main breaker calculations

[jimmy_mcquade]

Hi Everyone,

I asked this same question last week on the NEC forum and got a lot of
Good answers, but I am still confused. The system design has changed
some according to dad and I am now trying to understand this on top of
everything else.

The system is 208 3 phase and is a continuous running production line.
There are (12) 3 phase motors associated with a proposed MCC. (2) 7 ? hp,
(2) 5 hp, (2) 3 hp, (3) 1 ? hp, and (3) 1 hp motors.
Now the system will also include another (2) 20 hp motor fans on equipment
being purchased and (4) 20 amp, 120 volt plc control panels. Dad told me
that no 3 phase Motors would be run with less than a #12 wire (plant spec).

Table 430.247 to 430.250 gives the motor amps. 430.22 tells us the motor
overload protection, 430.22 is for the branch conductor size, 430.24 is the
feeder conductor, and 430.46 is the feeder protection.

For the (12) motors, I get a continuous load of 136.6 amps.
For the 7 ? hp motor, 24.2 amps, Motor overload current 30.25 (430.32 at
125%), #8 wire (table 310.16), 60.5 amps (70 amps)branch protection
(430.52 at 250%), 30.25amps (40 amps) feeder conductor (430.24).

The MCC main breaker would be 182.4 amps (430.62), 175 amps next size
down. Since this is a continuous load, the 80% breaker rule would apply and
the size would be 225 amp (225 * 80%) = 180 amps

You would also do the same thing for the (2) 20 hp motors and I think the
plc panels also.
The main breaker for all of these devices would be 426 amps (400 next size
down).
Since this is a continuous load, the 80% breaker rule would apply and the
size would be 500 amp (500 * 80%) = 400 amps. As dad says, you always
forget something, so go for a 600 amp service and breaker.

My questions are.
#1. Is this correct, #2. does the 80% breaker rule apply for the breakers,
and #3 if the branch protection for the 7 ? hp motor is 70 amps, wouldn?t
the wire size also be rated for 70 amps?

I cannot find a way to attach my spreadsheet, or i would attach it.
Thanks in advance,
Jimmy

 

[augie47]

Hi Everyone,

I asked this same question last week on the NEC forum and got a lot of
Good answers, but I am still confused. The system design has changed
some according to dad and I am now trying to understand this on top of
everything else.

The system is 208 3 phase and is a continuous running production line.
There are (12) 3 phase motors associated with a proposed MCC. (2) 7 ? hp,
(2) 5 hp, (2) 3 hp, (3) 1 ? hp, and (3) 1 hp motors.
Now the system will also include another (2) 20 hp motor fans on equipment
being purchased and (4) 20 amp, 120 volt plc control panels. Dad told me
that no 3 phase Motors would be run with less than a #12 wire (plant spec).

Table 430.247 to 430.250 gives the motor amps. 430.22 tells us the motor
overload protection, 430.22 is for the branch conductor size, 430.24 is the
feeder conductor, and 430.46 is the feeder protection.

For the (12) motors, I get a continuous load of 136.6 amps.
For the 7 ? hp motor, 24.2 amps, Motor overload current 30.25 (430.32 at
125%), #8 wire (table 310.16), 60.5 amps (70 amps)branch protection
(430.52 at 250%), 30.25amps (40 amps) feeder conductor (430.24).

#10 wire would be sufficient under most conditions

The MCC main breaker would be 182.4 amps (430.62), 175 amps next size
down. Since this is a continuous load, the 80% breaker rule would apply and
the size would be 225 amp (225 * 80%) = 180 amps




You would also do the same thing for the (2) 20 hp motors and I think the
plc panels also.
The main breaker for all of these devices would be 426 amps (400 next size
down).

I'm not coming up with those numbers. remember, even if you figure your 120v PLC cabinets at 20 amps (doubtful), the load could be distributed bewteen the 3 phases so the largest would be 40 amps on 1 phase.
I came up with about 404 amps, but since the PLCs would propaly be less than the 20 amps, 400 might work. From past experience I would go with the 500.

