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NEC Voltage Drop

11/08/2017 9:07 AM

To calculate Voltage drop in the conductors, what is the current value I need to consider, 100% of FLC (or) 125% of FLC?

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#1

Re: NEC Voltage drop

11/08/2017 9:22 AM

I would calculate for each and submit both on my homework assignment.

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#2

Re: NEC Voltage Drop

11/08/2017 12:11 PM

100%

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#3
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Re: NEC Voltage Drop

11/08/2017 12:47 PM

Maybe, it all depends on what one is expecting for a load condition that one is worrying about what kind of voltage drop will happen. Remember this Schneider circuit breaker tutorial chose nominal circuit breaker that will take about 150 seconds before tripping. A motor starter circuit might regularly fail to start and time out due to a low voltage condition at 125% of full current. There is no one answer from the sparse information provided.

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#4

Re: NEC Voltage Drop

11/08/2017 1:12 PM
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#5
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Re: NEC Voltage Drop

11/09/2017 10:51 AM

One reason would be to ensure that selective-tripping is functional. Especially when nearly identical trip-points are in series and starting currents/voltages exceed the limits of rated values. Also, there are two different ways to look at it. One from the supply side, and one from the load side. Sometimes I get objections when I say that you can use fuses for two different reasons: One to protect the load, and one to protect the supply. They usually think that if a fuse/breaker trips, that the whole circuit is protected because ALL of the current stops. But the problem is that, if there is a partial fault in the load, it could damage the load-side circuits without tripping the supply-side protection. Therefore, the load-side protection should be closer to the normal values of the load, while the supply-side protection should be closer to the capacity of the supply. Sometimes this is not a factor with selective-tripping, and gets forgotten. Also, having a larger safety margin might be desirable (for whatever reason), even if it's not needed. What's required/optimal/safe/over-kill is a judgement call. But whatever call is made, it still has to function properly within the larger system (which may or may not be adjustable).

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#6

Re: NEC Voltage Drop

11/09/2017 1:05 PM

I don't know the current value, what is it presently?

If it is 100% FLC, I would use that. If 130% FLC, I would take off running.

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#7
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Re: NEC Voltage Drop

11/09/2017 1:18 PM

Really? Maybe I misunderstood the whole purpose of the question. I thought it was just to get valid info to.... (???whatever). How could having valid info be "take off running" worthy? Am I missing something?

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#8
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Re: NEC Voltage Drop

11/09/2017 1:24 PM

You missed my joke.

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#9
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Re: NEC Voltage Drop

11/09/2017 1:53 PM

Oh. I never really had a keen sense of humor. I tried to get it calibrated once, but they thought I was joking.

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#10
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Re: NEC Voltage Drop

11/09/2017 2:32 PM

right...

current vs. present...

got it now?

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#11
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Re: NEC Voltage Drop

11/09/2017 3:13 PM

Yes, I currently got it at present. Now, I just have to put it in a safe place for tomorrow.

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#12
In reply to #2

Re: NEC Voltage Drop

11/09/2017 11:05 PM

Friend,

As Tornado has said, the NEC uses 100% of the FLC (full load current). The NEC uses 125% for calculations regarding the ampacity of a circuit and its overcurrent protection device for a continuous load (defined as a load that is on for 3 or more hours). Remember, however, that the NEC specifies a minimum standard of design/installation and encourages workers to exceed this standard. Therefore a calculation using 125% of the FLC will result in a lower energy loss balanced against a higher cost of the installation, along with better operation of the attached load.

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#13

Re: NEC Voltage Drop

11/10/2017 2:06 AM

100%

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#14

Re: NEC Voltage Drop

11/19/2017 12:05 AM

Usually, you have to check the minimum voltage in 2 situations:

1) at rated current [preferable 100%]

The minimum voltage for continuous function has to be as per NEMA MG1 [or IEC 60034-1].

2) start current

The minimum voltage for start has to assure a minimum torque as required.

If the load inertia it is in limits recommended by NEMA [or IEC] then a minimum 80% of motor rated voltage has to be present at motor terminals [for up to 100 kW induction motor] at start.

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