Mark V Electrical Requirement

[GG0452]

What are the proper/safe electrical circuit requirements for the Mark V.

For 40 years i have followed the SS specified electrical requirements that came with my 1983 Mark V 500 with an AO Smith 1 1/8 HP motor. These are; 15 amp minimum but 20 or 25 amp circuit is better if other equipment is used along with the Mark V. My circuit is 30 amps (10 gauge wire/ 2-15 amp receptacle in-line) and the shop vac (6.5 amps) and 500 watt (2 at 4 amps each but usually only use one) shop light are used on the same circuit. I have not fried the motor (it was just serviced and all is well) or tripped the breaker during use. And i never turned the speed dial down to D or lower before shutting the machine off.

I have recently read the Owners Manual for the Mark V 520 and they are now saying that i should have a dedicated 15 amp service for the Mark V and the receptacle and wire gauge should be for 20 or 25 amps for a bigger safety margin. And i should always turn the speed dial down to D or lower before i turn it off (what a pain that would be).

Anyone know why the change? Are the later motors less reliable?

[dusty]

I can not site reference but I do not believe this is a change. I believe Shopsmith recommendation has always been a dedicated 15 amp minimum circuit.

I have a 20 amp circuit(s) in my shop and I normally use it as dedicatd but on occasion I plug something in the same circuit along with the SS. I have never had a problem.

Again I say, I don't believe the Shopsmith or their recommendations have changed..

[BuckeyeDennis]

I added a new subpanel to my workshop recently, and came across this exact issue.

Per my Mark V manual: "The wire size (gauge) in the circuit you use must be rated for at least 15 amps. Using wire and receptacles rated to handle 20 or 25 amps will give you an even bigger safety margin. Do not use fuses or circuit breakers larger than 15 amps."

At first blush, this doesn't make much sense. But I found an article by Mike Holt, who lectures about the National Electrical Code (NEC), that sheds considerable light on the matter.

Per the NEC, induction motors should have a supply ampacity that is at least 125% of the motor's full-load current. Per the ratings plate of an Emerson motor from a Shopsmith, it's full-load current is 13.2A, and thus the supply circuit wiring should be sized for at least 1.25 * 13.2 = 16.5A. But induction motors have huge starting currents, several times greater than their rated continuous full-load current. So again per the NEC, the branch circuit breaker is supposed to be rated at 250% of the motor's full-load current (to avoid nuisance trips at startup). In this case, 2.5 * 13.2A = 33A, which then gets rounded down to a standard 30A breaker. That will still protect the supply wiring in the event of a short circuit. But crucially, in this scenario the motor is supposed to have its own independent thermal overload protection, to protect it from destructive overheating.

For whatever reason, Shopsmith did not design thermal overload protection into the Mark V itself. So instead, I believe they specify a 15A branch-circuit breaker so that it does double duty as both short-circuit protection and motor thermal overload protection. But now, the supply circuit is not as capable as the motor itself is when it comes to large, short-duration loads. Therefore Shopsmith says to crank the speed dial down before starting the motor, effectively reducing the load inertia on the motor. This allows the motor to come up to speed faster, and so reduces the duration of the large start-up current.

I measured the startup current on my Mark V at 54A, independent of the inertial load. You can see more than you probably want to know about that at this thread:

viewtopic.php?p=189429#p189429

Most of us these days have inverse-time breakers in our load centers, which act like time-delay (aka slow-blow) fuses. These can handle much larger short-term currents than the standard fuses typically installed back in the day. If I have a sanding disc mounted on my Mark V, I do turn down the speed dial before starting the machine. But with a saw blade mounted, I routinely start it up as Saw/Joint speed.

Based on all this, when I installed my new subpanel, I ran a 12AWG supply circuit for the Mark V. But I installed a 15A breaker to protect the motor, rather than a 20A breaker to match the wire ampacity. I'm admittedly unlikely to pull between 15A and 20A on my Shopsmith for long enough to fry the motor, but I figure it's better to be safe than sorry.

[GG0452]

My 1983 SS Mark V manual clearly states under Electrical Requirements page 2: "The amperage must be rated high enough to handle the load (in amps) of that motor, plus any other tools or appliances you may have plugged into the same circuit and running at the same time." If you run at 85 percent of the rated circuit capacity (as is a standard recommendation) then the circuit will need to be rated at 18 amps (rounded up to 20).

[GG0452]

From what i have read and comments made by repair technicians a big issue with ruining a SS motor is frying the start up coil which is not repairable. I don't think a 15 amp breaker will prevent that damage. If you stall your motor and not immediately shut the SS off the 15 amp breaker may save the day. If you run the motor under a very high load for an extended time it may not as there is no thermal protection.

[GG0452]

Just heard from SS Engineering about the Mark V electrical requirements. This is what they said.

