Motor horsepower and wire size

[Matt Sullenbrand]

Hello All,

I am beginning to set up my first permanent workshop in our recently purchased first house!! The workshop will be in the basement (definite pros and cons). I have done some wiring in the past and have worked alongside a few electricians and in the building trades as well, so wiring the workshop seems like a very doable option for me, along with insulating the ceiling, hanging resilient channel and drywall, doors, etc. I have read up on wiring in the past, but am reading the book Wiring Simplified for the first time, which is based on the NEC. My basic understanding of wiring for electric motors had been that for a 5hp motor, that tool should be on an independent 30a circuit with 10ga wire, and a 3hp on a 20a circuit with 12ga wire. A chart in this book indicated that, due to potential voltage drop, those wire sizes should both be increased by one gauge size each. All of my tools will be fed off of an independent 100a sub panel, which is fed by the main 200a service panel, and no tool will require wiring longer than 25 feet from the sub panel. This increase in wire gauge seems like it might be unnecessary since none of my tools are anywhere near the 100' max distance for the first wire gauge category, and since woodworking equipment does not typically have a pronounced startup load like a well pump might have. My budget does not have a lot of flex, much like many other people here, so I would rather not increase wire sizes if it is not necessary. Am I being pennywise and pound foolish here, or am I missing something? Any advice would be much appreciated.

Thanks,

Matt

[Thomas Hotchkin]

Matt
I am not a electrician, but I think you need to look at the motor's FLA (full load amp) rating and not HP rating, for your wire size. Your 100 amp sub panel, will it have enough 220 volt positions for all of your current and future tools? I have each large power tool on it's own circuit beaker, you can run out of spaces.

[Bill Neely]

The required ampacity is determined in the code book by motor type, voltage and horsepower. FLA isn't used.

You won't have enough voltage drop at 25 feet to worry about upsizing.

[John Coloccia]

Of course, you could always just do what I did: 10 gauge wire EVERYWHERE. I can go to any outlet and convert it to 120V, 20A all the way to 240V, 30A with nothing but an outlet and breaker swap. This has come in handy time after time after time after time. I hear you about budget. Do you have a friend who's an electrician, especially doing commercial jobs? Often times, you buy for the project, and it's just not worth it to try and store the leftovers. They end up in the dumpsters...spools of wire, heavy duty outlets, breakers, etc.

[Mike Heidrick]

How far is the sub panel from the main panel? Size that correctly for the amperage you will draw from the sub panel.

Once that is in do as you first said. run 10ga for your 220V circuits. If you will only run one 5hp tool at a time from each 220V circuit you can have more than one 220V outlet on each circuit.


Just run 12ga for all 120V circuits and install all 20amp outlets on each circuit you want.

[Peter Quinn]

I ran 10g for the 5hp circuits, 12ga for the 3hp circuits, 8g for the compressor ( that's a heavy start up and run)' 12 ga for all 110 circuits at 20A. None of my machines are far enough from the panel to suffer from voltage drop, I have experienced no ill effects. Aside from cost 8ga wire is hard to run and wire in general, I would not jump up to that casually. It's a good idea to run your plans by an electrician if possible even if doing the work yourself.

[Matt Sullenbrand]

Thanks much for the advice. I had expected that going up a gauge size was not necessary, and running 8ga is not something I was very interested in. But, I like the idea of running 10ga throughout. Always good advice here.

Best,

Matt

[Jim Andrew]

You might check the books of the machines you are wiring to. My 5hp small widebelt sander gave the size of wire as 8 gauge. Seemed a little much, but I put it in and have had no problems with the machine.

[Howard Acheson]

>>>> I think you need to look at the motor's FLA (full load amp) rating and not HP rating, for your wire size.

Correct, amperage is the factor that determines the wire size, not horse power.

