# calculation check



## gpop (May 14, 2018)

23.5hp about 18kw should have a fla of 26amp @ 480v 3 phase

2 centrifugal pumps. Line should take less then 3 seconds to prime and the pumps have a 10 second hard wired TDR. (pumps can not start at the same time)

main breaker feeding 2 sub breakers (one for each pump) using the tap rule. 

26A x 250% (pump 1)+ 26A (pump 2)= 88.5 plus 3A for the control panel = 91.5A

So 100 amp main breaker should be ok. 

Is that correct or is the engineer correct and it needs 150A


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## VELOCI3 (Aug 15, 2019)

Article 430 has so many exceptions that you almost have to read everything on it to get to the real answer


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## paulengr (Oct 8, 2017)

gpop said:


> 23.5hp about 18kw should have a fla of 26amp @ 480v 3 phase
> 
> 2 centrifugal pumps. Line should take less then 3 seconds to prime and the pumps have a 10 second hard wired TDR. (pumps can not start at the same time)
> 
> ...



250% is the Code maximum for a single motor but you are talking about a feeder (distribution) where we have more leeway. In that case we can use 26 x 600% + 26 A to avoid nuisance tripping during a stall of one pump or 182 A. You could use this as the ultimate trip level but a more refined answer is to look at breaker trip curves for the size that exceeds 182 A at 10 seconds.


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## gpop (May 14, 2018)

paulengr said:


> 250% is the Code maximum for a single motor but you are talking about a feeder (distribution) where we have more leeway. In that case we can use 26 x 600% + 26 A to avoid nuisance tripping during a stall of one pump or 182 A. You could use this as the ultimate trip level but a more refined answer is to look at breaker trip curves for the size that exceeds 182 A at 10 seconds.


I know this is less of a code question and more of a "what was he thinking" compared to what i was thinking question. 

It does seem like a valid reason. Of course i could counter that the sub breaker for the pump if sized correctly would clear before the main in a locked rotor situation. 

I just didn't want to waste time and effort pulling new wire with out have some sort of justification other than being over engineered.


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## oldsparky52 (Feb 25, 2020)

Motors were not my strong suit, but I thought you sized the feeders by using 1.25 x the highest motor rating then add 100% of the rest (when they can't start simultaneously). 

So that would be 26*1.25+26 = 58.5. So a 60-amp rated conductor would be the minimum size needed. 

What am I mistaken about?


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## paulengr (Oct 8, 2017)

oldsparky52 said:


> Motors were not my strong suit, but I thought you sized the feeders by using 1.25 x the highest motor rating then add 100% of the rest (when they can't start simultaneously).
> 
> So that would be 26*1.25+26 = 58.5. So a 60-amp rated conductor would be the minimum size needed.
> 
> What am I mistaken about?



An ampacity sizing is 125% of the FLA unless it’s a special case. That works with enough motors but in this case you will not coordinate with the motor nor the starter protection. That’s why Code sets a 250% of FLA size for the starter alone. I haven’t ever found a simple way to size for this situation.

Cooper (Bussmanm) has a great handbook on protection. It’s so good that the people behind the arc flash standards use it in their spreadsheets. It promotes fuses a lot but it’s a fantastic book explaining this topic with practical methods for designing without a $10,000 engineering software.


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## oldsparky52 (Feb 25, 2020)

paulengr said:


> An ampacity sizing is 125% of the FLA unless it’s a special case. That works with enough motors but in this case you will not coordinate with the motor nor the starter protection. That’s why Code sets a 250% of FLA size for the starter alone. I haven’t ever found a simple way to size for this situation.


Isn't that 250% used for SCP and OCP, not the feeder size? Would that apply to the feeder OCP or is it just between the last OCPD and the motor starter?


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## Bello (Oct 21, 2018)

oldsparky52 said:


> Motors were not my strong suit, but I thought you sized the feeders by using 1.25 x the highest motor rating then add 100% of the rest (when they can't start simultaneously).
> 
> So that would be 26*1.25+26 = 58.5. So a 60-amp rated conductor would be the minimum size needed.
> 
> What am I mistaken about?



This is how I would calculate this in Canada. 

Cable size at 125% of largest motor FLA plus all other motors FLA at 100%

Over current protection would be set at 175% of largest motor FLA (time delay fusing) plus 100% FLC of all other motors.

