FLA > 80% MOCP?

[HVACEE]

If we have a HVAC unit that has (9) 5HP fan motors, the calculations for this would be:

FLA: 68.4
MCA: 70.3
MOP: 77.9 or a 70A Breaker.

Now the MOP can't be less than the MCA in this case as it would probably cause nuisance trips. If we make it an 80A breaker, that seems reasonable, but what about 80% breaker rating? Would have to be a 90A breaker to be less than the 80% (72A).

Am I missing something?

 

[Dennis Alwon]

Is this a feeder? Sounds like your overcurrent protective device is much too low..

What is the voltage - single or 3 phase?

Article 430-62 is for feeder ocpd

 

[infinity]

If we have a HVAC unit that has (9) 5HP fan motors, the calculations for this would be:

FLA: 68.4
MCA: 70.3
MOP: 77.9 or a 70A Breaker.

Now the MOP can't be less than the MCA in this case as it would probably cause nuisance trips. If we make it an 80A breaker, that seems reasonable, but what about 80% breaker rating? Would have to be a 90A breaker to be less than the 80% (72A).

Am I missing something?

Can you show the calculation something doesn't seem right.

 

[HVACEE]

5 HP @ 480V = 7.6A per NEC

FLA = 7.6 A x 9 Motors = 68.4 A
MCA = 7.6 A x 125% + 7.6 A x 8 Motors = 70.3 A
MOCP = 7.6 A x 225% + 7.6 A x 8 Motors = 77.9 A next lower standard breaker is 70 A

 

[Dennis Alwon]

Your math is right. 70 amps overcurrent protective device and feeder must be 70 amps

 

[Dennis Alwon]

Why not just feed a 100 amp sub panel and then fed the 9 motors from that?

 

[HVACEE]

What about the 80% rule for the breaker? 68.4 A is definitely > 80% of 70 A...

 

[petersonra]

read carefully what 430.62 (B) says. 430.62(A) only applies to situations where you use the smallest possible feeder conductor in accordance with 430.24.

In any case i think you are not reading 430.62(A) correctly.

I agree you would take the FLC X 8.

However, what the code actually says for the 9th motor is this:

(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on 430.24 shall
be provided with a protective device having a rating or setting
not greater than the largest rating or setting of the branchcircuit
short-circuit and ground-fault protective device for any
motor supplied by the feeder [based on the maximum permitted
value for the specific type of a protective device in accordance
with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors],
plus the sum of the full-load currents of the
other motors of the group.

Lets look at 430.52.

The maximum CB size in T430.52 is 250% of FLC. And 430.52(C)(1) allows it to go to the next sized breaker.

5 HP @ 480V = 7.6A per NEC

8 motors = 60.8
250% of one motor is 19. the next sized breaker is 20a.


so for the smallest size conductor the code would allow for this feeder, you could not exceed 80.8 amps feeder OCPD.

But if you made the feeder conductors bigger there is no limit to the ocpd rating according to 430.62(B) as long as the conductors are protected.

 

[Ingenieur]

what are these for? a air cooled condensor unit? packaged cooling tower?

what does the control panel mfg say to supply it with
will they be staged or started simultaneously?

 

[Dale001289]

What about the 80% rule for the breaker? 68.4 A is definitely > 80% of 70 A...

As a general rule, the Code requires overcurrent protection for branch circuits and feeders to be sized at 125% of FLA. This compensates for the 80% rating of most molded case CB's (see UL 489, I think?).

 

[petersonra]

As a general rule, the Code requires overcurrent protection for branch circuits and feeders to be sized at 125% of FLA. This compensates for the 80% rating of most molded case CB's (see UL 489, I think?).

Not exactly.

For branch circuits

210.20 Overcurrent Protection.
(A) Continuous and Noncontinuous Loads. Where a branch
circuit supplies continuous loads or any combination of continuous
and noncontinuous loads, the rating of the overcurrent
device shall not be less than the noncontinuous load plus
125 percent of the continuous load.

