Machinery electrical hookups

[Bobby Robbinett]

The boss and I were having a discussion the other day about whether it is better to have a machine hardwired directly to its switch (or cut off box) versus having a plug and receptacle. We are installing a new 3 phase wide belt sander and he wants to put a 50amp plug and receptacle on it. He asked the electrician if that would be ok and the electrician said yes.

It would make sense to me that voltage loss would be lower by having the supply circuit line coming into the machine directly connected to the sanders internal connections with no plug or receptacle. The less parts in the chain the better, right? The electrician says it doesn’t matter and that either way will suffice. What are your opinions? 50amp 3 phase plugs and receptacles are not cheap but neither is the machine itself and my boss doesn’t want to skimp on stuff if it’s necessary but also at the same time wants to make it optimal.

[Alex Zeller]

I don't think you'll see any voltage loss by using a receptacle. Most of the equipment where I work was hardwired and now we are converting them over. With hardwired tools you have to deal with a breaker every time you need to do any servicing. If, for example, a tool needs to be moved for some reason you have to go through the extra work of disconnecting the wire and then deal with that wire. Dealing with the wire, for us, means looking out the breaker, putting wire nuts on the exposed copper, using electrical tape, posting a sign saying the circuit is powered off, and making sure the wire isn't a trip hazard. In a home shop you wouldn't go to those extremes.

Chances are once in place the odds on having to remove it are probably pretty low with a woodworking machine. So hardwiring it isn't as much of an issue. Your electrician should be able to tell you the code better than I can but for hardwired equipment we use sealtight conduit (I think any metal conduit or armored clad cable is fine) where as with a receptacle we can use SOOW. SOOW is more flexible so it can be better positioned to be out of the way. While we don't I'm pretty sure that an overhead drop with the proper strain relief can be SOOW. But the only ones I've paid any attention to all have a receptacle on them (either on the cealing or at the end of the cable).

So I think it comes down to what will work best. Cable on the floor can be a tripping hazard. A ceiling drop only works if the cable is not in the way.

[Jim Barkelew]

I'm not an electrician. I think there are two things to consider. If the machine might be moved maybe a plug makes sense with a few extra feet of chord. I use twist lock plugs for my 220V machines. if the machine is bolted to the floor hardwiring makes sense. Maybe a more important issue is business /insurance/OSHA considerations. I worked around large industrial machinery and lock-out/tag-out of power for maintenance was mandatory. The issue is the machine being powered up while someone else is working on it. Small shop might not have that issue.

Jim

[Bobby Robbinett]

I'm not an electrician. I think there are two things to consider. If the machine might be moved maybe a plug makes sense with a few extra feet of chord. I use twist lock plugs for my 220V machines. if the machine is bolted to the floor hardwiring makes sense. Maybe a more important issue is business /insurance/OSHA considerations. I worked around large industrial machinery and lock-out/tag-out of power for maintenance was mandatory. The issue is the machine being powered up while someone else is working on it. Small shop might not have that issue.

Jim

This sander is bolted to the floor and I highly doubt it will be moved, ever. Looks like the manual to the sander recommends hardwiring over plug and receptacle. So the boss man will likely go with hardwiring it. The old sander sat there for over 2 decades. We have dust collection hard piped there too.

[Jim Becker]

It's appropriate to have a local disconnect that isn't the breaker for a machine, so "either" the plug or a disconnect on the wall near the machine if hardwired is what I'd do. Hardwiring makes sense for a stationary machine literally bolted to the floor.

[Charles Lent]

The National Electrical Code for the USA allows flexible cords and plugs to be used for equipment that will be moved frequently, or in places where the equipment being connected vibrates excessively, but when you bolt a machine down, it is considered permanent equipment, and should be wired as if it was part of the building. I like the cord and plug for the ability to unplug and completely isolate the machine from power when doing any major maintenance because it's very easy to SEE that it is disconnected from power, but the NEC wants permanently installed tools to be connected directly to the building wiring with an electrical disconnect switch that can be locked off for maintenance. So to meet the code, it's best to hard wire that sander to the building power through a disconnect switch capable of being locked off. If he really wants to use a cord and plug, he should not bolt the sander to the floor. It could be considered "portable" then, even though it might take hand trucks or a forklift to move it. It is is a bit of a "gray" area of the code that most electricians don't know or understand, but a competent OSHA or local electrical inspector could fine him for.

Charley

[Frank Pratt]

Having a plug & receptacle won't make a bit of difference with regards to voltage drop.

[Bill Dufour]

code requires a disconnect visible from the machine. So you can use a cord and plug with the breaker mounted in a central panel anywhere. If you hardwire the switch box or breaker panel must be visible from the machine. Having all those boxes scattered around can be a bit confusing and hard to figure out in a emergency.
A clean dry shop should have no measureable resistance in a outlet conenction. Outdoors with green blades on the cord maybe some resistance.
Biill D.

[Rod Sheridan]

I prefer a padlockable disconnect switch, much more convenient than having to apply a clamshell and padlock to a cord end.......Rod

[Jerry Bruette]

code requires a disconnect visible from the machine. So you can use a cord and plug with the breaker mounted in a central panel anywhere. If you hardwire the switch box or breaker panel must be visible from the machine. Having all those boxes scattered around can be a bit confusing and hard to figure out in a emergency.
A clean dry shop should have no measureable resistance in a outlet conenction. Outdoors with green blades on the cord maybe some resistance.
Biill D.

