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Connect single pole load to double pole breaker
Can I connect 120 volt 30 amp single-pole load to 30 amp double breaker?
Answer: Yes. 30 amp requires 10 gauge wire. Always use solid copper wire, and make connections very tight.

Can I connect 120 volt 15 amp to 30 amp double breaker.
Answer: No.
Not unless you install 15 amp line fuse on the wire. The fuse must be easily accessible
and not concealed inside a box. A fuse can be installed at the load and still protect entire length of wire.

The best solution is always connect to correct size breaker, or install tandem breaker if panel does not have enough spaces.

Not enough space in breaker box
See inside main breaker box
Basic household wiring diagrams
Why ground wire is needed
Isn't a 30 amp double pole breaker is same as two 30 amp single pole breakers stuck together, so I can put 30 amps on each leg?
Answer: Yes, because the breaker draws 30 amps from each busbar.

If you load a panel up with only single pole breakers can you put 200 amps on each leg?
Answer: No.

200 amps on each leg would be 400 total amps.
If you draw 400 Amps, the main breaker will trip.
A 200 Amp 120/240Volt panel has maximum 200 Amp, no matter if loads are 120Volts or 240Volts.

Note, breakers are generally not fast-acting. They 
allow a certain amount of overvoltage before tripping, which means actual amperage can exceed the amp rating. However, chronically overclocking breakers will cause the breaker and busbar to get warm or hot, causing them to wear out over time. Use hand to test if breakers are running hot when under load. Breakers should not feel warm. Damaging the busbar causes permanent damage to the breaker box, and the breaker slot becomes non functional.

The safe maximum for breaker calculation is 80%. If breaker is rated 30 amp, then 30 x .8 = 24 amp safe maximum. 200 amp main breaker x .8 = 160 amp safe maximum. Add up wattage of loads on each breaker, and divide by voltage to get amperage at full load. Add up total amperage on all breakers to determine if electric service provides enough amps. Using this guideline will prevent long term issues and help protect investment.

Can I put 190 amps on one leg, and 5 amps on another leg?
Answer: No.
Unbalanced voltage can cause damage to motors, HVAC etc. Install 'voltage monitor' to protect vulnerable circuits.
Your single breakers should be evenly divided, with roughly half on one side and half on the other. The double breakers are snapped over both busbars so their location is unimportant for 'balancing the loads.' Leave top two breaker spaces on 1 side empty for installing Type 2 surge protection device.

Residential breaker box has 2 Hot busbars. Each busbar is 'out-of-phase' from the other busbar. Each busbar has 120 volt potential to Neutral (phase voltage) and 240 volt potential to the other busbar (line voltage). 240 volt breaker snaps over both Hot busbars, while single-pole breaker snaps over one Hot busbar. The breaker box should not be overloaded on one busbar, or one leg.
Unbalanced load on power company transformer can unbalance the transformer and electricity might not have the correct waveform, causing damage to motors, electronics etc.

How to wire voltage monitor
Type 1-2-3 surge protectors
What does out-of-phase mean
Troubleshoot household electric
See inside main breaker box
Basic household wiring diagrams
Question: Are both sides of the breaker 30 amp or one each one of them 15 amp ?
Answer: Both are 30 amp
You can make a 240 breaker by combining 2 single-pole breakers, but code requires a 'common bar' between the breakers so if one trips, then the other breaker also trips.

AC power reverses the direction of electrons 60 times per second... and the voltage rises and falls as electrons come to a stop, reverse direction and accelerate the opposite direction... despite electrons stopping, and voltage dropping to zero 60 times per second, the average voltage is above zero.
Each leg of a 240 volt circuit is out-of-phase with the other leg. This means each leg is delivering power to the load, and is independent of the other leg. Of course that is true because of how the power company generator works, and how single phase is delivered to home via the power company transformer. And so the electrons travel back and forth on the wire... and one leg has electrons going one direction when the other leg has electrons going the opposite direction.

If you pull 2 Hot wires from the same busbar, the electrons on each wire are traveling same direction as each other, so you can not use these two wires to get 240 volt. You must use a wire from different busbars so the electrons are moving opposite directions from each other.

Using power from both busbar increases total power. This is represented by the formula E = IR, or power (watts) = volts x amp. The formula shows if you change the voltage from single breaker 120 to double breaker 240, then the power (watts) goes up, assuming amps are the same. So 30 amp x 120 volt = 3600 watts, while 30 amp x 240 volt = 7200 watt, or twice the power (watt).
How is that possible? ... the answer is that both breakers are 30 amp... because both carry electrons that are being 'pushed' and 'pulled' the opposite directions down the wire, like pedaling a bicycle with two legs instead of one.
... so yes... the answer is that both breakers are 30 amp.
Figure volts amps watts
Formulas for ohm's law
What is electricity
How a generator works
Difference between single-phase and 3-phase
Electricity from power plant to end user
Single-phase water heater amp draw
Larger image
Refer to chart above.
1500 watt element.
12.5 amp @ 120 volt
or 6.3 amp @ 240 volt
So the amp rating drops by 1/2 for same wattage load.
This means that 240 volt breakers are designed to handle large appliances that cannot be supplied by 120 volt.
To illustrate how much more power 240 volt supplies to an appliance, we use the 4500 watt element with a rating of 240 volt.
When connected to 240 volts 30 amp double breaker, the element heats 16+ gallons per hour.
When same rated element is connected to 120 volt, it heats 1/4 the amount of water per hour, and a smaller breaker can be used. 
circuit breaker
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