Mainly aa electrical question on a home project. Please help.

I'm creating a heating system that will need heating elements.  I'm doing this project at home and I understand every home gets a total of 240VAC (two hot legs) from the power lines.  The tub that I am using is pretty big, 275 gallon to be specific.  When it comes to deciding which heating element to select, what matters most the volatge, wattage, or amperage?  I understand the highier the wattage the faster the heating process will reach it's setpoint, or am I wrong? I came across 120VAC and 250VAC heating elements.  What difference will the voltage make?  Amperage is also to figure out in order to determine the correct wire guage for the circuit?  Wire guage matter to avoid fires that may occure if the amperage is too high for the wire chosen correct?

 

Grade me.  A,B,C, or, F!

There is a lot more to this question than you may imagine. However, to put it simply, energy is what matters in terms of heating, so watt hours are what you are interested in, increase that and it heats faster, it is based around the specific heat of water. There are some calculators around, don't know how good they are, one here for example,
http://processheatingservices.com/water-heating-time-calculator/
The elements will be rated for a wattage at a given voltage and current, which must not normally be exceeded. So you need to get elements rated for the voltage you have available, which will be 120VAC, there will be a number of power ratings for that voltage, which determines the current they will draw. Ohms law v=i*r, and power, w=i*v, are the equations needed to solve all you want to know. These will all be rms values, which is what is needed with respect to power.
There are tables for the current capacity of wires, there is also the issue of voltage drop due to cable resistance to consider, see the info in the link below, for example
http://www.electricaltechnology.org/2013/09/electrical-wiring.html
Also be aware that water sitting in a tank can stratify, so the water at the top can be at a much higher temperature than at the bottom, this can be done both intentionally or not, depending on the application. If you need a more even temperature distribution, then stirring may be needed.This issue comes up when you consider where and how the temperature is sensed for control purposes, where the heater(s) is located, and the configuration of the tank.
The only other issue you have to deal with is safety, this voltage is lethal, so take care, no exposed wiring, and no possibility of the water coming into contact with live circuitry either. All metal surfaces that could potentially go live must be grounded, and earthleakage/breakers should be used as well.
Cheers,
Richard

I do understand ohms law and use it a lot at work. I need to know two of the four in order to figure out my answer.

So are you telling me that for example, a 120VAC 3,500watts vs 240VAC 7,000watts that the only difference between the two is the heating time (higher the wattage the faster the heating process)? Would it matter if I'm using 120VAC or 240VAC? I'm looking for making the best decision for energy saving (don't want a high electrical bill)! Energy providers charge by the wattage used and not the amount of amperage being pulled correct?

As for the water being hotter at the top, THANK YOU! I was just asking about that today! I know heat rises in the atmosphere, but I was wondering if that applied to liquid as well. I'll be using a SOLO brand digital temperature controller with a type K TC as the input sensor. I'm more concern about the temperature at the top not anywhere else, so it'll be best to place the TC sensor at the top since that's where I'm more concerned about correct?

When selecting the correct fuses for safety for the circuit i just need to add up all the wattages from the components being used in the circuit and divide it from the voltage being used (120 or 240)?

From your example, a 120VAC 3500W heater will take twice as long (all other things being equal) to heat the water as the 240V 7000W heater. They will consume the same amount of currrent (3500W/120V = 30A, 7000W/240V = 30A) but the 240V heater will deliver twice the heat energy per unit time.

Be aware that you will probably need to size the wiring and fuse above the actual requirement of the heater since the heater wattage and the fuse rating both have a +/- tolerance and the heater may draw more current immediately after being turned on than when operating in its steady state.

Additionally, thermal lagging is very important in terms of keeping your power bill down, even if the water is drawn off directly after reaching the controlled temperature. The loss even from a thermally lagged tank with standing water can be up to a few kWh per day.
cheers,
Richard