Motor Circuit Confusion

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Jon456

Jon456

Senior Member
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Colorado
This one is puzzling me.

I need to install a 115VAC branch circuit for some machinery. Here are the requirements and background info:

1. The building was spec'd by the GC to have three machines on this single branch circuit, with only one to be operated at a time.

2. Each machine has a momentary push-button switch to energize the motor. The motor would only be energized for a short period of time (45-50 sec) with long set-up times between operations (very low duty-cycle).

3. The motors are rated as follows: 1HP, 1 PH, FLA=11.2, 115V/60Hz, SF=1.6, SFA=15.0, Duty=5 Min.

4. Each motor's control box is factory-wired with a flexible 14AWG power cord that ends in a standard Type-B grounded plug.

5. I contacted the machines' manufacturer and spoke with their technical service dept. They recommended a 25A or 30A branch circuit (they said that when they do the installations, they typically use 30A breakers).

Calculations & Questions:

1. Conductors:
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Going through the NEC, it appears that 430.22(E) would apply. The
examples they give for "Classification of Service" are rather limited and
appear highly specialized. I'm guessing that my application would be classified as "Intermittent Duty". Does this seem right? If so, then my conductors shall have an ampacity of not less than 85% of the motor's nameplate current rating.

But which nameplate current rating do I use (FLA or SFA)? I'm going to assume I use the FLA. Then my conductors should be sized not less than:

11.2A * 85% = 9.52A

(I suppose this is how they are able to put a 14AWG cord on the motor.)

Since we'll have three machines on this branch circuit, then 430.24 applies. And for intermittent duty, then Exception 1 applies. So:

Sum = 9.52A + 9.52A + 9.52A = 28.56A

So that would mean a 30A breaker and 10AWG conductors from the distribution panel to the outlets are called for. Does this all seem correct?

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2. Motor & Branch Circuit Overload Protection
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For intermittent duty motors, I can apply 430.33: "...shall be permitted to be protected against overload by the branch-circuit short-circuit and ground-fault protective device, provided the protective device rating does not exceed that specified in Table 430.52."

From Table 430.52:
11.2A * 300% = 33.6A (Again I'm using the FLA rating)

So a 30A breaker satisfies this requirement. Everything look good so far?

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3. Motor Branch Circuit Short-Circuit & Ground-Fault Protection
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This is where I start to get confused. According to 430.53(A), two or more motors (not over 1 HP) shall be permitted on a nominal 120-volt branch circuit protected at not over 20A.

So how do we reconcile this 20A limitation with the requirement for a 30A breaker?

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4. Plugs & Receptacles
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As I mentioned, the power cords for these machines all have Type-B plugs. Is it Ok to put three 20A receptacles on this 30A branch, one to feed each machine? I've never seen a NEMA 5-15 outlet rated to carry 30A.

And what about GFCI? When I called the manufacturer's tech support, the guy there had never heard of GFCI on any installations of these machines. If GFCI is required, would I again use a 20A rated device?

Thanks again in advance for ANY assistance!

confused:
 
I'm not that familiar with intermittent duty motors, but I don't see how you can put a 20 amp receptacle on a 30 amp circuit.
 
infinity said:
I'm not that familiar with intermittent duty motors, but I don't see how you can put a 20 amp receptacle on a 30 amp circuit.
Click to expand...

I don't either. And I just discovered from this post that NEC Table 210.21(B)(3) requires a 30A receptacle on a 30A circuit.

So how does the manufacturer do this on their factory installs? I doubt they are cutting off their very own plugs.
 
Jon456 said:
4. Each motor's control box is factory-wired with a flexible 14AWG power cord that ends in a standard Type-B grounded plug.
Click to expand...

Just to be clear, the factory-installed power cord to this motor unit has a 115V/15A grounded plug (w/ two vertical spade pins & a round ground pin), not a 115V/20A grounded plug (w/ one vertical spade pin, one horizontal spade pin, & a round ground pin).

This is perplexing.
 
electricman2 said:
I dont think you have any choice. As long as you have a 1-15P plug, you are limited to a 20A OCPD.
Click to expand...

And I suspect a 20A breaker would be sufficient, except:

1. It appears that a 30A breaker is required by NEC (according to my calculations).

2. I'm still a bit concerned about the in-rush current. Incidentally, I've called the manufacturer's tech support three times about this and each time I've gotten a different answer about the actual in-rush current: one person told me 28A peak (but I get the feeling he didn't really know), another told me 19A peak, and the last guy told me 23A peak. The power unit for this machine is actually manufactured by a sub-contractor. I asked if I could contact that company but the manufacturer of the machine said they couldn't give out their contact information.

What I don't understand is why the manufacturer of these machines put us in this quandary to begin with. On one hand they recommend 25A or 30A breakers (and they stated they only use 30A breakers when they perform the installs). But on the other hand, they fit the machines with 15A plugs.

If I do end up installing 20A breakers, I'll likely run 10AWG for the branch circuit anyway (just in case).
 
Jon456 said:
If I do end up installing 20A breakers, I'll likely run 10AWG for the branch circuit anyway (just in case).
Click to expand...
In my opinion, that would be a waste. You're more likely to need 30a breakers for starting current than you are 30a conductors for running current.

