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Discussion Starter #1
I just bought a dust collector. The vendor says it draws 20a and requires a 40a circuit. I asked around and most users put in a 30a circuit and never have a problem.

I wired it up temporarily and found it draws 140a for less than a second and then quickly drops to 20a.

I looked up Square D's trip curve. They say that 5x the rated load will trip a breaker between 4 and 12 seconds. So my 140a is less than 5x the 30a rating, and only occurs for a split second. So, a 30a breaker ought to be okay, right? Even if my meter is reading low, it will take 210a for 2 seconds; and it couldn't possibly draw 210a.

140a seems like a lot of current and I am a little surprise they design breakers to stay open that long. Since it sounds odd to me, I just want to check to see that I am understanding it right.

For bonus points, if someone wants to explain how a 3hp motor can draw 20a, I would like to know. I thought maybe that was with no load, but it is 20a while working the machine. Seems really really high.
 

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Discussion Starter #3
They put the plate on the back of the motor, so my picture of it isn't very good.
3hp, 60hz, 220v, 22a, 3450rpm, 1 phase, continuous, 40C, Class F.

It came without a cord. For testing it I hardwired it, but planned on putting cord and outlet in. It is only 15' from the breaker box; I suppose hardwiring might be an option. What material is suitable for a whip to the control box?

I swapped the 40a breaker out for a 30a and started it 3 times without any problems. Oddly it only drew 19.2a on the 30a breaker compared to 20.4 on the 40a breaker; same cable. Maybe it was a bit more broken in?
 

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The breaker size has nothing to do with what the unit is drawing for amperage. If you are going direct to the panel there are lots of options depending on what is allowed by the authority having jurisdiction in your area.
 

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If the manufacturer states it must be on a 40A circuit, it must be on a 40A circuit.
 

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What model # dust collector is this? The motor nameplate says 22 amps continuous so I believe the minimum branch circuit wiring for a single motor @ 125% is 27.5 amps.
 

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What model # dust collector is this? The motor nameplate says 22 amps continuous so I believe the minimum branch circuit wiring for a single motor @ 125% is 27.5 amps.
You size the OCP and min. branch circuit wiring off the HP rating...

3 HP @ 240 v. = 17 AMPS

17 AMPS X 250% = maximum breaker size, = 40 Amp breaker

17 AMPS x 125% = minimum branch circuit wiring, = 21.25 #12AWG is good for 25 amps in this scenario,

This of course is for hard wiring the motor.
 

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Discussion Starter #8
What model # dust collector is this? The motor nameplate says 22 amps continuous so I believe the minimum branch circuit wiring for a single motor @ 125% is 27.5 amps.
I wasn't aware there was a rule like that. Thanks.

Grizzly G0441
http://www.grizzly.com/products/3-HP-Cyclone-Dust-Collector/G0441

The same motor is used in the Shop Fox W1816. There they call for a 30a circuit. They used to sell the identical cyclone, and that was on a 30a circuit also.
http://pics.woodstockint.com/specsheets/w1816_ds.pdf

I can't see how the breaker could affect the current also, but it was consistant. I guess I should go back to the 40a and see what happens. Actually, now that I think of it, I measured the 40a at the circuit box, and the 30a at the machine; but that couldn't possibly matter.
 

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What model # dust collector is this? The motor nameplate says 22 amps continuous so I believe the minimum branch circuit wiring for a single motor @ 125% is 27.5 amps.
You are not allowed to use the nameplate to size the branch circuit wiring, you have to use the NEC.
 

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If the manufacturer states it must be on a 40A circuit, it must be on a 40A circuit.
You know this, buz, but it's worth saying for others: What buz says is true for sizing the conductors; you must comply with the code for wiring the circuit that the nameplate says it requires. It's nearly always OK to underfuse a circuit, though. So you must wire a circuit sized for 40A. However, you can then put a 30A breaker on the circuit, and if it'll start on that without tripping, no harm done.
 

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You know this, buz, but it's worth saying for others: What buz says is true for sizing the conductors; you must comply with the code for wiring the circuit that the nameplate says it requires. It's nearly always OK to underfuse a circuit, though. So you must wire a circuit sized for 40A. However, you can then put a 30A breaker on the circuit, and if it'll start on that without tripping, no harm done.
Except voiding the warranty. :whistling2:
 
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Discussion Starter #13
You size the OCP and min. branch circuit wiring off the HP rating...

3 HP @ 240 v. = 17 AMPS

17 AMPS X 250% = maximum breaker size, = 40 Amp breaker

17 AMPS x 125% = minimum branch circuit wiring, = 21.25 #12AWG is good for 25 amps in this scenario,

This of course is for hard wiring the motor.
Thats interesting. I can use #12 and a 40a breaker if I hardwire. There would only be about 25' for cable, so the voltage drop would be trivial. I think a 40a breaker is overkill; I can use a 30a breaker can't I?
I know I can't just run the romex to the switch box; I would have to run romex to a junction box and run some sort of whip to the switch box. What are the requirements for that?

I take it that everything changes if I go to a plug. What do I need then?
I appreciate the help.
 

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I would be sure to hard wire it. You will need to run some sort of conduit to the dust collector I would run mc wire or liquid tight. And I would also do a 40 amp circuit to the dust collector.
 

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Discussion Starter #15
I would be sure to hard wire it. You will need to run some sort of conduit to the dust collector I would run mc wire or liquid tight. And I would also do a 40 amp circuit to the dust collector.
Do I run MC from the panel to the switch box, or can I run romex to a junction box and MC from the junction box to the switch box?

When you say a 40a circuit, are you saying #8, or the #12 that stickboy says is adequate,with a 40a breaker?
 

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For bonus points, if someone wants to explain how a 3hp motor can draw 20a, I would like to know. I thought maybe that was with no load, but it is 20a while working the machine. Seems really really high.

Sounds about right !
1 hp = 750w.
3 x 750 = 2250w.
2250w / 120v = 18.75a.

Close enough !
 

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MEASURE ONCE, CUT TWICE
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Thats interesting. I can use #12 and a 40a breaker if I hardwire. There would only be about 25' for cable, so the voltage drop would be trivial. I think a 40a breaker is overkill; I can use a 30a breaker can't I?
I know I can't just run the romex to the switch box; I would have to run romex to a junction box and run some sort of whip to the switch box. What are the requirements for that?

I take it that everything changes if I go to a plug. What do I need then?
I appreciate the help.
Last time I checked, #12 needs to be protected by a maximum 20 amp breaker.
I would use emt if surface mounting, from the panel to a box and cabtyre for your "whip" to the motor.
 

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Last time I checked, #12 needs to be protected by a maximum 20 amp breaker.
What did you "check" to find that? That is a common misconception among DIY'ers who don't read the ENTIRE code.

Mark
 

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Thanks for pointing out my error.

Enlighten us DIY'ers as that's why we're here.

Thanks.
Pugsy
Stickboy's post #9 above cites the code sections and includes a diagram.
 
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