6 Disconnect Rule v.s. Single Main Disconnect

[xxmrwhitexx]

Hello, I have looked over the forums and though the six disconnect rule is discussed the following question has not been addressed that I am aware of. I am aware of the codes surrounding when and how to apply the 6 disconnect rule; my question is "What are the benefits to going with a single main disconnect?" The scenario I am running into is regarding an industrial application. We are replacing an old 5kV service with a new 480/277V secondary service. The new main switchboard is to be a 1600A board. The original design showed a 1600 Amp main in the incoming section and (3) feeder circuit breakers in the distribution section. The cost that came back on this gear higher than the client was anticipating. (Poor expectation management by others) So in order to drop the cost I sent out an alternate design showing the removal of the main circuit breaker while utilizing the 6 disconnect rule. The main switchboard price was cut in half and was able to be reduced to 1 section. I have a discussion coming up with the owner to explain the pro's and cons of this decision. The cons I have came up with for the 6 disconnect rule are 1. It reduces flexibility for future expansion later down the road by limiting the amount of feeders that can be derived from that board. 2. there is a minor safety concern when shutting down the building due to the multiple switches (Unqualified guy hits 1 of the 3 switches thinking the building is down before servicing...zap) Beyond that I cannot come up with anything. Does it impact how a fault is cleared? What justifies the additional cost of the gear to go with a single main disconnect if it is justifiable at all?

 

[Senior member]

Are you Pricing option 2 with all 6 switches installed?

 

[bob]

1. It reduces flexibility for future expansion later down the road by limiting the amount of feeders that can be derived from that board 2. there is a minor safety concern. 3. Does it impact how a fault is cleared? 4. What justifies the additional cost of the gear to go with a single main disconnect if it is justifiable at all?

1. I disagree. You have 3 additional spaces for expansion. The limiting factor is the 1600 amp capacity of the panel. 2. Very minor. Use qualified personnel 3. It should not. 4. When you want to use more that 6 breakers. Some panels will have a 1600 amp main 24 or more spaces for additional breakers. By using the 6 breaker positions, you will need to add an additional breaker to feed a sub panel with more breakers for expansion.

 

[xxmrwhitexx]

I ran this by another PE and he brought up the fact that there is a possibility of overloading the service entrance conductors as the switchboard fills up. Since its often that the maximum total capacity of the feeders could exceed 1600 amps. Basically there is no overload protection for the service entrance conductors.

On top of that I ran into a scenario in Arizona where the utility actually won't allow the six disconnect rule since they provide, install and maintain the secondary service conductors. Same reason as stated above.

Thanks for the reply!

 

[kwired]

When adding new switches down the road a new load calc should need to be done to prevent overloading the service conductors.

If each main is less than 1000 amps GF protection equipment is not required and probably has some impact on cost, beyond the fact that a 1600 amp switch will cost more than say a 600 or 800 amp switch does.