how many intermittent motors allowed on a circuit?

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hardworkingstiff

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Wilmington, NC
Looking at a job with 22 boat lifts on a dock. There is no way that all 22 lifts will operate at the same time. The motors are "intermittent" motors (and so is the use). They are single phase, 1 H.P. and will be wired to a 208 circuit.

It is my understanding that I need to count each motor connected to a ciruit. Anyone know of a section of the code that would prove me wrong? I'd love to wire all the lifts to one 100-amp circuit (overcurrent protection will be at the controller). I doubt there would ever be more than 3 boat lifts in use at the exact same time.
 
I think it would be tricky to interlock the motors so only a couple would be on at the same time - but I've never looked into it.

A couple things to think about: sunshine, repeated use, and what if the dock gets a lot busier in coming years?

Some lakes around here were fairly dead a few years ago, now they're all a lot busier. :)
 
430.22(E) Other Than Continuous Duty. Conductors for a motor
used in a short-time, intermittent, periodic, or varying
duty application shall have an ampacity of not less than the
percentage of the motor nameplate current rating shown in
Table 430.22(E), unless the authority having jurisdiction
grants special permission for conductors of lower ampacity.
Click to expand...

This is one of those cases where the AHJ might take pity on you and agree that a 100 amp feeder is adequate.

It might be simpler to just put in a 150 amp feeder. Is there a huge price difference?
 
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I don't think there would be any real possibility of overloading a 100A circuit. They won't even be under much load on the motors while lowering the boats, only when raising them. Even if the lake gets busy, each lift will be dedicated to one boat. I can't imagnine anyone raising and lowering their boat more than twice a day.

But I don't see any way the code allows a single 100A circuit. I would be tempted to call them appliances. However, I think 220.18 might prevent that.

I think your AHJ should take pity on you also.


Steve
 
I never understood table 430.22(E). Many of the numbers are more than 100%. So does that mean we would have to provide an even larger circuit than a normal motor?

Steve
 
You maight check the nameplates and see if the motors are listed as intermittent duty in that case table 422.22(A) may be applicable. I would seem that boat lifts are comparable to elevators (maybe). If the manufactures recognize these motors as continuious duty per the note to the table then you may be confined to sizing the feeder conductors per 430.24.
 
The best 430.26 will give you is 85% of the total load of all the motors (from my very limited understanding of this table). In my opinion, I think that's overly conservative for 22 motors when each one is only going to run for 5 min. a day. But I guess you have to follow the NEC.
 
What is the possibility of running it off a 3? circuit? Three pole breaker instead of two pole, one more wire, and divide the motors up 8, 7, and 7 among phases.
 
Look at 220.61. If you are talking about having a 100A feeder, and then running a full size branch circuit to each lift, then I think 220.61 allows this. That is, it allows a feeder that is smaller than needed to run all the motors at one time.

Notice 220.60 only says that it is "unlikely" that two or more loads would run at the same time. I would make a conservative estimate of how many might run at once (maybe 5 or 6) and see if your AHJ will approve a feeder that size.

Steve
 
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