[Bill Adamsen]

Breakers protect the wiring - so choice of breaker drives the minimum awg. Of course you want to make sure the breaker allows the motor to start and run to the service level required. One alternative I was encouraged to consider (forum members here and at Practical Machinist) would be to use a single circuit of 8awg (40amp breaker) with fused disconnects for each machine. For a single person shop where only one machine is used at a time, this could have merits. Of course you'd want to make sure that if the dc was running along with perhaps a planer, and the air compressor kicks in, that the circuit continues to function. I have a 5hp air-compressor on dedicated 8awg (40amp), and I always catch my breath when it kicks in.

[John Goodin]

If you are buying wire at a Borg look at pricing for larger amounts as well. Just went into one to but 100' of 12 AWG. It was only 15 bucks less than a 250' roll. If the 10 gauge is similarly priced it may make it pretty cheap to follow John C's good advice above.

[Keith Hankins]

Wow you will get a ton of responses. Read your manual, as it should tell you the amp rating. If the plate/manual calls for 20A them you can do a 12. In most cases, 3hp and not an extream run, 12Awg/20A breaker recepticle/breaker (keep them right). 5hp is generally 10awg. The big advantage of going with 10 even for a 3hp, is if you upgrade later, you don't have to rewire. No need to "up" the size above what's called for, unless you want to just spend the money. I've wired 5 shops now.

[Ole Anderson]

Wiring boxes with 12 ga is bad enough, 10 ga is a real pain.

[Matt Sullenbrand]

Thanks again for the comments. The collective knowledge is very much appreciated!

[John Coloccia]

Wiring boxes with 12 ga is bad enough, 10 ga is a real pain.

Tucking is certainly a bit more cumbersome, and you need to watch the box fill. I used stranded wire in exposed conduit. That makes things easier. Incidentally, if you twist the stranded wire counter clockwise before wrapping it around, say, a ground screw, it won't come apart when you tighten it. It also helps to leave the insulation around the end instead of pulling it clean off. None of my terminations are around screws, though. I won't use an outlet or plug that doesn't have a proper clamping termination of some sort.

[Anthony Whitesell]

Hello All,

...This increase in wire gauge seems like it might be unnecessary since none of my tools are anywhere near the 100' max distance for the first wire gauge category, and since woodworking equipment does not typically have a pronounced startup load like a well pump might have...



What others have said about wire gauge, size, amps, is all correct. I'm going to make two comments to the quote above.

(1) 3% voltage loss occurs at (nearly) 50' for each max amperage-min wire gauge specification. Ie., 15A in a 14g, 20A in 12g, etc. each reach the 3% voltage at or just past 50 feet (IIRC, 50-65'). I'm not sure what the 100' refers to, but the fine print note in the actual NEC refers to 3% voltage drop.

(2) I'm not sure what is leading you to conclude that WW tools (motors) do not have the same startup loads/impact that a well pump does. My well pump is actually much better (less load and/or less noticeable impact) than most of my WW tools. I use only one tool at a time with DC on a separate circuit. When I fire up the bandsaw, the light dim. That doesn't happen with the well pump. Both well pumps and WW tools contain large motors and the startup current calculations are based on factors with no differentiation between the motor for a well pump and a motor for anything else. I write this is sincerely as possible and am curious as to what has lead to your conclusion.

[Chris Friesen]

Breakers protect the wiring - so choice of breaker drives the minimum awg.

Just a quibble...this doesn't apply to motor circuits.

In a motor circuit the motor overload controls protect against long-term but minor overcurrent. Thus the FLA of the motor (or the HP, depending on what rules you're using) drive the conductor size. The breaker protects against short-term shorts, which allows the breaker to be oversized by up to 400%.

In practice the oversized breaker is rarely necessary for home woodworking equipment, so it's simpler to treat it as a general purpose circuit and put a 20A breaker on #12 wiring, and a 30A breaker on #10 wiring.


Note that as a general rule of thumb a typical 3HP motor will need a 20A 240V circuit, and a 5HP motor will need a 30A circuit. I've never had to upsize the conductor to the next higher size, but I'm in a double garage rather than a large shop with long wiring runs.