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  • Amp Draw Question

    I wish I were more knowledgeable about electrical stuff but I'm not so here is the dumb question of the day. When a motor is rated at x number of amps, is that rating the actual amp draw all the time when the motor is running or something else? The reason I ask is that I'm looking at buying a compressor that is rated at 15amps and the circuit in my garage is on a 15amp breaker. Will I be tripping the breaker every time I turn the compressor on?
    Teach your kids about taxes..........eat 30 percent of their ice cream.


  • #2
    Re: Amp Draw Question

    Originally posted by BadgerDave View Post
    I wish I were more knowledgeable about electrical stuff but I'm not so here is the dumb question of the day. When a motor is rated at x number of amps, is that rating the actual amp draw all the time when the motor is running or something else? The reason I ask is that I'm looking at buying a compressor that is rated at 15amps and the circuit in my garage is on a 15amp breaker. Will I be tripping the breaker every time I turn the compressor on?
    That is the draw of the motor while running. On start up the motor will draw more current.

    The amp rating on breakers is the amperage required to be run continuously to trip the breaker. It would take a instant draw of 10,000amps to trip a breaker.

    Both those points said. The breaker shouldn't trip on start up(In theory, though depending on the breaker it might anyway) but it would trip after running, as a continuous run of 15 amps should trip it.

    IMHO you need a smaller compressor or a larger circuit.

    Comment


    • #3
      Re: Amp Draw Question

      I agree... generally the amp rating is the "running" or normal operational load amperage draw. Start-up will require a short "surge". Back when one used fuses, you would have to use a "slow-blow" in order to use a 15-amp device, otherwise a normal fuse would burn out.

      Circuit breakers are designed to handle short amounts of load over the rating (as I understand it, anyway).

      I hope this helps,

      CWS

      Comment


      • #4
        Re: Amp Draw Question

        You can probably get by with that compressor on a 15 amp circuit as long as you don't have anything else on it. The amp draw will exceed the 15 amp when the compressor starts under load, but usually, if it is rated 15 amp, it will run on a 15 amp circuit. The actual normal operation will probably be less than 12 amps. I have a 15 amp compressor, a 13 amp table saw, and a 15 amp circular saw, all which run fine on a 15 amp circuit. I can't start any of them sometimes or in the case of the table saw really load up the motor, if I have the 7 amp shop-vac running at the same time, but a light bulb or fan doesn't seem to bother the operation. Most breakers will take a momentary surge exceeding their rated amps to allow for motor (inductive) loads.

        Many of the tools are rated a peak amps, not normal load.

        Go
        Last edited by Gofor; 08-26-2010, 10:19 PM.
        Practicing at practical wood working

        Comment


        • #5
          Re: Amp Draw Question

          Here's more info than you probably wnat but you might find it useful in terms of understanding.

          The nameplate rating of the motor is, believe it or not, primarily a thermal rating. The speed of an AC induction motors depends primarily on the line frequency (60 Hz here in the US). Which is why you see motors almost always rates at 3450 or 1725 rpm - these speeds are the number of winding poles in the motor... a 4 pole winding will gove you a nominal 1800rpm speed and a 2 pole winding will give you 3600. THe 3450 and 1725 numbers account for what is called "slip".... the motor tries like hell to run synchronously but comes up just a little short.

          The point is that as load increases, this type of motor will slow down a little bit but not too much. The winding poles have to stay in pretty close sync to the line frequency. So, it will simply draw more current (and change the phase angle between voltage and current) as it struggles to maintain speed. As load increases and it pulls more current, the motor necessarily gets hotter.

          The nameplate rating, in your case 15 amps, means that the motor is designed with winding insulation, bearing grease, etc that can cope with the heat generated at that current. Most often the motor plate will also specify a temperature... which is the ambient at which the ratings apply. You will also often see a service factor - typically "1.15" or "1.30" - which means that the motor can stand tolerate overloading to 115% or 130% of the ratings... although life will be shortened.

          Line frequency is very stable in the US. But line voltage varies a bit from the power company and also depending on the draw. With an AC motor, if voltage drops, the current will increase to make up for it... the motor, remember, doesn't want to slow down. This, btw, is why you don't want undersized or too-long extension cords, especially to an induction motor. The small cord causes a voltage drop... which increased the current draw... and the motor then runs too hot. Even if it doesn't burn out, you're shortening its life if you use "wrong" extension cords.

          Back to the topic... In your case, the compressor pump presents a very predictable load to the motor. The designers matched the pump to the motor and selected the pulleys (if it's a belt drive) so that the current would get close to the rated power, but not exceed it. No responsible designer exceeds the motor nameplate rating. The real question is, did they match the pump so that the nominal motor draw is really close to 15A? Or is the nominal draw something less, despite the nameplate rating? Unfortunately you're not going to know this from the specs. Therefore, you can't be sure that you aren't going to be tripping the breaker every time your home AC cuts in, there's a momentary drop in line voltage, and the compressor current spikes to maintain speed. But it's pretty unlikely. Nominal current draw is likely to be around 10-12 amps with a 15 amp nameplate-rated motor.