Since this is a continuous load, the 80% breaker rule would apply and the
size would be 500 amp (500 * 80%) = 400 amps. As dad says, you always
forget something, so go for a 600 amp service and breaker.

My questions are.
#1. Is this correct, #2. does the 80% breaker rule apply for the breakers,
and #3 if the branch protection for the 7 ? hp motor is 70 amps, wouldn’t
the wire size also be rated for 70 amps?

The conductor would be sized by 430.22, 24.2 *1.25 =30.25, normally a #10

I cannot find a way to attach my spreadsheet, or i would attach it.
Thanks in advance,
Jimmy

I would agree with your dad (of course). Things are often forgotten or soon added to these projects, so unless you are on an extrremely tight budget, go large.

 

[Smart $]

... As dad says, you always
forget something, so go for a 600 amp service and breaker.

Missed other thread and have not looked for or at it...

If this is a service, what about lighting and gp receptacles, hot water heater, bathroom circuit, heating and/or air conditioning, etc.?

I cannot find a way to attach my spreadsheet, or i would attach it.

Embed it in a Word document, then attach the Word document.

 

[jimmy_mcquade]

Augie47,

I am attaching my calculations in word format
copied from excel.
I'm using dad's laptop, so i hope it works correctly.
This is my first attachment. Everyone, please feel free to
see if i did my work correctly.

Thanks again,
Jimmy

 

[jimmy_mcquade]

Let's try again.

 

[augie47]

Reviewed it and rechecked my math. For the motors, including the 20 HP, I arrived at a Feeder SC-GF protection of 344.5. Assuming you feed your PLC from all 3 phases, that would put 2 on 1 phase so add 40 amps (,max)
fro 384.5. If you want to add the continuous factor we then have 480, next standard size 500.

 

[jimmy_mcquade]

Augie47,

Thanks, for the reply.

the plc panels are 120 volts each and distributed over the 3 phases.

You said, if i want to apply the continuous factor, then we have 480 amps.

Does that mean the 80% rule does not apply for these continuous loads that
will be running 12 hours a day (12 hour shifts)?
Regards,
Jimmy

 

[augie47]

Augie47,

Thanks, for the reply.

the plc panels are 120 volts each and distributed over the 3 phases.

You said, if i want to apply the continuous factor, then we have 480 amps.

Does that mean the 80% rule does not apply for these continuous loads that
will be running 12 hours a day (12 hour shifts)?
Regards,
Jimmy

In one way I wish you had not asked that as I know I'm going to look foolish, but until you posed the question I had never considered it on motor circuits. 430.24 allows us to use 12% of the largest motor without any "continuous" multiplier on the others and yet it states motors of continuous duty.
Conductor ampacity in 210.3 allows for 125% of load for continuous duty, but 210.2 refers us to Art 430 for motor ampacity.
Overcurrent protection is based on conductor amapcity and 430.51.

Bottom line is I need to rely on someone with more Code" smarts to answer those questions.
(a) If several motors are operating in a continuous fashion (3 hours or more) where does the Code address an increase in conductor size over what is calculated by 430.24
(b) In situations such as yours, when determine Feeder Short-Circuit/Ground Fault protection due you factor in a "continuous load" multiplier.

As is often the case here, I find I don't understand as much Code as I should. Let's see what the experts say.

 

[One-eyed Jack]

In one way I wish you had not asked that as I know I'm going to look foolish, but until you posed the question I had never considered it on motor circuits. 430.24 allows us to use 12% of the largest motor without any "continuous" multiplier on the others and yet it states motors of continuous duty.
Conductor ampacity in 210.3 allows for 125% of load for continuous duty, but 210.2 refers us to Art 430 for motor ampacity.
Overcurrent protection is based on conductor amapcity and 430.51.

Bottom line is I need to rely on someone with more Code" smarts to answer those questions.
(a) If several motors are operating in a continuous fashion (3 hours or more) where does the Code address an increase in conductor size over what is calculated by 430.24
(b) In situations such as yours, when determine Feeder Short-Circuit/Ground Fault protection due you factor in a "continuous load" multiplier.

