lipo c rating calculation

[cogwheel1]

What is the formula to calculate the neccessary C rating on lipo batteries for a given motor/ESC combo.? I have the motor specs, and voltage I want to run. I know the formula to calculate the Packs capacity in amps. Also, is there a formula to calculate how big of a ESC you need. This may sound stupid, but in reviewing different setups, it seems that most people are running speed controllers with ratings way higher than the specs for their motor require, ie: the motor may have an 45 amp max rating and I see ESC installations of 120amp. I realize it's better to err high but that seems like overkill. If a motors maximum amp rating is 60 amps, wouldn't it be sufficient to have a 70 amp ESC with a burst of 80 or 100 amps?

[Jeepers]

Its not overkill to run an esc much larger than needed. just easier on the wallet in the long run! Motors can pull many more amps than rated. overpropped, bind in the driveline etc.

[cogwheel1]

lipo C rating needed for any given aplication

That makes sense I guess, except I'm going the maximum 2 motors possible for this hull I'm modifying the first time, and to buy 2 ESC with twice the amp rating I need is just too costly!
How about that formula on the C rating needed on lipo's?
Would you just use the formula for calculating the packs amperage and go from there, ie; if you had a motor needing 60 amps and had a pack that was a 3000 mah 35C, then the capacity of the pack would be 3X35=105 amps---more than enough. Or for an example of one that wouldn't work, 2000mah 12C which would be 2X12=24 amps, basically 1/3 of what you would need.
Is that correct?

[CornelP]

That is correct, just calculate it with a minimum 20% spare for every part. If the motor draws 50A, get a min 60A ESC and a 75A battery. The bigger the spare, the cooler everything runs. Getting an ESC twice the required amps is an investment... in safety. It is not twice the cost, but will save you from buying a replacement when the first one fries... :)

[785boats]

cogwheel 1
Although you are right with the formula, it would be wise to use packs with a larger capacity as a 3000mah pack won't give you a very long run time. Don't forget, it is wise to leave about 20% capacity in the packs at the end of a run.
A 5000 mah pack at 25C will give you 125A but a longer run time at your original load.
As for the ESC rating I think that double what you 'think' the load will be is compulsory. Nine times out of 10 you will draw more current than you think.
The rating on a motor is the max efficient rating. It will draw more current than that if the prop is too big, or the boat is running very wet, or a binding drive line, or battery capacity/rating too low, etc. A combination of two or more of these usually ends in smoke if the ESC is only rated to just above what you 'thought' the max amps were going to be.
The only way to know is to measure it with a data logger, but by then you've already bought the ESC.
One of our club members put a data logger in a boat with an 8XL, CC120A ESC & a prop that most people were using for oval racing that they had been using most of the year with no temperature problems. Can't remember the size, sorry. In 5 laps the current draw never went below 130 Amps & was spiking up to 155A.
They also put the logger in a boat with the Aquacraft SV 27 package which is rated at 45A . Again the set up was a generally accepted one that had been running with no heat problems. Readings of 70A were logged on each lap under full load.
Both boats were well set up & running nice & loose & on good 4s packs.
Hope this helps with your ESC size decision.
Paul.