          However, every time the AC motor kicks on, it has to spin up the motor and whatever it's attached to (pump in this case). Remember that this motor wants to get to line frequency, so it will accelerate as hard as it can. In so doing, but depending on the design of the motor start winding and circuit, and even if the nominal draw is less than 15A, you may pull more than 15 amps. Could be several amps higher. With a compressor, the duration of the startup draw can be enough to trip your breaker. the point of all this is that it depends on the design, NOT on the nameplate rating.

          So the bottom line is that its a crapshoot - might be just fine. If we're talking a 120V circuit, that 15 amps, assuming a power factor of 0.8 is about 2 hp (forget the advertised hp rating...). It might work, but for a compressor of this size I think a 15A circuit is undersized. I would say that if it's a US made compressor, cal the manufacturer. If it's chinese... no way to tell but my experience is that they most often have an undersized pump and thus pull far less than the ratings (and underperform their specs). Again, there is no way to know for sure.... Do you have any spaces left in the panel? If it's an attached garage, stringing some romex is pretty easy, and will give you a dedicated circuit for your compressor which is always the best way to go.

          If you give it a try and have trouble, whatever you do, DON'T just change the breaker! Your 15A circuit is most likely wired with 14g... you need 12g for a 20A run.

          Comment


          • #6
            Re: Amp Draw Question

            Originally posted by Andy_M View Post
            ...........................................
            If you give it a try and have trouble, whatever you do, DON'T just change the breaker! Your 15A circuit is most likely wired with 14g... you need 12g for a 20A run.
            Of course, that is one of the "solutions" I was considering if I can't get the compressor to run right on the existing circuit. Thanks for that heads up. Fortunately, the breaker box is located in the garage so adding a circuit, if I have to, shouldn't be that big of a deal, I hope.
            Teach your kids about taxes..........eat 30 percent of their ice cream.

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            • #7
              Re: Amp Draw Question

              Didn't mean to offend by stating the obvious about the wire guage... but other folks read these posts and who knows what people are likely to try to do!

              Comment


              • #8
                Re: Amp Draw Question

                rule of thumb is the load on a breaker shouldn't exceed 80% of the rating.

                so a 15 amp should only run at about 12 amps. and a 20 should run about 16 max.

                the biggest demand is when the compressor is already pumped to pressure and has to refill.

                i would install a 20 amp dedicated circuit.

                rick.
                phoebe it is

                Comment


                • #9
                  Re: Amp Draw Question

                  Originally posted by Andy_M View Post
                  Didn't mean to offend by stating the obvious about the wire guage... but other folks read these posts and who knows what people are likely to try to do!
                  Sorry if I wasn't clear about who the slap on the head was meant for as it was meant for me! Sometimes these smiley's can turn around and bite one right in the.......... I seriously WAS considering that alternative and you saved me and those "other folks" out here from making a huge mistake. Again, sorry for not being clear.
                  Teach your kids about taxes..........eat 30 percent of their ice cream.

                  Comment


                  • #10
                    Re: Amp Draw Question

                    Originally posted by PLUMBER RICK View Post
                    rule of thumb is the load on a breaker shouldn't exceed 80% of the rating.

                    so a 15 amp should only run at about 12 amps. and a 20 should run about 16 max.

                    the biggest demand is when the compressor is already pumped to pressure and has to refill.

                    i would install a 20 amp dedicated circuit.

                    rick.
                    Rick, the 80% sizing rule applies for continuous loads, which are typically defined as 3 hours or longer of continuous running... with some exceptions for specific equipment. A plug-in type air compressor falls, I believe, under "intermittent load". Thus the breaker could be sized at 100% (unless the start draw causes a trip of course). If the circuit isn't dedicated, you have to do a load calc, in which continuous and intermittent loads are treated differently.

                    But yes a 20A dedicated circuit is best. It's never too wise to max out a circuit... allowed by code or not. Since Dave's panel is already in the garage.... seems like an easy fix and an hour well spent!

                    Comment


                    • #11
                      Re: Amp Draw Question

                      Badger Dave,
                      If you need a hand with a 20-amp circuit for that compressor,
                      let me know. You're only about 2.5 hours away and I could make a run down to ya and pull power from that load center to a dedicated 20-amp box for you. Or if you like, I'll send you my phone no. and I can walk you through that without much prob. over the tele.
                      Cheers,
                      Jim Don

                      PS. If you decide to do it yourself, get some BX with 12 gauge inside and a four-square metal box with a single gang outlet cover. This will allow you a larger box to work in with the wires. And still be able to close up the four-square without have to fight the wires directly behind the receptacle. The BX is a little easier for a novice to work with so you won't need a bender, while still protecting the wiring if it is strung along a wall. The wire always needs mechanical protection if it is not run inside of a wall.

                      Comment


                      • #12
                        Re: Amp Draw Question

                        JimDon, thank you for the very generous offer. If push does come to shove and I have to run a new circuit, I do have a relative who is a sparky that I can impose upon to help me. Actually, I'll probably end up helping him................ by just staying out of his way. Again, thanks for the offer that was very nice of you.
                        Teach your kids about taxes..........eat 30 percent of their ice cream.

                        Comment

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