Dual Motor circuit design

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2Broke2Sleep

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Florida
I am working on a boat lift that has two 3/4 Hp (40 degrees C) motors for a boat lift. I'm wiring it for 220V single phase from a CB to a fused disconnect. I did my calcs and came up with no. 12 AWG wire (minimum) with 15A dual-element time delay fuses in the disconnect, max circuit breaker size 35A. Since I was provided with 10AWG thhn for the job that's what I will be pulling. Distance to motor controller is not far. FLA is 5.8A, FLC 6.9A. I wasn't positive where to round my values up or down when determining my overload, conductor size, and ground fault calcs so I rounded down to get my fuse and CB size... Anyone see any problems here?
 
Jraef

Jraef

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As to overload devices, all of the diagrams in that tech guide do not show an overload relay. That's probably because they use a motor that is thermally protected (TP on the nameplate of the motor if not spelled out). If so, you don't need to worry about that part, it's already covered.
 
chris kennedy

chris kennedy

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Miami Fla.
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60 yr old tool twisting electrician
Jraef said:
As to overload devices, all of the diagrams in that tech guide do not show an overload relay. That's probably because they use a motor that is thermally protected (TP on the nameplate of the motor if not spelled out).
Click to expand...

I've done dozens of these and the Gem has contactors (no OL) and the motors are NOT TP. Thats why I keep stressing to wire as per the controller instructions. I have had a few of these motors smoke within the first week of use, don't need warranty issues.
 
Jraef

Jraef

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chris kennedy said:
I've done dozens of these and the Gem has contactors (no OL) and the motors are NOT TP. Thats why I keep stressing to wire as per the controller instructions. I have had a few of these motors smoke within the first week of use, don't need warranty issues.
Click to expand...

Wow. Seems odd. No OL relay AND no TP motor would be a code violation... One or the other, even both is OK, but neither is not.

Oh wait, maybe because it is limited duty cycle? I don't have my code here to check though, but it might fall into one of the exceptions.
 
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2Broke2Sleep

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Location
Florida
So is my design, as redundant as it may be sufficient? The disconnect is already in place upstream of the motor controller...... The only wiring on my part is the CB, branch circuit conductors, conduit, and fuse selection... Existing was a 30 A CB with 30A Dual element time delay fuses. Homeowner used dryer circuit... Installed sub panel and additional receptacle circuit for post the controller and disconnect are mounted on.
 
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GoldDigger

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Jraef said:
Wow. Seems odd. No OL relay AND no TP motor would be a code violation... One or the other, even both is OK, but neither is not.

Oh wait, maybe because it is limited duty cycle? I don't have my code here to check though, but it might fall into one of the exceptions.
Click to expand...

My impression is that it falls within an exception (did not read to see which one though.) None of the lifts (boat, auto, etc.) that I have seen incorporate any overload safety. The instructions do not always even give any guidance as to allowed duty cycle over any time period.

I can see a potential safety issue with the lift stopping/disabling unexpectedly on overload, and the controls are generally strictly manual or have automatic operation at most for one raise or lower cycle.

If you experience several burnouts either the unit is badly designed or the operator is doing something unexpected.
 
kwired

kwired

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Location
NE Nebraska
Jraef said:
Wow. Seems odd. No OL relay AND no TP motor would be a code violation... One or the other, even both is OK, but neither is not.

Oh wait, maybe because it is limited duty cycle? I don't have my code here to check though, but it might fall into one of the exceptions.
Click to expand...
I know nothing of what is typical of boat hoists, but limited duty cycle did cross my mind.

Not sure you can have say a 30 amp breaker protecting both motors in such case though, I could see a 15 or even a 20 amp breaker for each motor though.
 
drcampbell

drcampbell

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I would think that a boat hoist would need a protection scheme that disconnects power from BOTH motors in the event of a fault.
If one motor stops and the other motor keeps turning, the boat is going to tilt and be at risk of falling out of the sling.
 
kwired

kwired

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Location
NE Nebraska
drcampbell said:
I would think that a boat hoist would need a protection scheme that disconnects power from BOTH motors in the event of a fault.
If one motor stops and the other motor keeps turning, the boat is going to tilt and be at risk of falling out of the sling.
Click to expand...
That could be easily accomplished with control components for each motor contactor, can be simple or complex depending on how much you want to put into it and what potential failures you may want to monitor. Simple might just leave you shut down with boat at a funky angle partly in/partly out of the water. Complex may see one issue and be able to respond with certain actions to correct some situations without complete shut down.
 
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GoldDigger

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drcampbell said:
I would think that a boat hoist would need a protection scheme that disconnects power from BOTH motors in the event of a fault.
If one motor stops and the other motor keeps turning, the boat is going to tilt and be at risk of falling out of the sling.
Click to expand...
Reading the fine print on the placatd, we learn that the "Level" switch disables one motor and lets you run the other motor up and down to level the boat. And warns you that if the disabled hoist rig is the one with the limit switches attached you have to be very careful when using the Level switch!

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iquestionit

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Location
Danbury, CT U.S.A.
First I would ask if the disconnect can be seen when standing in front of the controller? If not, you will need to bring the disconnect next to the controller.

If this was an enclosed system you will need the fla of the system meaning motors + controller. This controller has the option of using both or one and interchanging. There is a nameplate somewhere or this system is not UL listed nor recommended for use in US. And the feed circuit will need gfci protection unless provided by the controller. I, myself, would not guess which schematic to use but would feel secure using a 15amp circuit to the controller since it is what will protect the motor and no UL listed device I am aware of uses or requires less than a 15a. Fuses are the way to go, though.


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Jraef

Jraef

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Found it:

430.33 Intermittent and Similar Duty. A motor used for
a condition of service that is inherently short-time, intermittent,
periodic, or varying duty, as illustrated by Table
430.22(E), shall be permitted to be protected against overload
by the branch-circuit short-circuit and ground-fault
protective device, provided the protective device rating or
setting does not exceed that specified in Table 430.52.
Click to expand...
Then in table 430.22E:
Short-time duty
operating valves,
raising or lowering
rolls etc.
Click to expand...
 
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