Exception: Where the assembly, including the overcurrent devices
protecting the branch circuit(s), is listed for operation at 100 percent of
its rating, the ampere rating of the overcurrent device shall be permitted
to be not less than the sum of the continuous load plus the noncontinuous
load.

For feeders:

215.3 Overcurrent Protection. Feeders shall be protected
against overcurrent in accordance with the provisions of Part I
of Article 240. Where a feeder supplies continuous loads or any
combination of continuous and noncontinuous loads, the
rating of the overcurrent device shall not be less than the
noncontinuous load plus 125 percent of the continuous load.
Exception No. 1: Where the assembly, including the overcurrent devices
protecting the feeder(s), is listed for operation at 100 percent of its
rating, the ampere rating of the overcurrent device shall be permitted to
be not less than the sum of the continuous load plus the noncontinuous
load.

Note this is the minimum size because of the phrase shall not be less.

basically if you use a 100% rated breaker you can go to 100% FLC.

If you do not use a 100% rated breaker it is 100% of the noncontinuous load plus 125% of the continuous load.

 

[Dale001289]

Not exactly.

For branch circuits



For feeders:



Note this is the minimum size because of the phrase shall not be less.

basically if you use a 100% rated breaker you can go to 100% FLC.

If you do not use a 100% rated breaker it is 100% of the noncontinuous load plus 125% of the continuous load.

That’s why I said “In general”.
100% rated breaker are a special order.


Sent from my iPhone using Tapatalk

 

[kwired]

What about the 80% rule for the breaker? 68.4 A is definitely > 80% of 70 A...

That is general rules in 210 and 215, motors it changes a little. For one thing you already have to multiply motor FLA by 1.25 so that is already factored into minimum conductor ampacity no need to multiply by 1.25 again. Overcurrent device - most common values is 175% for time delay fuses or 250% for inverse time breakers. You can go next standard size higher. If that won't let motor start you have the ability to go even higher.

5 HP @ 480V = 7.6A per NEC

FLA = 7.6 A x 9 Motors = 68.4 A
MCA = 7.6 A x 125% + 7.6 A x 8 Motors = 70.3 A
MOCP = 7.6 A x 225% + 7.6 A x 8 Motors = 77.9 A next lower standard breaker is 70 A

You must have at least 70 amp conductor, 6 AWG is less then 70, 4 AWG is good for 85 - 4 it is unless other adjustments come into play.

Feeder overcurrent protection for inverse time breaker can be 250% largest plus all others. gives you 79.8 which can be rounded up to 80, which is less then the ampacity of 4 AWG. I'd likely put it on a 90 just in case it wouldn't hold while starting most/all of them all at once. 4 AWG can be protected by next standard size of 90 pretty much in any case.

 

[Dennis Alwon]

Here is an interesting US Chart of average resistivity in ohms/meter. I am surprised that some areas in Texas have a 33 ohms/meter. I assume this means that an 8' ground rod which is almost about 2.5 meters would have a resistivity reading of 13 ohms.

https://www.rd.usda.gov/files/UTP_Bulletins_1751F-802.pdf

Page 59 is where the chart is located. I have no idea as to how accurate this is...

 

[Dale001289]

That is general rules in 210 and 215, motors it changes a little. For one thing you already have to multiply motor FLA by 1.25 so that is already factored into minimum conductor ampacity no need to multiply by 1.25 again. Overcurrent device - most common values is 175% for time delay fuses or 250% for inverse time breakers. You can go next standard size higher. If that won't let motor start you have the ability to go even higher.



You must have at least 70 amp conductor, 6 AWG is less then 70, 4 AWG is good for 85 - 4 it is unless other adjustments come into play.