As is often the case here, I find I don't understand as much Code as I should. Let's see what the experts say.

I will be interested in the replys also. I have always thought you would be imposing on the feeder if you did not consider the continuous load and adjust accordingly.

 

[Smart $]

Let's try again.

From what I gather, your numbers are a bit high. Looks like you are compounding your allowances. See attached... but I could be wrong

Also, I should emphasize my numbers are for determining minimum size and rating. There is nothing prohibiting you from going larger.

PS: To embed an Excel spreadsheet in a Word doc, copy in Excel and Paste Special as an Excel document in Word.

View attachment 4239

 

[Smart $]

...

Bottom line is I need to rely on someone with more Code" smarts to answer those questions.
(a) If several motors are operating in a continuous fashion (3 hours or more) where does the Code address an increase in conductor size over what is calculated by 430.24
(b) In situations such as yours, when determine Feeder Short-Circuit/Ground Fault protection due you factor in a "continuous load" multiplier.

...

I don't consider myself an expert in the area of motors, but my responses to your questions are as follows:

a) It doesn't. For the most part, the Code considers in general any motor as continuous duty in the sense you imply. For circumstances where "a motor used in a short-time, intermittent, periodic, or varying duty application" we use 430.22(E) and its factor table, or other requirements as they apply. Note just in Table 430.22(E) the percentage factor can range from as low as 85% to as high as 200% depending on motor type and conditions of usage.

b) I've already typed and deleted three responses to your question because it can be taken several different ways. In general, when it comes to motors and calculating their load on feeders, throw out the concept of a blanket continuous load factor or multiplier, such as one would use for general illumination. The factoring is always applied on an individual motor load basis.​

 

[augie47]

so in the OP's situation, where all of the motors listed are operated continuously, once we apply the 250% to the largest motor for selecting our SC/GF protective device, there is no Code requirement to address the "continuous load" ?

(that's the way I read it and why I stated the OP "could" and not "should" add the multiplier, but I thought I had overlooked something)

 

[One-eyed Jack]

so in the OP's situation, where all of the motors listed are operated continuously, once we apply the 250% to the largest motor for selecting our SC/GF protective device, there is no Code requirement to address the "continuous load" ?

(that's the way I read it and why I stated the OP "could" and not "should" add the multiplier, but I thought I had overlooked something)

I am in agreement that this is what the code appears to say. Consider 1000 total hp with the largest being 100hp. I would be very reluctant to size a feeder with only 25 hp cushion.

 

[Smart $]

so in the OP's situation, where all of the motors listed are operated continuously, once we apply the 250% to the largest motor for selecting our SC/GF protective device, there is no Code requirement to address the "continuous load" ?

(that's the way I read it and why I stated the OP "could" and not "should" add the multiplier, but I thought I had overlooked something)

That's correct as long as it's a single motor circuit.

 

[augie47]

That's correct as long as it's a single motor circuit.

Single motor circuit ? What if it's only motors, but multiple motors ?

 

[One-eyed Jack]

That's correct as long as it's a single motor circuit.

The OP has 12 motors on a single feeder with branch circuits for individual motors. My question and I believe Augie's is, do we consider all of the motors as continuos and up the size of the feeder accordingly. 125% of the largest motor and the fla of 12 motors with all of the motors under full load continuously just does not seem to be enough for the feeder.

 

[Smart $]

Single motor circuit ? What if it's only motors, but multiple motors ?

More hoops to jump through

Refer to 430.53 in general, but note 430.53(B) in particular.

 

[One-eyed Jack]

More hoops to jump through

Refer to 430.53 in general, but note 430.53(B) in particular.

Not multiple motors on branch circuit. 12 motors on feeder. Call it a service feeder to an MCC.

 

[Smart $]

Not multiple motors on branch circuit. 12 motors on feeder. Call it a service feeder to an MCC.

