Auto switching shopvac - looking for confirmation on load calculations

**JontyG** · 15th December 2010, 05:33 PM

Hi,

Just looking to get some input on the maximum load that a built in master/slave switch on a shopvac can handle.

I may be better off posting this in an electrical forum, but I know this is a common issue that many of you would have encountered

I have a 1400W shop vac that has a built in master/slave switch, rated for a max 1000w tool (master device). I gather this is because it would normally be used on a 10A circuit, hence the maximum load is 2400W.

However, if I was to use it on a 15A circuit (which I already have), would I be able to use a power tool up to 2200W? I have based this on the following calculation:

Max load on 15A circuit: 15A x 240V = 3600W (VA)
Max rating for master device: 3600W - 1400W = 2200W

Does that seem right?

Are there likely to be any issues with the circuitry in the shopvac that would prevent it from handling a tool over 1000W?

BTW, I want to be able to autoswitch my drop saw (rated at 7.2A or ~1730W) with the shopvac.

Thanks,
Jonty

**malb** · 15th December 2010, 06:56 PM

The unit would be designed to cater for loads to 2400W total (Master device and Slave Vac) because that is the limitation for standard domestic circuits in OZ. Because of this design limitation, specific elements of the unit such as the the power cord, plug, master unit socket, Master/Slave module, and the ON/OFF and Manual/Auto switch will be specified to cope with a 10A combined load.

Having access to a 15A outlet to connect through will not overcome these design limitations. It is possible to attempt to operate the unit with higher rated devices as the Master, but you would be operating beyond the supplied ratings for the unit and hence beyond the protection of manufacturers/distributors warranty provisions and liability potential in the event that using the unit in the manner you wish were to create an incident which threatened life or property.

That said, I have done a similar thing with a 2HP RAS as Master, but prefer not to as the saw power is limited by the additional burden of operating both units, leaving the saw prone to stalling if even modestly overloaded.

**JontyG** · 16th December 2010, 06:31 AM

Thanks for the input. Much appreciated

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**JontyG** · 16th December 2010, 07:30 AM

I just realised that I often have my drop saw and vac plugged into the same 10A outlet (double adapter). This would result in ~13A draw, which I guess is also not an ideal scenario.

May look at getting one of the JayCar master/slave switches to give me more capacity.

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