Thread: current demand of power tools.

Motors do have a starting circuit that does draw very high current for a fraction of a second on start up. During normal running the current draw will be that at which the motor is rated at. The continuous rating is what the wiring needs to support to prevent fire from wires overheating, the large draw at the start is not a problem as it is only being drawn for a short time.

Hi Soundman,

I don't have the gear to be able to measure but would sure like to hear your observations. Haven't manged to blow the breakers at the board yet but I often wonder when I fire up the 2HP DC connected to the 3HP Router in the Triton Router Table!!!:confused:

Jamie

q9
I am well aware of the concepts & principles of start up currents, peak loadind ect.

What I am looking for are actual figures on a range of machines used in the industry.

barnsey
most circuit breakes will carry more than their rated load for a extended period. How long is a little more complicated.

However is is NOT good practise to allow that situation to continue.

A 3hp router & a 2hp extractor should at least run on seperate power points preferably seperate circuits.

If you use as a basis for constant draw calculations the figure of 1HP=720W approx (sometimes stated as 750W or 746W)

Now as a rule of thumb anything with a motor that can start under a light load you should allow at least double the running wattage. Rememebering that this is a general rule, there are some motors that will draw 3 to 3.5 times the running power.

So base on the above a 1HP motor under a light startup load will draw approx 1500W and if we convert that back to amps that give us

1500/240 = 6.25A startup current.

So if we do the calculations based on Soundmans equipment we get

3HP router startup up power draw is: 2 * (3*720) = 4320W
4320/240 = 18Amps initial current draw.

2HP Extractor is : 2 * (2*720) = 2880W
2880/240 = 12A intitial draw

As you can see if both of these are on the same circuit and are started together, there goes the breaker/fuse.

From the above you can easily see that carefully planning of circuits and their respective loads is needed to ensure that you not having to run back to the switchboard to reset breakers.

One thing I would like to point out here though is that the above is meant to be a rough rule of thumb guide and not a detailed or highly accurate explanation.

If in any doubt seek professional advice. Most local electrical supply authorities are more than willing to assist in this area of planning.