Compressor auto drain to pressure switch (120/240V)

[67King]

Want to make sure I'm not missing anything. Have a 5.2 horse, 80 gallon compressor. Setting up everything in the new house, I have mounted it on an outside wall by the garage/shop. At the last house, I wasn't as good as I should have been about draining it since it was outside. Anyway, to alleviate that, I'm planning on an auto drain. Now, since I bought it, I've replaced most of my pneumatic tools with M12 and M18 stuff, so it doesn't get used that often. So, I don't want to have the auto drain connected and on all of the time, as it is conceivable that it would completely drain the tank between uses.

So my plan is to wire it to the pressure switch. Two questions along that line. First of all, is there any problem with that on that end of the circuit (i.e. the pressure switch or the auto drain)? I have already run 10/3 wiring to the compressor so I will have a neutral for it since it is a 120V, and the compressor is 240V. Obviously only the hot wire would come from the switch. SPecs on one that I have found state it only draws 4mA. I assume all of them will be similarly small, regardless of which one I get.

Second question is on the supply end. I assumed that I could just feed the drain off of the hot sides of the supply. Believe that is how ovens and other appliances that have 120/240V setups (i.e. a 240V that also requires a neutral). But I'm not positive. The breaker itself (30A, so way oversized for my needs) won't be affected by a slight current difference between the two poles, will it? In other words, am I overlooking anything?

 

[czarjl]

I would think you could wire something like this https://www.amazon.com/dp/B07DPFMXJ5?tag=atomicindus08-20 automatic drain valve with timer in parallel with the motor after the pressure switch. When the motor is running the solenoid would see power and drain on whatever interval the timer is set to. It may take a bit of trial and error to get the timer set to so that it drains at about the end of the run cycle.

 

[mm08822]

A 240v drain would have been simpler and more easily to connect off the load side of ps.

I would run the neutral to the ps and come out of a spare opening for a cable to the drain valve.
Ps volume is small so extra wires may be a real PIA.
IIRC, drain valve connections expect a small cord size like 16-3. That is too small to be put on a 30A circuit.

A 30a cb won't even know that the drain valve is hooked up across 1 pole. So that is a non-issue.

This all assumes you have a typical ps with the integral on/off lever and unloader.

Timing would have to be such that it opens long enough before the compressor shuts off between cut-in and cut-out pressures.

Pics ?

 

[czarjl]

A 240v drain would have been simpler and more easily to connect off the load side of ps.

Agree, but the linked was a quick grab to show what is out there.

 

[mm08822]

Agree, but the linked was a quick grab to show what is out there.

I was typing while you posted. It was an fyi for the OP.

BTW, I have that same valve in 240v, still in the box for the last 5 yrs! I'll get to it.....

 

[67King]

Timing would have to be such that it opens long enough before the compressor shuts off between cut-in and cut-out pressures.

Pics ?

Yeah, good point. Have a 2 pole switch between breaker and ps, can just wire off one of those leads and not have to worry about it.

Was so hung up on the soft start being after the ps that I didnt think that the bleeder could be ahead, just after my indoor switch

 

[mm08822]

Soft start!!
So this is where all the details matter.

If you wire it just after this newly mentioned switch, it will always be on which is what your original concern is, no? It needs to cycle in a coordinated manner with the run cycle if you keep power to the compressor 24/7.

If you turn the switch when you are done needing air, then it should be OK.

How do you plan to operate it?

 

[matt_i]

You mentioned you don't use it much due to battery tools....a non-electrical solution would be to drain the tank to a lower point simply by gravity and let water accumulate there in a more "sacrificial piece" of plumbing. Think of a 1/4NPT to 2" NPT bell reducer, ******, bell reducer back to ball valve that sat lower than the tank and was connected back to the tank with a piece of NiCopp or other seamless tubing. You might only have to go once a year to drain it out.

I didn't see it mentioned above using these words, but essentially I think you are proposing the auto-drain power to be wired in parallel with the motor windings (if 240vac) and would use one of the 240vac lines feeding the motor (T1 or T2) plus neutral to intermittently power the auto drain (if 120vac). I think then the key would be to get the "dwell" time on the auto drain set to be shorter than a typical compressor cycle, or ideally it would "purge" as soon as its powered on. That said you might be able to use something like an Asco valve (powered by AC) that simply is ON while the compressor motor is running, and meter the flow with a needle valve.

 

[67King]

Soft start!!
So this is where all the details matter.

If you wire it just after this newly mentioned switch, it will always be on which is what your original concern is, no? It needs to cycle in a coordinated manner with the run cycle if you keep power to the compressor 24/7.

If you turn the switch when you are done needing air, then it should be OK.

How do you plan to operate it?

Yeah, sorry, plan to go solar here soon, so going to add a soft start. And yeah, that has to go directly to the motor, downstream of any other switches (breaker, double pole wall switch, pressure switch).

