Series

hhsting

Senior Member
I understand I had some version of this previously but was little confused about previous situation.

Please see attached sketch region label #1 is series rated with bkr #1 and bkr #2. Region label # 2 is series rated with bkr #2 and bkr#4.
Region label #3 is series rated with bkr #4 and bkr#7.

Assume motors full load exceed 1%.

Following questions:


1. I am guessing but not sure but regions label #1,#2,#3 should not be series rated because of NEC 2014 section 240.86(C). If not then which regions should not be series rated NEC 2014 section 240.86(C)?

2. Fault current of motors can travel anywhere there is fault. If fault is between region #1 I am not sure how it would work. Utility fault current would go downstream and motor fault current go upstream and thus cancel some of the utility fault currents or not? If allowed to be series rated Region #1 then what is logic behind it

3. If fault in between Region #3 then motor faults current and utility fault current would add so line side of panelboard breaker would see motor fault currents as well as utility. If Region is allowed to have series rating and not fully rated 240.86(C) then would not the panelboard breaker be melted given it is less than available fault current? Yet if allowed to be series rated then what is logic behind that allowance?

 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
You need to draw out the actual fault current paths.
For example:
Breaker #1 will never have to interrupt both utility and any motor produced fault current at the same time.
Breaker #4 will never have to interrupt both its source fault currents (utility plus motors from other circuits) and its load produced fault current at the same time.
Breaker #7 will never have to interrupt its load produced fault current at the same time as its source.

240.86(C) is about fault current being added between the two series rated device. The intent is to account for fault current which may not be limited by the dynamic impedance produced by an upstream device in a series rated system.
For example:
The line side fault current into the series combination of breaker #4 and breaker #7 is the utility + all motors in the system not on this circuit.
The 240.86(C) current allowance would be the motor loads connected between the load side of breaker #4 and the line side of breaker #7.
 

hhsting

Senior Member
You need to draw out the actual fault current paths.
For example:
Breaker #1 will never have to interrupt both utility and any motor produced fault current at the same time.
Breaker #4 will never have to interrupt both its source fault currents (utility plus motors from other circuits) and its load produced fault current at the same time.
Breaker #7 will never have to interrupt its load produced fault current at the same time as its source.

240.86(C) is about fault current being added between the two series rated device. The intent is to account for fault current which may not be limited by the dynamic impedance produced by an upstream device in a series rated system.
For example:
The line side fault current into the series combination of breaker #4 and breaker #7 is the utility + all motors in the system not on this circuit.
The 240.86(C) current allowance would be the motor loads connected between the load side of breaker #4 and the line side of breaker #7.
I still don’t follow. Lets take a look at Bkrs #2 and bkrs #4 which are series rated.

Let me understand What you are saying is between bkrs 2 and bkrs 4 utility source current and motors source currents are Not being added so 240.86(C) does Not apply?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Let me understand What you are saying is between bkrs 2 and bkrs 4 utility source current and motors source currents are Not being added so 240.86(C) does Not apply?
No, that is not what I said.

I said 240.86 deals with motor fault currents occurring between series rated devices.

Assume some currents and see how they flow through the protective devices into a chosen fault location.
 

hhsting

Senior Member
No, that is not what I said.

I said 240.86 deals with motor fault currents occurring between series rated devices.

Assume some currents and see how they flow through the protective devices into a chosen fault location.
Assume fault between bkr 1 and bkr 2. Call that fault location x. Assume also bkr #1 and bkr #2 series rated.

All motor fault currents would move up and utility fault down to the fault location x.


Would NEC 2014 section 240.86(C) apply or not in case above?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
240.86 always applies when fault current is being injected between two devices using a series rating.

My point is the magnitude of the injected current varies throughout the system.

At your fault point, how much current is breaker 1 interrupting (only the utility). How much is breaker 2 interrupting (only the back feed from the motors). Current is not being injected between these two breakers.
 

hhsting

Senior Member
240.86 always applies when fault current is being injected between two devices using a series rating.

My point is the magnitude of the injected current varies throughout the system.

At your fault point, how much current is breaker 1 interrupting (only the utility). How much is breaker 2 interrupting (only the back feed from the motors). Current is not being injected between these two breakers.

Ok. Assume their is fault load side of bkr #4 and bkr#4 is series rated with bkr #2, bkr#4 is series rated with bkr#7.

Bkr#4 line side would have current injected from utility + motors.

So then bkr #4 cannot be series rated with bkr#2 and bkr#7?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Do the math. I assume you have the data.
What currents are flowing from where? How much current is allowed per 240.86?

Just because a series rating is not possible on one part of a circuit does not mean a different series rating is not possible on a different combination of devices.
Series ratings can not be cascaded and are usually limited to two devices. Manufacturers of multi-metering equipment often have series rating for three devices, have you checked any of their application notes?
 

hhsting

Senior Member
Do the math. I assume you have the data.
What currents are flowing from where? How much current is allowed per 240.86?

Just because a series rating is not possible on one part of a circuit does not mean a different series rating is not possible on a different combination of devices.
Series ratings can not be cascaded and are usually limited to two devices. Manufacturers of multi-metering equipment often have series rating for three devices, have you checked any of their application notes?
Ok now designer is saying its meter stacks with feeder breakers. Yes it is multimeter equipment i.e meter stack with series rating three. Please see attached sketch.

Bkr#2 is series rated with bkrs #3 to bkrs #24.

Each if the meter stacks bkrs #3 to bkrs #24 are series rated with downstream breakers. So bkr #3 is series rated with bkr #25, bkr#4 is series rated with bkr #26 and so forth.

Please see attached sketch has Fault locations #1,#2,#3. Assume each if these location 240.86(C) has FLA of motors greater than 1%.


Can you have three series rating devices then and be compliance with 240.86(C) at the fault locations given in attached sketch? Can you please explain further. I really need help in this

 
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