Feeder overcurrent protection for inverse time breaker can be 250% largest plus all others. gives you 79.8 which can be rounded up to 80, which is less then the ampacity of 4 AWG. I'd likely put it on a 90 just in case it wouldn't hold while starting most/all of them all at once. 4 AWG can be protected by next standard size of 90 pretty much in any case.

The 125% of FLA for motor branch circuits is based on a typical TEFC motor Service Factor of 1.15 - which drives the thermal (or electronic) overload device set typically at 125% of Motor FLA. Explosion proof motors typically have a SF of 1.0 and a tighter tolerance for OL settings, nonetheless the circuit is basically sized at 125% to match the overloads.
The CB is normally ‘high magnetic’ only for short circuit protection.
Cutler Hammer for example standardizes its HMCP trip settings at 1300% of FLA for todays high efficiency motors.


Sent from my iPhone using Tapatalk

 

[Dale001289]

Here is an interesting US Chart of average resistivity in ohms/meter. I am surprised that some areas in Texas have a 33 ohms/meter. I assume this means that an 8' ground rod which is almost about 2.5 meters would have a resistivity reading of 13 ohms.

https://www.rd.usda.gov/files/UTP_Bulletins_1751F-802.pdf

Page 59 is where the chart is located. I have no idea as to how accurate this is...

I have seen this chart before but not all the calculations and studies - very impressive stuff — thanks for sharing.
Those who think Grounding is simple...simply don’t know Grounding.


Sent from my iPhone using Tapatalk

 

[kwired]

The 125% of FLA for motor branch circuits is based on a typical TEFC motor Service Factor of 1.15 - which drives the thermal (or electronic) overload device set typically at 125% of Motor FLA. Explosion proof motors typically have a SF of 1.0 and a tighter tolerance for OL settings, nonetheless the circuit is basically sized at 125% to match the overloads.
The CB is normally ‘high magnetic’ only for short circuit protection.
Cutler Hammer for example standardizes its HMCP trip settings at 1300% of FLA for todays high efficiency motors.


Sent from my iPhone using Tapatalk

To add to that, for other then 100% rated devices - the reason we size conductors at 125% of continuous load is because of termination temp rating. Typical ocpd sinks some heat into the conductor meaning terminations will be the hot spots and need more material there to sink that heat into. Otherwise we typically have conductors with 90C insulation connected to 75C rated terminals - so the conductor can take it, the terminal is the reason for the additional 25%.

intermittent duty applications is a different game - some you situations you can have conductor ampacity of only 85% motor rated current others you need 200%.

 

[Dale001289]

To add to that, for other then 100% rated devices - the reason we size conductors at 125% of continuous load is because of termination temp rating. Typical ocpd sinks some heat into the conductor meaning terminations will be the hot spots and need more material there to sink that heat into. Otherwise we typically have conductors with 90C insulation connected to 75C rated terminals - so the conductor can take it, the terminal is the reason for the additional 25%.

intermittent duty applications is a different game - some you situations you can have conductor ampacity of only 85% motor rated current others you need 200%.

True - but ‘heat sink at terminals’ applications is more for thermal-magnetic CB applications vs magnetic only. Intermittent operations many times get lost in the shuffle. It pays (literally) to investigate all motor loads since not always continuous duty.


Sent from my iPhone using Tapatalk

 

[kwired]

True - but ‘heat sink at terminals’ applications is more for thermal-magnetic CB applications vs magnetic only. Intermittent operations many times get lost in the shuffle. It pays (literally) to investigate all motor loads since not always continuous duty.


Sent from my iPhone using Tapatalk

But at same time motors is a different ball game then most other applications and we are sent to art 430 instead of following the general rules for motor circuit conductors and overcurrent protection on those circuits. The conductor still gets sized at 125% for majority of continuous duty applications even if it will be on an instantaneous trip only breaker. I suppose the fact that overload protection can be 125% is a contributing factor to that, but at same time the rule for conductor ampacity doesn't change when you have a 1.0 SF motor.