Breaker -> switch inside garage -> PS -> soft start -> motor

Plan to manually turn the switch on and off when I need air, so I do NOT plan to keep the power on 24/7. If I forget to turn it off.....which I have been guilty of doing at old house (as well as the ball valve for the air line), then it won't really matter because the compressor will kick on if the bleed drops the air pressure enough, and I won't constantly have an empty tank that takes a while to get back up to pressure. Really what I am aiming to do is ensure the auto drain is not powered if the power to the compressor is off.
So I best way is probably:
Breaker -> inside switch-> (split off auto drain here) =
Top branch -> PS -> soft start -> motor
Bottom branch -> auto drain

Hope that makes sense, I tried to draw it out but the format wouldn't take it.

You mentioned you don't use it much due to battery tools....a non-electrical solution would be to drain the tank to a lower point simply by gravity and let water accumulate there in a more "sacrificial piece" of plumbing. Think of a 1/4NPT to 2" NPT bell reducer, ******, bell reducer back to ball valve that sat lower than the tank and was connected back to the tank with a piece of NiCopp or other seamless tubing. You might only have to go once a year to drain it out.

I didn't see it mentioned above using these words, but essentially I think you are proposing the auto-drain power to be wired in parallel with the motor windings (if 240vac) and would use one of the 240vac lines feeding the motor (T1 or T2) plus neutral to intermittently power the auto drain (if 120vac). I think then the key would be to get the "dwell" time on the auto drain set to be shorter than a typical compressor cycle, or ideally it would "purge" as soon as its powered on. That said you might be able to use something like an Asco valve (powered by AC) that simply is ON while the compressor motor is running, and meter the flow with a needle valve.

I got really bad about draining it at the old house, so I'm hoping to not do the same thing. And yeah, I'm hoping it will purge as soon as it is powered on. I do not know if it will or not, if it will, I'm golden. But I could also just set the valve to only be for about a second, but cycle every few minutes or so. I know more frequently folks open for a few seconds, but every 30-45 minutes. Unless I'm bead blasting or painting, I don't anticipate it being on for that long.

 

[wyliesdiesels]

Want to make sure I'm not missing anything. Have a 5.2 horse, 80 gallon compressor.

what is the HP rating as stated on the motor nameplate not tank label?

So my plan is to wire it to the pressure switch. Two questions along that line. First of all, is there any problem with that on that end of the circuit (i.e. the pressure switch or the auto drain)? I have already run 10/3 wiring to the compressor so I will have a neutral for it since it is a 120V, and the compressor is 240V.

10/3 NM-b is not code compliant for a 5HP motor.

Obviously only the hot wire would come from the switch. Specs on one that I have found state it only draws 4mA. I assume all of them will be similarly small, regardless of which one I get.

Second question is on the supply end. I assumed that I could just feed the drain off of the hot sides of the supply. Believe that is how ovens and other appliances that have 120/240V setups (i.e. a 240V that also requires a neutral). But I'm not positive. The breaker itself (30A, so way oversized for my needs) won't be affected by a slight current difference between the two poles, will it? In other words, am I overlooking anything?

actually a 30a breaker could be too small for a 5HP motor due to in-rush current and possible nuisance trips.

Are you hardwiring this compressor?

 

[67King]

what is the HP rating as stated on the motor nameplate not tank label?



10/3 NM-b is not code compliant for a 5HP motor.



actually a 30a breaker could be too small for a 5HP motor due to in-rush current and possible nuisance trips.

Are you hardwiring this compressor?

I'm a little confused here. 5.2 horse is 3.9kw, which at 240V is 16 Amps. Yes, I know it spikes at startup, but code is for operating current, not startup. And on edit, I'm putting a soft start on it, as well, to protect my inverter when convert to solar. And 10 gauge is rated for 30 amps. Not sure where you are seeing otherwise?

Motor reads 5.0, 22 amps (which is obviously higher than 16, still well under 30). I actually ran 50' THHN through metal conduit, and swapped to Romex at a junction box once I got it to the ceiling close to where I'm tying in. Electrician ran four NEMA 14-30 receptacles into my workshop when the house was built. I'm running to one of those, but taking out the receptacle, and hard wiring it to the new wiring (which is also 10/3) he ran. Compressor was on a 30 amp circuit with 10/2 for 19 years at my old house, and never had a trip. I'm also adding a soft start.

I know the routing is odd. Workshop was supposed to have been left unfinished, but drywall guys did not get that message. I had the electrician put a 30 amp circuit on each corner because I didn't know where my lift would go, and I wanted to just run the wiring to the ceiling. He put the receptacles there for inspection. I had planned to pull the wiring down when I figured out where the lift would go, but since they finished the workshop, I'm just running conduit, rather than tearing out drywall.

