Adding an additional load to a MCC switchboard (verifying the capacity of busbar) 1

[kply_231]

Hello,
I am trying to add an additional load to a MCC switchboard that has a bus bar rating of 800A. I want to verify if adding the load will lead to exceeding the capacity of the bus bar rating or not. I am in Canada and the code that I follow is Canadian Electrical Code part 1.

Sum of the full load amps of all connected loads after adding the new load is coming out to be 736A. Does this mean I am okay, or do I need to multiply 736A by 125% to check the bus bar capacity. I am confused because when a cable is selected, Canadian Electrical Code part 1 specifies to multiply 125% to the full load amps, but not sure if that is applicable even for a busbar.

Below is the link to a different discussion thread on this similar topic -
thread238-420754
In one of the posts in the above discussion thread, I noticed that @waross mentioned about adding 25% of the largest motor, 135% of the capacitor loads, 25% of the largest transformer and 135% of the transformer feeding capacitors to the sum of the full load amps of all connected loads. Below is the snip of the reply from @waross that I am referring -

Can someone provide me the reference clause number to the rule that @waross has mentioned his reply?

 

[wroggent]

I am not familiar with the CEC but in the NEC the overcurrent protection must sized in the same manner as the feeder, and the MCC bus bars are required to be rated not less than the overcurrent protective device. You could serve the MCC from a LVPCB in a switchgear at 100% of the MCC bus bar rating if you desired (when following NEC, I don't know about CEC).

 

[jraef]

Most MCCs I see are such that if you added the FLA of the connected devices, it usually exceeds the MCC bus rating. So it's common practice to size the MCC bus for the EXPECTED load, i.e. you KNOW that some of the loads will not be on at the same time and that few of them will likely be running at full rated current. There is no official document allowing this however and if you ask an MCC mfr for their suggestions, they will fall back on the feeder rules described by waross...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Padlock01]

I would suggest you put a recorder on the MCC for a day or two to establish what your existing load is.
Then confirm whether a load factor has been applied to your new loads.
Then together with your new and existing loads see whether you exceed the busbar rating.
As we do not know the supply transformer details and whether you have exceeded the rule of thumb
Loading Transformer only 80% capacity.

 

[kply_231]

@wroggent: My concern is not about the rating of overcurrent protection device in the MCC switchboard, but the rating of the main feeder bus bar in the MCC.
@jraef & @padlock01: I am not sure what code you follow, but Canadian Electrical Code part 1 has provision for applying demand factors on the loads to permit a lower percentage of the sum of FLAs of all connected loads in a switchboard provided that I have data of the most recent 12-month period. Refer rule 8-106 in Canadian Electrical Code part 1. It says as below -

"Where additional loads are to be added to an existing service or feeder, the augmented load shall be permitted to be calculated by adding the sum of the additional loads, with demand factors as permitted by this Code, to the maximum demand load of the existing installation as measured over the most recent 12-month period, but the new calculated load shall be subject to Rule 8-104 5) and 6)."

 

[waross]

I would start by reviewing the power billing invoices during the previous 12 or 24 months.
The bills will show the maximum demand for each month.
This will also indicate whether the main service has adequate capacity for the additional proposed load.
You may be able to isolate other known loads to determine the maximum demand on the MCC in question.
or
You can recalculate the existing loads with the appropriate demand factors to see if there is extra capacity available.
Review the capacity of any transformers and or feeders supplying the MCC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[kply_231]

Hello waross.
Can you please give me the reference to the rule in Canadian Electrical Code part 1 that you mentioned in this post -

thread238-420754:

"Add 25% of the largest motor, 135% of the capacitor loads, 25% of the largest transformer and 135% of the transformer feeding capacitors to the sum of the full load amps of all connected loads"

 

[waross]

CEC 2015 Edition
See:
Section 0 Object Scope and Definitions;
Feeder — any portion of an electrical circuit between the service box or other source of supply and the branch
circuit overcurrent devices.
Section 8 — Circuit loading and demand factors
Scope
8-000 Scope
8-104 Maximum circuit loading (see Appendix B)
8-106 Use of demand factors (see Appendix B)
8-210 Other types of occupancy
26-208 Conductor size for capacitors
26-258 Conductor size for transformers
28-108 Conductors — Two or more motors
28-110 Feeder conductors

Tables:
Table 14
Watts per square metre and demand factors for services
and feeders for various types of occupancy
(See Rules 8-002 and 8-210.)

Appendix B Notes on Rules
Rule 8-106(10)
Section 28, Example to determine motor conductors and protection, Feeder conductor ampacity
Diagram of circuits, control, and protective
devices for motors
I have omitted rules pretaining to Elevators, fire pumps and other types of loads not generally fed from an MCC.
I hope that this helps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[kply_231]

@waross: Thanks a lot.
Rule 28-108 & 28-110 of Canadian Electrical Code part 1 is what would apply in my situation.