Not sure what code requires the disconnect to be visible from the machine. I've worked in several factories that had motor control centers (MCC rooms) that were climate controlled and full of disconnects that were not visible from the machines.

As far as the plug and receptacle goes, just because its unplugged doesn't mean it's locked out. You have to secure the plug in a lock out device.

[John K Jordan]

Voltage drop results from the insufficient wire diameter and/or excessive wire length for a given current. There are voltage drop calculators online.

A plug/receptacle won't reduce the voltage unless it is defective, oxidized, or quite dirty. These conditions might cause arcing or heat. The same conditions can exist for improperly made connections when direct wiring.

I like to have a plug/receptacle for a positive disconnect when working on a machine, temporarily moving it, etc. Twist lock connections may be better for some environments.

[Alex Zeller]

One of the biggest problems we have is stranded wire connected to terminals. Either the screws are not tight enough or they don't capture all the stands. The result is the contact area is much smaller than the gauge of the wire which then heats up the connection. Heat and resistance go hand in hand when one goes up so does the other. Sooner or later the the connection gets bad enough where impacts the tool. We often use a thermal camera to look for them before they cause damage.

[Bill Yacey]

Stranded wire can be tinned with solder, essentially making a solid conductor at the connection point. The more surface area of contact with the conductor, the lower the resistance of the connection, with less tendency towards heating.

Just about any plug / receptacle introduces higher resistance then the conductors themselves because the contact surface area between the plug and receptacle is usually quite small, and the subsequent heating from the higher resistance weakens the spring pressure in the socket which in turn further exacerbates the situation.

[David Walser]

... Looks like the manual to the sander recommends hardwiring over plug and receptacle. So the boss man will likely go with hardwiring it. ...

I recently installed my new lathe. It's manual, too, said to hardwire the lathe. However, my shop only has one 240 circuit and I, currently, have two machines that use that voltage. So, I went with a plug -- just like I did with the lathe the new one replaced. I strongly suspect that part of the manual was written by the company's attorneys, not its engineers.

[Rollie Meyers]

A plug/receptacle used for a disconnect is required to be rated for the HP of the equipment, for motors above 3 HP, they are super expensive making a safety switch pretty economical in comparison, pretty much of all the cord & plug connected single phase 5 HP equipment does not meet code. 430.109(F)

[Charles Lent]

On larger plugs with arc shields we drilled a hole in the edge of the arc shield for a padlock to lock them out. Some of the smaller twist locks have a small hole in the ground pin that we enlarged to be able to use a small padlock through it. In either case, the padlock prevents the plug from being inserted. Add the lockout tag and it complies. If the disconnected plug on smaller equipment is in full view by the person servicing the equipment a lockout is not required. Unplug the machine and tape the plug to the machine cabinet in full view and you have complied. If the machine will be left unmanned for any time, it's best to lock the machine off with a lockable container for the plug, unless a lock can be applied to the plug as above.

Charley

[Charlie Velasquez]

A plug/receptacle used for a disconnect is required to be rated for the HP of the equipment, for motors above 3 HP, they are super expensive making a safety switch pretty economical in comparison, pretty much of all the cord & plug connected single phase 5 HP equipment does not meet code. 430.109(F)

NEC wrt motors is an entirely different animal. You must account for ground fault protection, short circuit protection, and over-current protection. All at the same time.
.... AND make sure that the over-current protection allows the motor to start .... at maybe six times its running amps.
Each part has its own table to determine conductor size and circuit breaker or fuse size, and sometimes it can lead to weird pairings... a 14g wire on a 30 amp breaker, for instance.

[Charlie Velasquez]

On larger plugs with arc shields we drilled a hole in the edge of the arc shield for a padlock to lock them out. Some of the smaller twist locks have a small hole in the ground pin that we enlarged to be able to use a small padlock through it. In either case, the padlock prevents the plug from being inserted. Add the lockout tag and it complies. If the disconnected plug on smaller equipment is in full view by the person servicing the equipment a lockout is not required. Unplug the machine and tape the plug to the machine cabinet in full view and you have complied. If the machine will be left unmanned for any time, it's best to lock the machine off with a lockable container for the plug, unless a lock can be applied to the plug as above.

Charley

The lockout is good, but it is that arc shield that gives the plug it's hp rating. A disconnect must be able to safely interrupt power from the motor. Motors are just generators backwards. A spinning motor can sustain an arc when disconnected. so, arc shields, enclosed pin and sleeve, spring disconnect, whatever to contain/control the arc.

Edit: as Rollie said, switch is easier

[Rollie Meyers]

NEC wrt motors is an entirely different animal. You must account for ground fault protection, short circuit protection, and over-current protection. All at the same time.
.... AND make sure that the over-current protection allows the motor to start .... at maybe six times its running amps.
Each part has its own table to determine conductor size and circuit breaker or fuse size, and sometimes it can lead to weird pairings... a 14g wire on a 30 amp breaker, for instance.

But you do not get to use those "weird" pairing with a receptacle, it would need to be hardwired.