A? 400A?)
- Type of cooking equipment
- Type of water heater
- Other HVAC equipment, if any
- Type of clothes dryer
and many other little details. My hunch is you're OK, but that's based on "this isn't a really old house that kept getting expanded but never had a utility service/panel upgrade (heavy-up)". So that is something that may be worth checking.
And finally: economics.
Electric resistance heaters are really inexpensive to buy and install. They are really expensive to run. If they are really running most of the time for 5 - 7 months, that's a lot of electricity. Let's say 50% duty cycle for 6 months:
- 4000W x 180 days x 24 hours/day x 0.5 = 8,640 kWh per heating season.
Electric rates vary a lot. And you need to include not just the base rate but also all the various fees that get tacked on by utilities and governments. A quick search says average US rate 0.23/kWh. YMMV.
8,640 x 0.23 = $ 1,987 per year.
So what are the alternatives? You have to first figure out how much heat. 4,000W = 13,648 BTU. So that's a rough target. Probably anything in the 12,000 - 16,000 BTU range is worth considering.
In a quick search, I found some 15,000 BTU mini-split systems in the ~ $2,000 price range. Those actually provide both heating and cooling, so if you have an old air conditioning system for the summer this may save you some money there as well due to more efficient cooling. The actual power (= cost) needed gets a little more complicated. These units will produce more heat inside when it is warmer outside - i.e., their efficiency varies with temperature. A good quality mini-split can produce 2 - 3, or even 4 times as much heat as an equivalent resistance heater. This is referred to as COP or Coefficient of Performance.
If you can cut that $2,000 heating bill in half, saving $1,000 per year, the payback for the new system is $1,000 per year. Unfortunately, a $2,000 system may cost quite a bit more by the time you factor in installation costs, but it is something worth considering.