Slightly OT

How do I calculate wire and breaker size for 2 motors starting simultaneously?
Thanks

Bump
I thought that I read somewhere that it wasn't as simple as doubling.
Or is it that simple?
Thanks

are they on the same circuit or different circuits?
are they in the same control panel, mcc buckets?
motor voltage and size? are they the same horsepower?
assuming standard motors, not vfd driven motors.
distance from starter to motors? voltage drop may apply.
ambient temperature range, (hottest air temp), temperature derating may apply for the wire.
are the motor wires in a multi conductor conduit, if yes, derating factors apply.

there are lots of things to consider.
james

Hi, a lot of that is in the NEC, section 430. There is a flow diagram that will tell you which sections cover which parts of a motor circuit. You have to read carefully, because section 430 will refer one to other sections and tables as far as overcurrent, branch and feeder sizes, wire and conduit sizing, etc. Wish you the best.

Thanks
I have a copy of the code and have read 430 many times in the last 40 years.
They are both 1.5 hp on a saw. When the operator pushes the green button both starters pull up and energize the two motors. Since both start at the same time my question was what size breaker and wire should I use for the feeder and what size fuse should I use in the disconnect.
I used a 30amp breaker and wire. At start up I'll throw a clamp on it and see where I am.
Malbe I'm overthinking this thing.

get the FLA of the largest motor x 125%, then add the amps of the second motor to get conductor size, breaker size, etc...

You mention 'Both starters pull up', so I'm assuming you have a separate branch for each motor.

For a 1.5HP motor, (assuming 480VAC) use three of the following fuses for each circuit:
LPJ-6SP, TCF6, LP-CC-10, FRS-R-6

Use minimum #14 AWG from the motors to the contactor (#12 recommended)

You have a 3HP load + control voltage, I'd use a LPJ-10SP, TCF10, LP-CC-15, or FRS-R-10 for the main disconnect with #12AWG.

Lets get started here:


430.24 says that in this case you need to have a conductor ampacity determined by:
(1.25 * Largest motor) + (Sum of FLC of other motors) + (1 * noncontinuous non-motor-load) + (1.25 * continuous non-motor-load).



I checked the exceptions and didn't see a note for your situation. I'm guessing you don't need a code cite to size your wire



On to the Overcurrent protection:
430 Part III gives your motor overload requirements. 432(C) says to size the overload based on the motor service factor. SF >= 1.15 or temperature rise 40C or greater is at 140% and all other motors at 130% max. I'm assuming you are using starters/overloads on them. Each motor needs its own overload. Sorry, but I don't have a cite on this one and not sure where it is at.



Ground fault and short circuit detection:
430.53(C) has the information regarding your specific situation. There is a bunch in there, but you use table 430.52. In particular, you want 430.53(C)(4) where it says to use the following:


(table 430.52 value of highest rated motor) + (FLC of all other motors) + (all other loads on circuit)


If the above is less than the calculated ampacity of conductor sizing, you use the ampacity of the conductor size.


I took the more liberal route here because you are starting two motors at the same time always.