3 HP wiring for my dust collector and starting surge

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Forum topic by dmorrison posted 02-27-2012 10:03 PM 2217 views 0 times favorited 7 replies Add to Favorites Watch
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151 posts in 3504 days

02-27-2012 10:03 PM

Changed the setup for my shop and moved the dust collector. I have a 3HP Transpower single stage unit. Being that it is 3HP it is wired to a 220volt 30AMP circuit. I used 12 gauge wire for the long run of wire, about 35 ft, from the outlet to the remote control relay box.

I put a amp meter on the unit and it draws 10 amps while running. So the 30amp circuit is overkill and the 12 gauge wire is fine.

Here my question for all the electrical guru’s out there. I attached the amp meter to the wire and the starting surge is 60AMPS. The surge lasts about 2 seconds

Seems like a awful lot of amps. Is this normal for a 3HP motor to have this kind of starting amperage.

The unit runs fine. Circuit breakers do not pop. All is well. I just never put an amp meter on it while starting.


7 replies so far

View canadianchips's profile


2616 posts in 3239 days

#1 posted 02-28-2012 03:54 AM

One horsepower electric is defined as 746 watts, so three horsepower is 2238 watts. Divide that by 240 volts, and you get about 9.325 amperes. Now you have to consider the power factor, which is the ratio of apparent power to true power, caused by the inductive (reactive) load of the motor causing the current to be out of phase with respect to voltage. You did not say what the power factor was, so lets assume it is 0.8. That makes the current about 11.65 amperes.

That’s running current, at full load. Starting current is often four times higher.

11.65×4 = 46.62 starting.

-- "My mission in life - make everyone smile !"

View Paul Stoops's profile

Paul Stoops

352 posts in 2803 days

#2 posted 02-29-2012 02:57 AM

That doesn’t seem out of the ordinary to me. The starting surge only lasts for a very short time. It takes a lot of power to get the fan impeller up to speed.

However, if I read your post correctly, it sounds as though you are running 12ga. wire from a 30A breaker. I think that could be a safety hazard, since 30 amp circuits are normally wired with 10ga. wire. I believe that 12ga wire is rated at only 20 amps.

I agree that the 12ga wire is suitable for your DC motor load, but that circuit should be protected with a 20 amp breaker instead of a 30 amp. I would change out the breaker as soon as possible.

With your present setup, the 30 amp breaker will never trip until you have overloaded the current carrying capacity of the 12ga wire by 50%. That is not a good way to test your smoke alarms….................!!!

-- Paul, Auburn, WA

View Grandpa's profile


3261 posts in 2917 days

#3 posted 02-29-2012 03:36 AM

I agree with Paul. The breaker is really in the circuit to protect the wiring in the wall and prevent fires inside the wall. If the wiring is overloaded with amps then it can burn and the breaker will not trip. In reality the wire becomes the fuse. Not something you want burning inside the wall. 12 ga. is rated for 20 amps. Probably the best thing would be to replace the wiring with 10 ga and use a 30 amp receptacle. That is where I would go with this system. In fact I am getting ready to install some wiring for a collector that I have ordered and this is what I plan to use. 30 amp CB with 10 ga wire. I am getting excited about it. The electric panel for the system came 2 days ago and the filters came today. I am getting ready for it.

View Paul Stoops's profile

Paul Stoops

352 posts in 2803 days

#4 posted 02-29-2012 03:57 AM

Altho Grandpa’s approach has merit, I think a couple of factors need to be considered, as well. The cost of copper wire has skyrocketed recently, so changing his wiring from 12ga. to 10ga. involves a serious investment—plus the higher cost receptacle.

More than that, I would wonder about putting a fairly low current draw device (10A) on a high current circuit. The motor would have to draw 30 amps for a long enough time to trip the breaker, so I wonder if the motor insulation would withstand that without sustaining some damage. Personally, I would feel safer simply changing the breaker to a 20 amp unit. However, if you plan to use other tools on the same circuit at the same time, the higher rated circuit could be an advantage, particularly if the tools start at the same time.

Regardless of which way you decide to go, the breaker amperage rating and the wire current carrying capacity must be the same: i.e. 20A breaker w/12 ga. wire, or 30A breaker w/10ga. wire.

-- Paul, Auburn, WA

View dmorrison's profile


151 posts in 3504 days

#5 posted 03-28-2012 06:05 PM

Paul Stoops
I agree the wiring in the wall to the 30 amp outlet is 10 ga. wire. It is the wire leading from the DC to the outlet. I did go with 12 gauge wire. Just as a lamp that is rated at 15 amps has a cord designed for 15 amps. the outlet it’s plugged into is probably a 20 amp circuit. At least in my shop.



View BlankMan's profile


1490 posts in 3595 days

#6 posted 03-28-2012 07:38 PM

Dave, inrush current can be 4 to 8 times the FLA current and is equivalent to locked rotor current. NEMA has some standards that can help narrow down what it really is on your motor. Most motor manufacturers put a NEMA Code letter on the motor’s nameplate that is an indicating code.

Take a look at your motor and see if there is something like “Code – J”, that’s off a 2HP Marathon motor I have. If there is let me know what the letter is and I can calculate the range the inrush current would be in for that motor.

I had a similar problem with my air compressor when I first got it, empty it would start up fine but when starting to recharge it would pop the 20A breaker it owned all alone. Turns out to get around that I had to install a 30A breaker wired with 10ga to a 20A slow-blow fuse that was then properly sized to protect the motor taking into account it’s service factor. The 20A breaker couldn’t handle the inrush, the 30A could. Came up with this solution after dealing with Marathon (the AC motor mfg), Square D (CB mfg), and Bussman (whose fuse I used) engineers.

-- -Curt, Milwaukee, WI

View Fred Hargis's profile

Fred Hargis

5244 posts in 2735 days

#7 posted 03-28-2012 09:40 PM

I think you are seeing what’s pretty normal on the inrush at startup, and certainly the #12 cord is fine, although that is a lengthy extension. The 30 amp circuit may be overkill, but that’s what I ran to my tools that are 3 HP or up. I think you’re in good shape.

-- Our village hasn't lost it's idiot, he was elected to congress.

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