MLO Service Panel and Conductor Size

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Electriman

Senior Member
Location
TX
Greetings,

I am working on a 800A, 480VAC MLO service panel. I was wondering what size of conductor I should select for this service. I was wondering if I should size the cable based on 125% of 800A or based on 800A since there is no main breaker to trip.

Second question, this service is 480V 4 wire, do I need to de-rate the cable since I have 4 current conductor cable?

Thank you in advance.
 
jumper said:
Here is a thread that will help get you started.

http://forums.mikeholt.com/showthread.php?t=109839

After reading it, I would then ask any questions that need further clarification.
Click to expand...

I looked at the thread but that thread is talking about a breaker. In my application there is no 800A breaker and it is a MLO panel. I did not see any thing also to talk about 4 Wire system when you have to consider 4 current caring conductors into account. But the length of my cables are 100' and it would be routed via over head conduit.
 
1. What is the calculated load of the service? I am guessing since you have no main disconnect, you have 6 or less breakers in the service panel.

2. Are you sure the neutral is a CCC? Are the majority of the non-linear and using the neutral?
 
jumper said:
1. What is the calculated load of the service? I am guessing since you have no main disconnect, you have 6 or less breakers in the service panel.

The calculated load is less than service. The owner wants to room for future growth. and this panel has 2 load breaker one 400A and one 200A that feed the two other panel with main breaker.

2. Are you sure the neutral is a CCC? Are the majority of the non-linear and using the neutral?
I always consider neutral as a CCC. But in this case neutral carrying the unbalance loads. Do we have any where in code that mentions where neutral is not a CCC?
Click to expand...

1. What is the calculated load of the service? I am guessing since you have no main disconnect, you have 6 or less breakers in the service panel.

The calculated load is less than service. The owner wants to room for future growth. and this panel has 2 load breaker one 400A and one 200A that feed the two other panel with main breaker.

2. Are you sure the neutral is a CCC? Are the majority of the non-linear and using the neutral?
I always consider neutral as a CCC. But in this case neutral carrying the unbalance loads. Do we have any where in code that mentions where neutral is not a CCC?
 
Electriman said:
Greetings,

I am working on a 800A, 480VAC MLO service panel. I was wondering what size of conductor I should select for this service. I was wondering if I should size the cable based on 125% of 800A or based on 800A since there is no main breaker to trip.

Second question, this service is 480V 4 wire, do I need to de-rate the cable since I have 4 current conductor cable?

Thank you in advance.
Click to expand...

generally you would want to start with a load calculation per article 220.

then look at 230, especially 230.90.
 
Assuming you did the load calculations (100% non-continuous + 125% continuous) to size the 400 amp and 200 amp branch breakers that would mean you calculated load is not greater than 600 amps so you can size your conductor anything over 600 amps (perhaps 2-350s) or you can size them to allow you full use of your 800 amp panel (2-600s ?)

It would be highly unlikely your load would qualify as one with a major portion as non-linear so it is doubtful the neutral would be counted as a current carrying conductor.
 
Electriman said:
I am working on a 800A, 480VAC MLO service panel. I was wondering what size of conductor I should select for this service. I was wondering if I should size the cable based on 125% of 800A or based on 800A since there is no main breaker to trip.
Click to expand...

For the smallest conductor size you would use the calculated load. if you do not have the calculated load or you are leaving room for the future in my opinion the inspector could require you size the conductors to the total of the current ratings of the service disconnect breakers.
 
iwire said:
For the smallest conductor size you would use the calculated load. if you do not have the calculated load or you are leaving room for the future in my opinion the inspector could require you size the conductors to the total of the current ratings of the service disconnect breakers.
Click to expand...

Breakers or rating of the MDP?

I could see sizing the service conductors to the max rating of the MDP.
 
jumper said:
Breakers or rating of the MDP?

I could see sizing the service conductors to the max rating of the MDP.
Click to expand...

Actually if the calculated load is not available in my opinion the total amperage of the service disconnects cannot exceed the bus rating.
 
jumper said:
I could live with that.

The exception for the 2-6 disconnects states that the calculated load is required for the sum of the breakers to exceed the bus rating.
Click to expand...

Yeah, its not like I can point to a direct section that says it like I did.

But I look at it like this, the service conductors and bus bars need to be protected from overcurrent and there are only two ways to do that.

Use an OCPD or know the load will not exceed the rating.
 
Electriman said:
1. What is the calculated load of the service? I am guessing since you have no main disconnect, you have 6 or less breakers in the service panel.

The calculated load is less than service. The owner wants to room for future growth. and this panel has 2 load breaker one 400A and one 200A that feed the two other panel with main breaker.

2. Are you sure the neutral is a CCC? Are the majority of the non-linear and using the neutral?
I always consider neutral as a CCC. But in this case neutral carrying the unbalance loads. Do we have any where in code that mentions where neutral is not a CCC?
Click to expand...
The neutral is a current carrying conductor, but for the purpose of conductor ampacity adjustments for number of conductors in a raceway, it is not always considered to be contributing any more heat to the raceway then the ungrounded conductors it is a balance for. Three phase four wire wye, it only carries unbalanced current from the other conductors, unless there is a significant amount of non linear load in the neutral conductor.
 
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