Non continuous Motor demand factor

[julianov]

If we have a panel with only 2 motors, these are the only loads of the panel. One motor has a Full Load Amps (FLA) of 50 A, and the other has an FLA of 25 A. According to NEC Article 210.19(A)(1)(a), the total ampacity for the main feeder panel is calculated as follows:

50*1.25 + 25 = 87 A

Therefore, we need a feeder that can handle 87 A. However, here's my question: what happens if the motors only operate for 3 hours per day? Since these are water pumps, they run 3 times for 1 hour each day.

 

[infinity]

Therefore, we need a feeder that can handle 87 A. However, here's my question: what happens if the motors only operate for 3 hours per day? Since these are water pumps, they run 3 times for 1 hour each day.

Doesn't really matter how long they run. Unless they're set up to be interlocked and can only run one at a time your calculation is correct.

 

[augie47]

The term "continuous" when referring to motors is not the same as the NEC"continuous" referring to3 hours or more.
It makes more sense when you view 430.22(E)

 

[julianov]

Thanks. I have a question that's been on my mind: why is it that with motors, we're advised not to consider less than 100% of the motor plate? For instance, when it comes to a water pump in a dwelling unit. However, for an oven with a rating of 4KW (as indicated on the plate), we can consider only 80% of that load by 220.55. It seems inconsistent to me. Especially considering that when someone uses an oven, it often runs continuously for more than 30 minutes at a time. As the oven is in a branch circuit, it would be working with 20% more current than the conductor ampacity.

 

[augie47]

Keep in mind the NEC definition of continuous load "
Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

An oven does not meet that definition as during a cook time the load will vary.

 

[julianov]

Thanks.

I think I understand now. The continuous load rule applies to every load except motors, is that correct? But why is this the case? What distinguishes a 2-hour motor from a 2-hour oven?

Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

(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.

 

[NoahsArc]

The oven heating element will switch off, and probably be in an off state far more often than the on state. Turn your oven on right now and test it, once it's heated up it's going to be off mostly.

A motor will run constantly, even if it's not FLA the entire time. It does not get a break. If it does, it's 430.22E table.

An oven is more like a periodic or intermittent duty device, by motor terminology.

 

[wwhitney]

Upon reflection, I believe the 125% factor for conductor sizing on motor loads has nothing to do with continuous loading. A conductor's ampacity is already a continuous rating. The 125% factor is to provide for additional conductor capacity to account for the larger than FLA startup currents.

Now here is a potential corner case in the conductor and OCPD sizing rules for motor loads: let's say I have a number of motors with an FLA of 12A and which will, in fact, run fully mechanically loaded, drawing the full FLA, and do so continuously. If I have a feeder supplying 11 such motors, then the minimum feeder ampacity will be (1.25 + 10) * 12 = 135A. The maximum OCPD will be (2.5 + 10) * 12 = 150A, a standard size.

But if I had 11 non-motor continuous loads of 12A each, the minimum OCPD size would be 1.25 * 11 * 12 = 165A; next standard size up is 175A. So in theory if our 150A motor feeder OCPD is in a densely packed panel in a warm ambient environment, the 150A breaker could nuisance trip. Not due to motor startup current, but due to the continuous loading.

In practice I gather this is very unlikely to happen, as the FLA values from the Article 430 tables are already fairly conservative, so the fully mechanically loaded motor is unlikely to draw the full FLA value.

Cheers, Wayne

 

[julianov]

Thanks @wwhitney but just a question, why if you have 11 motors, you multiply by 10 here?:

(1.25 + 10) * 12 = 135A

Now here is a potential corner case in the conductor and OCPD sizing rules for motor loads: let's say I have a number of motors with an FLA of 12A and which will, in fact, run fully mechanically loaded, drawing the full FLA, and do so continuously. If I have a feeder supplying 11 such motors, then the minimum feeder ampacity will be (1.25 + 10) * 12 = 135A. The maximum OCPD will be (2.5 + 10) * 12 = 150A, a standard size.

 

[wwhitney]

Thanks @wwhitney but just a question, why if you have 11 motors, you multiply by 10 here?:

(1.25 + 10) * 12 = 135A

Because for the minimum conductor size, you take one of the largest motor present at 125% of FLA, and the rest (10 others) at 100% of FLA.

Cheers, Wayne

 

[GeorgeB]

why is it that with motors, we're advised not to consider less than 100% of the motor plate?

An unhappy fact is that modern high efficiency motors draw more current at partial load than older ones did. Note that I said more CURRENT, not consume more POWER. I don't have any specifics at hand here, but a new motor that draws 100FLA might draw 75A at 50% power. The older motor might draw 103FLA but 60A at 50% power.

Power factor is the culprit.

It's been a number of years since I assembled data on this, but Reliance (now Baldor) had detailed downloadable data sheets on their "standard" products, and this comparison could be made. Baldor is now part of ABB; I don't know if the corporate push to give users less information exists there.