I'd like to know whether the motors contain their own OCP, so we can discuss the Art. 240 exceptions to Table 310.15, and whether they apply to receptacles as well.

In other words, if we supplied an AC unit through a plug and receptacle, can we still apply 430 and 440 when selecting branch-circuit conductors, devices, etc?
 
benaround said:
With only one motor running at any time, IMO 430.24 ex.3, would allow for a 20a OCPD to be used.
Click to expand...

Exception 3 states "Where the circuitry is interlocked so as to prevent operation of selected motors or other loads at the same time..."

In my case, the only "interlock" is the operator. Granted, the motors each independently actuated by a momentary push-button switch (release the button, the motor stops) so it would be impossible for one man to actuate more than one machine at a time. But technically, a second person could push the button on a second machine at the same time (and a third person at the third machine...). I realize this is extremely unlikely (and there will be signs posted to warn against operating more than one machine at a time), but if we are to go by the "letter of the code", then what is unlikely is not the same as what is permissible.

In any case, 430.24 Ex.3 only applies to sizing conductors. It does not apply to overload protection or to short-circuit and ground-fault protection.
 
LarryFine said:
In my opinion, that would be a waste. You're more likely to need 30a breakers for starting current than you are 30a conductors for running current.
Click to expand...
I agree that the conductors should never have to handle more than 15A continuous current. But if I did need to install 30A breakers (due to tripping of 20A breakers from in-rush current) wouldn't I be required to have 10AWG wire for the branch circuit? How could I put 30A breakers on a 12AWG circuit?


LarryFine said:
I'd like to know whether the motors contain their own OCP, so we can discuss the Art. 240 exceptions to Table 310.15, and whether they apply to receptacles as well.
Click to expand...
No, the motors have no OCP. This is from a label on the motor:

"Warning - Motor contains no thermal protector. Separate overcurrent protection must be provided to prevent burnout and possible fire hazard from overloaded or stalled motors."

To be sure that wasn't just a label attached by the motor manufacturer (Emerson) and that OCP was added by the machine manufacturer, I opened the cover to the control box mounted to the motor. That contained only a couple of capacitors, the push-button switch, and the wiring to the plug.
 
Last edited:
Wait! I think we may be onto something with this issue of overcurrent protection...

According to 430.42(C):

Cord-and-Plug Connected. Where a motor is connected to a branch circuit by means of an attachment plug and receptacle and individual overload protection is omitted in 430.42(A), the rating of the attachment plug and receptacle shall not exceed 15 amperes at 125 volts or 250 volts. Where individual overload protection is required as provided in 430.42(B) for a motor or motor-operated appliance that is attached to the branch circuit through an attachment plug and receptacle, the overload device shall be an integral part of the motor or the appliance. The rating of the attachment plug and receptacle shall determine the rating of the circuit to which the motor may be connected, as provided in Article 210.

In my application, the first part (in green) applies. The second part (in blue) does not apply because my motors are rated at one HP. The question is, does the third part (in red) apply to the entire section, or just to the second part. The way I interpret it, it applies to the entire section. But correct me if I'm wrong.

So the first part (green) explains why the manufacturer placed 115V/15A plugs on these motors. And the third part (red) would appear to limit my branch circuit to 20A OCP with 15A receptacles (because a 15A device can be plugged into a 20A branch circuit). Is that correct?

So what about the manufacturer's recommendation for a 30A breaker (with appropriately-sized conductors)?
 
benaround said:
Jon, how about 430.62(B) will that help?
Click to expand...

Hmmm... it does seem like it would apply. But I'm not sure of the definition of a "motor feeder". Would that just be any branch circuit that supplies a specific motor?

That raises another question I have: the NEC mentions "permanently" and "non-permanently" installed motors. Where are those terms defined? In my case, we will be bolting the machines to the floor (implying permanently installed). But the motors are corded with plugs (implying non-permanently installed). These machines are available with caster options (which we are obviously not installing) for rolling on the floor if that helps clarify how these machines are defined.
 
Jon456 said:
No the motors have no OCP. This is from a label on the motor:

"Warning - Motor contains no thermal protector. Separate overcurrent protection must be provided to prevent burnout and possible fire hazard from overloaded or stalled motors."
Click to expand...
In this case you will have to provide a separate OL relay or fuse in the circuit to each motor in addition to the branch circuit breaker.
 
electricman2 said:
In this case you will have to provide a separate OL relay or fuse in the circuit to each motor in addition to the branch circuit breaker.
Click to expand...

No, not for intermittent duty motors.

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 does not exceed that specified in Table 430.52.
 
So no consensus on the sizing of this circuit? I really need the advice of some of you code experts here.

I'm ready to install conduit and the size of conduit coming off the distribution panel is dependent on the size of the conductors I'll run for this circuit. If 12AWG then I can use 1/2" EMT, but if 10AWG then I'll need to step up to 3/4" EMT. (I'm running some other circuits in this same conduit).
 
I hate to use the more expensive 3/4" conduit & 10AWG wire if I really don't need to. But I guess I have no choice since I can't seem to get an authoritative answer to these questions. :(
 
You could just pull 3 20A circuits and be done with it. Each motor gets it's own circuit. Already figured the 3 motors on one circuit myself but didn't take into account the 15A plugs.
 
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