My understanding of rules 28-108 and 28-110 is that the cable size will not change due to a change in over current protection unless they are tap conductors over 7.5 meters in length.


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## Bello (Oct 21, 2018)

My understanding of rules 28-108 and 28-110 is that the cable size will not change due to a change in over current protection unless they are tap conductors over 7.5 meters in length.



Just noticed that Tap conductor ruling was stated in OP post.


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## Quickservice (Apr 23, 2020)

paulengr said:


> 250% is the Code maximum for a single motor but you are talking about a feeder (distribution) where we have more leeway. In that case we can use 26 x 600% + 26 A to avoid nuisance tripping during a stall of one pump or 182 A. You could use this as the ultimate trip level but a more refined answer is to look at breaker trip curves for the size that exceeds 182 A at 10 seconds.


Stalled compressor.... same effect.


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## HertzHound (Jan 22, 2019)

gpop said:


> I just didn't want to waste time and effort pulling new wire with out have some sort of justification other than being over engineered.


So what wire size is existing? It seems like if you had a 100A Feeder before, your good to go. You mentioned 100A increasing to 150A. This post seems to go back and forth on wire and breaker size. Two different things.


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## Dennis Alwon (May 9, 2009)

Here is the code section

The OP did it correctly, IMO 




> 430.62 Rating or Setting — Motor Load.
> (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
> ...


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## Dennis Alwon (May 9, 2009)

Unfortunately if the plans show 150 amps then that is what you need to do. The engineer may be figuring that one may need to use the exception to T430.52 which allows you to use more than 250%


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## gpop (May 14, 2018)

HertzHound said:


> So what wire size is existing? It seems like if you had a 100A Feeder before, your good to go. You mentioned 100A increasing to 150A. This post seems to go back and forth on wire and breaker size. Two different things.



existing is 150A breaker in mcc then around 300' of 1/0 to the panel.

Panel main is 100 then a few inches to 2 x 30 amp breakers and a few fuses.

One thing i should mention. If the main breaker trips out of hours the estimated cost to reset is $800 to $1000 as 3 teams will be automatically dispatched after the plc fails to respond or reports main power/back up power failed.


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## oldsparky52 (Feb 25, 2020)

gpop said:


> existing is 150A breaker in mcc then around 300' of 1/0 to the panel.
> 
> Panel main is 100 then a few inches to 2 x 30 amp breakers and a few fuses.
> 
> One thing i should mention. If the main breaker trips out of hours the estimated cost to reset is $800 to $1000 as 3 teams will be automatically dispatched after the plc fails to respond or reports main power/back up power failed.


So you have an existing feeder that is rated for 150 amps and the engineer wants you to change the 100 amp breaker to a 150 amp breaker. 

What's the concern? Does the 150 not fit in the space the 100 is in? Did you miss the expense on your bid?

Sounds like the engineer wants to do the best job of keeping the $800 to $1,000 expense you mentioned at bay.


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## gpop (May 14, 2018)

Dennis Alwon said:


> Unfortunately if the plans show 150 amps then that is what you need to do. The engineer may be figuring that one may need to use the exception to T430.52 which allows you to use more than 250%


There's no harm in pushing back and asking for there load calculation. We have to approve the prints which means asking questions. 

The original breaker is a Sq-d which to the eye looks like a 150 as it has HG150 in big letter printed on it. It is a 100A which is written in small letters near the bottom. So this may just be a case that the engineer thought it was 150A so he left it that size.


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## gpop (May 14, 2018)

oldsparky52 said:


> So you have an existing feeder that is rated for 150 amps and the engineer wants you to change the 100 amp breaker to a 150 amp breaker.
> 
> What's the concern? Does the 150 not fit in the space the 100 is in? Did you miss the expense on your bid?
> 
> Sounds like the engineer wants to do the best job of keeping the $800 to $1,000 expense you mentioned.


Cascade tripping would be preferable.


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## oldsparky52 (Feb 25, 2020)

gpop said:


> Cascade tripping would be preferable.


Wouldn't the 150 offer that scenario better than the 100?


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## gpop (May 14, 2018)

oldsparky52 said:


> Wouldn't the 150 offer that scenario better than the 100?


No idea unless i check the trip curves.

150 breaker - wire - 150 breaker is a crap shoot which trips first.


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## Bird dog (Oct 27, 2015)

gpop said:


> Cascade tripping would be preferable.


It's also referred to as coordination of OCPDs.


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