My reply was to augie's post, as it seemed to address multiple motors on a single "branch" circuit.

The OP has 12 motors on a single feeder with branch circuits for individual motors. My question and I believe Augie's is, do we consider all of the motors as continuos and up the size of the feeder accordingly. 125% of the largest motor and the fla of 12 motors with all of the motors under full load continuously just does not seem to be enough for the feeder.

Well it all starts with...

220.50 Motors. Motor loads shall be calculated in accordance
with 430.24, 430.25, and 430.26 and with 440.6 for
hermetic refrigerant motor compressors.

So we go to...

430.24 Several Motors or a Motor(s) and Other
Load(s). Conductors supplying several motors, or a mo-
tor(s) and other load(s), shall have an ampacity not less
than 125 percent of the full-load current rating of the high-
est rated motor plus the sum of the full-load current ratings
of all the other motors in the group, as determined by
430.6(A), plus the ampacity required for the other loads.

FPN See Annex D, Example No. D8.​

Exception No. 1: Where one or more of the motors of the
group are used for short-time, intermittent, periodic, or
varying duty, the ampere rating of such motors to be used
in the summation shall be determined in accordance with
430.22(E). For the highest rated motor, the greater of ei?-
ther the ampere rating from 430.22(E) or the largest con-
tinuous duty motor full-load current multiplied by 1.25
shall be used in the summation.

Exception No. 2: The ampacity of conductors supplying
motor-operated fixed electric space-heating equipment
shall comply with 424.3(B).

Exception No. 3: Where the circuitry is interlocked so as to
prevent operation of selected motors or other loads at the
same time, the conductor ampacity shall be permitted to be
based on the summation of the currents of the motors and
other loads to be operated at the same time that results
in the highest total current.

430.25 Multimotor and Combination-Load Equipment.
The ampacity of the conductors supplying multimotor and
combination-load equipment shall not be less than the minimum
circuit ampacity marked on the equipment in accordance
with 430.7(D). Where the equipment is not factorywired
and the individual nameplates are visible in
accordance with 430.7(D)(2), the conductor ampacity shall
be determined in accordance with 430.24.

430.26 Feeder Demand Factor. Where reduced heating of
the conductors results from motors operating on duty-cycle,
intermittently, or from all motors not operating at one time,
the authority having jurisdiction may grant permission for
feeder conductors to have an ampacity less than specified in
430.24, provided the conductors have sufficient ampacity
for the maximum load determined in accordance with the
sizes and number of motors supplied and the character of
their loads and duties.

FPN: Demand factors determined in the design
of new facilities can often be validated against actual
historical experience from similar installations. Refer to
ANSI/IEEE Std. 141, IEEE Recommended Practice for
Electric Power Distribution for Industrial Plants, and
ANSI/IEEE Std. 241, Recommended Practice for Electric
Power Systems in Commercial Buildings, for information
on the calculation of loads and demand factor.​

...where I've highlighted extended references.

125% of largest motor plus 100% of remaining motors should be adequate for feeder sizing... and the code says what it says. We also have to take into account this is to determine the minimum size feeder. As I said earlier, there is no prohibition from going larger...

...the biggest question is whether the minimum size feeder and its compliant ocpd will permit such extreme condition as all motors starting simultaneously....??????

 

[skeshesh]

The additional explanatory notes after 430.24 (2005 handbook) states "Where the selection of a feeder protective device of higher rating or setting is based on the simultaneous starting of two or more motors, the size of the feeder conductors is required to be increased accordingly" - and a few paragraphs later "except where two or more motors may be started simultaneously, the heaviest load that a power feeder will ever be required to carry occurs when the largest motor is started and all the other motors supplied by the same feeder are running and delivering their full-rated horsepower."
Also example D8 in the annex D is worth a look.
I believe if simultaneous start is possible you need to size all equipment and feeders as such. As stated Smart $, there's nothing agains prohibiting upsizing the conductor so.... If you have the motor start up curves you can try to play around with them and see what kind of values you get.