running two 15A compressors on a 30A circuit ?

[michaelwoodcock]

Hey guys, as the title says I'm looking to run two 15A compressors on my dryer circuit. I have an old compressor with questionable tank integrity and it's almost cheaper to buy a whole new compressor than a new tank for the old one. I would do a few things like have one compressor switch on after the other, is this do-able? or would I be tripping the curcuit, shortening motor lifespan, or both?

My dryer circuit has the two bridged 30A breakers, this means it's 30 amps right? or do the amps add up? I'm kindof new to house wiring & such

Thanks,
Mike

 

[michaelwoodcock]

just to add more details: I hope to use the old compressor head and pump to pump up the new one. To double the air flow. I want to get rid of my old tank because it's of questionable integrity.

 

[RECox286]

Invest in a clamp-on amp meter to investigate how many amps each compressor is drawing. You may be surprised to find the starting and running
loads may warrant different thoughts about the setup as you envision it.
I'm sure that others will chime in with their thoughts.

Uncle Bob

 

[smothers33]

How are you planning on wiring them? I'm not sure exactly what you want to do but from the sounds of it you want to run 2 15a compressors at once. Both off 1 30a circuit. If I'm right you should be able to do it but 2 Things... 1 you will probably need to hook them up in such a way that 1 switch turns both on otherwise you'll have all 30a going to 1 compressor before turning on the second which would break your compressor circuit. 2nd as stated above youl need to invest in a amp meter to test the initial startup ampdraw.

 

[Charles (in GA)]

Save your pennies, buy a new compressor.

Charles

 

[Sureshot]

If I understand what you are trying to do I would sell the old compressor and apply the money to a better new one. How much air volume do you need?

 

[andywander]

... 1 you will probably need to hook them up in such a way that 1 switch turns both on otherwise you'll have all 30a going to 1 compressor before turning on the second which would break your compressor circuit.

Sorry, but that is just wrong. The 30A circuit will not "force" 30A to the single compressor. the first compressor will only draw 15A, as it is rated to do. When you switch the second one on, it will also draw 15A, per it;s rating.

A 30A circuit means it is CAPABLE of supplying 30A, but the amount that is actually drawn depends on the equipment that is drawing it.

 

[TwoInch]

i didnt understand that either, as he mentioned amp draw in the next paragraph.... maybe just a brain fart? that would mean all our lamps would blow out everytime we turn em on, as each breaker would be throwing full amperage to them. electricity doesnt work like that.

 

[vintagefan]

Wire a relay to the pressure switch on each compressor, that disables the pressure switch on the other if engaged.

That way only one will turn on at a time, and they will fill alternately, and automatically.

 

[RivennHewn]

Invest in a clamp-on amp meter to investigate how many amps each compressor is drawing. You may be surprised to find the starting and running
loads may warrant different thoughts about the setup as you envision it.
I'm sure that others will chime in with their thoughts.

Uncle Bob

What he said.

Start up usually draws more power than rated.

 

[Sureshot]

So you want to run two compressors filling one tank?

I would skip it as my previous post says but if you insist set the start pressure on the second unit at 20 psi below the first so it will only run one except in high demand situations where it would be needed.

I suspect if you are using your dryer circuit to run your compressor you are not an industrial user and likely not needing the huge volume. If you really need it run another circuit as I would be inclined to say the starting draw on the second compressor will trip the circuit.

 

[jam0o0]

electric motors sometimes pull twice as much current at start up compared to running current. get a meter that can record instant amps. a digital $20 meter will not work for this.

 

[smothers33]

Sorry, but that is just wrong. The 30A circuit will not "force" 30A to the single compressor. the first compressor will only draw 15A, as it is rated to do. When you switch the second one on, it will also draw 15A, per it;s rating.

A 30A circuit means it is CAPABLE of supplying 30A, but the amount that is actually drawn depends on the equipment that is drawing it.

yeah I was thinking about it at work and it didn't make too much sense to me either. I was thinking he was trying to hook them up in series with one another but still would not work how I was thinking

 

[michaelwoodcock]

Hey thanks for the replies guys, so many it'd be impractical to quote them all individually, but to answer a few questions:
Yes, going to be using both compressor pumps & motors to fill one new tank
I don't feel good about selling my old compressor as it may blow up at any time due to rust
I have a moderate air requirement. I sometimes run out of air and it bothers me to no end so I plan on going overkill. I run pneumatic polishers, die grinders, sandblasters, impacts, ratchets, and I think that's pretty much it at the moment.

going to buy a clamp on multimeter this weekend and I'll do some investigation.

 

[eljefino]

You can run four compressors in theory as the dryer is 240V x 30 A. If it were 240x15 (not made) you could rewire with each outlet/compressor on opposite phases. Similarly a 120x30 (not made) would just let you plug them both in naturally.

But since you say you don't know what you're doing I don't think this whole thing is such a dandy idea. If this dryer outlet is somewhere convenient like your garage you could have an electrician put in a subpanel and you'll have lots of power to drool over.... 4 x 15 amp 120 circuits to be specific.

If nothing else, plug each compressor into a power strip that has its own fuse/breaker. The dryer breaker won't protect; it's too big.

 

[michaelwoodcock]

^ the compressors are both 240v, so I don't think I can run on opposite phases.

 

[RECox286]

What you need to do essentially, and it is not all that difficult, once you know the

starting loads, is to offset the start pressure of each machine so they will not start

simultaneously. Starting load is usually about 2 to 3 times running load, but it is

not more than a 'flash in the pan" event. I think what the OP wants to do is doable

and will be safe to do with little effort, once the variables are taken into account,

and as long as the 30A breaker is not overloaded while both machines are

running. Otherwise, I would suggest running another 30A service to run the

other machine, or to upgrade with a 50A subpanel and be done with the

whole mess.

Further thought: if the receptacle next to the dryer is a dedicated 20A

120v for the washing machine, then it could be changed out to a 20A 240V

receptacle driven from the panel with a 2 pole 20A breaker and used to

run the second machine (or, whichever is the lower of the running loads)

and would not require any wiring changes between the panel and receptacle

location.

Uncle Bob

 

[Steevo]

A friend had a body shop that had a 120 gallon horizontal compressor in it. The compressor had two motors and pumps on top, both filling the same tank. Both pumps fed into a "T", then into a single check valve into the tank, and the pressure relief line from both heads went to a "T", then into the single pressure switch.
The pressure switch activated two (2) motor starters, one for each motor, and each motor starter had independent power feeds and breakers.
We thought about bringing it to his house when he closed the shop, but the motors were 3-phase, so that wasn't realistic. The plumbing and separate circuits are what I remember about it, though.

 

[MFortie]

You can run four compressors in theory as the dryer is 240V x 30 A. If it were 240x15 (not made)

???

Am I missing something? 15 amp, 240 volt outlet: