# 800 amp feeder #500 vs #600 kcmil

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#### Sparky5150

##### Member
My company has standardized on parallel #600 kcmil for 800 amp feeders.

In the olden days it was normal to use parallel #500.

I was wondering if #500 is still permissable or if there is a specific code that required the change to #600?

Thanks!
Sparky

#### don_resqcapt19

##### Moderator
Staff member
If the 500s are on the load side of an OCPD and if the calculated load is less than 760 amps they are fine. If the conductors are on the secondary of a transformer or if the calculated load exceeds 760 amps, 600s are required.

#### jumper

##### Senior Member

A pair of #500s are good for 760 amps, so if the load is that or less, 240.4(B) allows the next size breaker.

#### jumper

##### Senior Member
Well if Don is gonna bring in 240.21(C), I will throw in that you need #600 if the conductors are taps under 240.21(B)

#### charlie b

##### Moderator
Staff member
And I will add that if nobody has bothered to calculate the load, and if the intent is to use an 800 amp panel with 800 amps of overcurrent protection, then a pair of 500's is not adequate. You have to prove the load is below 760, or you have to assume it is between 760 and 800.

#### jumper

##### Senior Member
And I will add that if nobody has bothered to calculate the load, and if the intent is to use an 800 amp panel with 800 amps of overcurrent protection, then a pair of 500's is not adequate. You have to prove the load is below 760, or you have to assume it is between 760 and 800.
Well gee Charlie, I thought I kinda said in a way that in post #3.
Don did also.

#### charlie b

##### Moderator
Staff member
Well gee Charlie, I thought I kinda said in a way that in post #3. Don did also.
No you didn't, Derek, and neither did Don. :happyno: Neither of you addressed the possibility (and, I suspect, a not uncommon circumstance) that nobody actually performed a load calculation. Consider a core and shell installation, with no tenants yet contracted to occupy the space. You put an 800 amp panel into what will eventually be a machinery space or a commercial laundry space or a pinball palace. You can't calculate the load that the eventual tenant will install in that room. So you must presume that the load will exceed 760, and you must use 600's. That was my point.

#### jumper

##### Senior Member
No you didn't, Derek, and neither did Don. :happyno: Neither of you addressed the possibility (and, I suspect, a not uncommon circumstance) that nobody actually performed a load calculation. Consider a core and shell installation, with no tenants yet contracted to occupy the space. You put an 800 amp panel into what will eventually be a machinery space or a commercial laundry space or a pinball palace. You can't calculate the load that the eventual tenant will install in that room. So you must presume that the load will exceed 760, and you must use 600's. That was my point.
Okay, sounds good to me.:thumbsup:

#### iwire

##### Moderator
Staff member
Neither of you addressed the possibility (and, I suspect, a not uncommon circumstance) that nobody actually performed a load calculation.
The NEC requires a load calculation so of course it is done.

#### stevebea

##### Senior Member
If the 500s are on the load side of an OCPD and if the calculated load is less than 760 amps they are fine. If the conductors are on the secondary of a transformer or if the calculated load exceeds 760 amps, 600s are required.
Don, could you give me the NEC article you are refering to regarding transformer secondary conductors and overcurrent protection. I had a similar situation last week on a 800 amp service. I ordered 600's but someone in the office trumped me and ordered 500's. Thanks!

#### jumper

##### Senior Member
Don, could you give me the NEC article you are refering to regarding transformer secondary conductors and overcurrent protection. I had a similar situation last week on a 800 amp service. I ordered 600's but someone in the office trumped me and ordered 500's. Thanks!
240.21(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary, as specified in 240.21(C)(1) through
(C)(6). The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.

#### stevebea

##### Senior Member
240.21(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors
feeding separate loads, shall be permitted to be connected
to a transformer secondary, without overcurrent protection
at the secondary, as specified in 240.21(C)(1) through
(C)(6). The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.
Thank you very much kind sir! :thumbsup:

#### Sparky5150

##### Member
Thanks again men!!!

This is a service entrance from the load side of the utility company transformer to an office building for future tenants.
There is no way to accurately prove it will calc out to be less than 760 amps so #600's it is.

Sparky

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