"When we moved from AO Smith to Emmerson motors, Emmerson required us to list the new electrical requirements for their motor warranty.
I truly believe turning the speed down to “D” or slower with every stop, so it is started in the slow range, is important to the life of the Emmerson motor and to your switch. My 1979 AO Smith motor is still running fine because I turn down the speed dial before shutting it off EVERY TIME. The prolonged starting current surge in the motor, starting at “R” for sawing will, over time, degrade and short out the insulation on the motor windings as well as cause the switch to arc and stop working properly.
NOTHING is made today like it was made in 1983. Yes. you need to follow this rule. I just have gotten in the habit of turning the speed dial a couple quick turns before turning off the switch.
I would also upgrade to the larger extension cord. This larger cord will help the performance of everything plugged into it because it will allow easier flow of electricity with less resistance and less heat build up.
Your 30-amp circuit in your shop is fine and does not need to be upgraded. The reason for the “15-amp dedicated circuit” is to prevent new owners from running their dust collector on the same circuit as they are running their Mark, thus overloading that circuit.

Jim McCann
Engineering, Quality & Customer Repair
Shopsmith LP"

[RFGuy]

Good feedback from Jim and also good post above by Dennis. I can't remember if it was in my owner's manual or just something that those who sold my Dad on his Mark V taught us, but I have always turned the speed dial down before turning it off. As long as you keep the worm gear and speed control quadrant clear of dust and lubricated it really isn't a pain to do so IMHO. When too much dust gets built up there, then it becomes hard to turn the speed dial and a pain if that happens. Others on the forum in past posts have indicated that it is no problem to leave the speed dial at high speed for switching it off and turning it back on. From a safety standpoint, but also as an electrical engineer, I never agreed with this based on my understanding of everything involved. The sheaves form a transmission, so I have to ask would anyone put their car in 4th gear and try to start the engine? It just never made sense to me to apply a heavier load to an electrical motor on startup if not needed. Yes, it is a small inconvenience and takes a small amount of time to spin the dial back down each time, but I just choose to do it.

[chapmanruss]

I highly recommend following the advice from Jim McCann. He has been with Shopsmith a long time and is one of their most senior (time employed) employees. As noted, his position in the company is Engineering, Quality & Customer Repair. As Jim said,

I just have gotten in the habit of turning the speed dial a couple quick turns before turning off the switch.

This is an easy habit to use to and also works those moving parts of the Speed Changing system at the same time. It is one of the things I have always followed with the Conventional Headstock. You will notice the need for maintenance sooner if you are regularly changing the speed. When the dial starts to get harder to turn it's time to clean and lubricate the parts. If you leave the speed at the same higher position most of the time you will not notice that and also shorten the life of the motor and switch.

[DLB]

Your 30-amp circuit in your shop is fine and does not need to be upgraded. The reason for the “15-amp dedicated circuit” is to prevent new owners from running their dust collector on the same circuit as they are running their Mark, thus overloading that circuit.

Jim McCann
Engineering, Quality & Customer Repair
Shopsmith LP"

I get what Jim is saying, right up until that last part. A 30 amp circuit is okay, but a 20 amp circuit is not? And a 15 amp is? Unless you have an older motor in which case the 20 amp (or 25 amp) circuit is preferred and 15 amp is the minimum.(?) And the manual's requirement for a 15 amp maximum CB is actually a requirement for a dedicated circuit, though the manual doesn't actually say that (and explicitly refers to other tools or appliances plugged into the same circuit)? This wasn't enlightening as to what the power requirement actually is.

My understanding of the NEC is that a 30 amp circuit with 15 amp outlet receptacles is probably unacceptable. That doesn't necessarily mean a 15 amp style outlet, but one actually rated at 15 amps. Many of these types are rated for a 20 amp circuit, but I think finding one rated at 30 amps would be a challenge.

Most homes have a combination of 15 amp and 20 amp circuits with outlet receptacles that the Shopsmith power cord will plug into. I've used both and I find that the Mark takes longer to start (reach normal motor speed) on a 15 amp circuit than it does on a 20 amp at most loads. So, while I think it will work on either one I prefer using it on the 20 amp circuit. (As Dennis points out, that's because of the wire, not the CB rating.) I'll turn it off with the switch if it doesn't start quickly rather than hoping the CB prevents further damage. I also use spin-up time to determine if I think I need to lower the speed before shutting off. In my shop with a short distance to the CB panel on a 20 amp (AWG 12) circuit, there is no appreciable difference in spin-up time until above Saw speeds. OTOH put that on a 15 amp circuit with 75 or 100 feet of AWG 14 and it might not start at all at that speed.

- David

[GG0452]

My 30 amp circuit has 10 gauge wire (see above first posting). And in spite of the comments to the contrary I never tripped the breaker or caused damage to the motor (it was just serviced and no issues noted) or fried the power switch. So it might just be the older AO Smith motor is very rugged compared to the newer motors, or i just was lucky. We will never know.

After doing every thing wrong, according to the later SS owners manual (mine is from 1983 or older), and in the interest of lessening any undue strain on a 40 year old motor, or tempting fate, I have decided to change my ways. I spent the $400 and had a 20 amp circuit (12 gauge wire) installed in my garage so that it is closer to the Mark V (and with the 2nd outlet i can dedicate it to the SS when in use). I upgraded my extension cord to 12 gauge and it is only 25 feet long. AND i will try to remember to turn the speed down before i shut it down.