 

[wyliesdiesels]

I'm a little confused here. 5.2 horse is 3.9kw, which at 240V is 16 Amps. Yes, I know it spikes at startup, but code is for operating current, not startup. And on edit, I'm putting a soft start on it, as well, to protect my inverter when convert to solar. And 10 gauge is rated for 30 amps. Not sure where you are seeing otherwise?

code requires the wire for a motor circuit to be sized based on the table FLC amps. for 5HP this is 27a. 27 x 1.25 = 35a rated wire. #10 NM is limited to 30a.

Motor reads 5.0, 22 amps (which is obviously higher than 16, still well under 30). I actually ran 50' THHN through metal conduit, and swapped to Romex at a junction box once I got it to the ceiling close to where I'm tying in. Electrician ran four NEMA 14-30 receptacles into my workshop when the house was built. I'm running to one of those, but taking out the receptacle, and hard wiring it to the new wiring (which is also 10/3) he ran. Compressor was on a 30 amp circuit with 10/2 for 19 years at my old house, and never had a trip. I'm also adding a soft start.

NEMA plugs and receps are not rated for 5hp so its good you are swapping it to hardwired.

The 10/3 NM should be changed to #8 or #10 THWN if you want it to be code compliant.

 

[67King]

code requires the wire for a motor circuit to be sized based on the table FLC amps. for 5HP this is 27a. 27 x 1.25 = 35a rated wire. #10 NM is limited to 30a.



NEMA plugs and receps are not rated for 5hp so its good you are swapping it to hardwired.

The 10/3 NM should be changed to #8 or #10 THWN if you want it to be code compliant.

Interesting. So why would I not use 22amps is the motor is spec'd to 22A? Not trying to sound argumentative, I was unaware that motors required different amperage calculations.

And yes, I have a couple of EV's, I know that NEMA stuff can't handle the heat for prolonged loads (unless it is the EV spec 14-50 receptacle)!

Edit, I really appreciate your feedback on this.

 

[wyliesdiesels]

that is the code requirements. dont know the reasoning

as far as the NEMA receptacles for motor loads. it has nothing to do with heat and instead has to do with disconnecting the motor while running which can create a nasty arc flash and the plug is not rated for the HP.

 

[mm08822]

As I understood it from 20'ish years ago, high efficiency (energy star) motors were being installed everywhere using conductors sized to these lower full load amp (fla's).
That was fine, but when a motor failed, the next guy put an el cheapo motor in of same hp but was running at higher fla's. The conductors were never checked or at least never replaced with larger size.

So using FLC values in the NEC to size motor conductors from the onset assures the next guy doesn't screw it up.

 

[67King]

The 10/3 NM should be changed to #8 or #10 THWN if you want it to be code compliant.

Actually I just double checked. The stuff I ran was dual rated, THHN/THWN-2, which from what I've found appears to be good to 40A at 90C in a dry environment? I do have a short run of Romex, which would be only 30A.

As I understood it from 20'ish years ago, high efficiency (energy star) motors were being installed everywhere using conductors sized to these lower full load amp (fla's).
That was fine, but when a motor failed, the next guy put an el cheapo motor in of same hp but was running at higher fla's. The conductors were never checked or at least never replaced with larger size.

So using FLC values in the NEC to size motor conductors from the onset assures the next guy doesn't screw it up.

Interesting. That does make sense. But I can't say that I saw an Energy Star sticker on the compressor anywhere!

 

[BillK]

I dont have time to search but when I bought my machine shop business the compressor that was there was outside and it had some type of mechanical automatic drain ? It operated at a particular pressure if I remember correctly and each time the compressor cycled and the pressure went up and down it would activate the drain. We were in that building for at least 6 years and I never remember ever touching it. Might want to look into something like that ?

 

[mm08822]

Interesting. That does make sense. But I can't say that I saw an Energy Star sticker on the compressor anywhere!

I wasn't implying you had a high efficiency motor, but seems like a decent motor none the less. 22A is ~25% less than 28A.

 

[wyliesdiesels]

Actually I just double checked. The stuff I ran was dual rated, THHN/THWN-2, which from what I've found appears to be good to 40A at 90C in a dry environment? I do have a short run of Romex, which would be only 30A.

you didnt list the gauge so it means nothing and you dont size circuits based on the 90° column. thats for derating only.

 

[67King]

you didnt list the gauge so it means nothing and you dont size circuits based on the 90° column. thats for derating only.

I kept it at the same size as the rest of the wiring, 10 gauge.

 

[mm08822]

I kept it at the same size as the rest of the wiring, 10 gauge.

For a new install, per current NEC, 10s are under-sized. But, based on 22a x 125% (the old method), you're good.
Also, you have run history from a previous location with the same equipment.
I wouldnt worry about it at this point.

 

[wyliesdiesels]

I kept it at the same size as the rest of the wiring, 10 gauge.

#10 THWN is good for 35a so...