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I want to join 3 resistors connected in parallel into one. Like this enter image description here

If for the total resistance I use

$$R_t = \frac{R}{3}$$

But what about the power of them?

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You are correct, a single 50 Ohm resistor with a 30W power rating.

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    \$\begingroup\$ If the paralleled resistors are very close to each other - so they will heat each other - the power rating should be reduced somewhat. The power rating of a single resistor will assume it is not heated by its surroundings. \$\endgroup\$ – Peter Bennett Feb 10 '16 at 1:07
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Just a note, if all of the paralleled resistors are the same value just divide the total number of resistors by the resistance value of one resistor then take the reciprocal of your answer. In your case(3/150ohms=.02) then (1/.02=50ohms).

If calculating paralleled resistors with different resistances you must add the reciprocal of each individual resistor, then calculate the reciprocal of your answer. For example 3 resistors, 1pc-200 ohm 1pc-50 ohm 1pc-1k ohm then (1/200ohm+1/50ohm+1/1000ohm=.026)(1/.026=38.4615384615 ohms).

The allowable power dissipation is always the sum of their respective ratings regardless if in series or parallel. Common sense tells us that heat will not dissipate as fast when bundled in parallel or not properly used thus changing the wattage or power capability for long term loads.

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    \$\begingroup\$ If they have different values and you connect them in parallel, they will run at different powers. The total power rating will be limited by the resistor that hits its power rating first. So not as simple as the total rating is the sum of the ratings! \$\endgroup\$ – Neil_UK Feb 10 '16 at 8:05
  • \$\begingroup\$ I have found that there are the Caddock resistors of 50Ohm 30W value that in one TO-220 unit can do the job. \$\endgroup\$ – Roman Feb 10 '16 at 11:47

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