# Question

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#### hhsting

##### Senior Member
Calculated demand load comes to 122A. The breaker used is 125A.

Would the cable size be 1/0 awg Al or 2/0 awg Al at 75C? I think its 1/0 awg Al but engineer says 2/0 awg Al should be the size

#### augie47

##### Moderator
Staff member
If it meets the requirement of 310.15(B)(7) then 1/0. If not, 2/0

Why do you think 1/0 ?

#### electrofelon

##### Senior Member
Calculated demand load comes to 122A. The breaker used is 125A.

Would the cable size be 1/0 awg Al or 2/0 awg Al at 75C? I think its 1/0 awg Al but engineer says 2/0 awg Al should be the size
Can you explain why you think 1/0 is ok?

#### electrofelon

##### Senior Member
Are you sure you didn't mean to discuss the title track off the 1970 moody blues album "a question of balance"??

#### d0nut

##### Senior Member
Why do you think 1/0 would be acceptable at 75C?

Also, if this is under your work as a plan reviewer, why would you care if the conductors were upsized?

#### hhsting

##### Senior Member
Can you explain why you think 1/0 is ok?

Two reasons:

1. NEC 2017 section 240.4(B)

2. I am not sure but this is five stories building with dwelling units. Would 310.15(B)(7) apply? Is it multi family dwelling unit? Feeder is 3 phase 208/120V

#### augie47

##### Moderator
Staff member
Two reasons:

1. NEC 2017 section 240.4(B)
Not applicable in this case. You have a 122 amp load so your conductor must be rated at at least 122 amps 215.2
2. I am not sure but this is five stories building with dwelling units. Would 310.15(B)(7) apply? Is it multi family dwelling unit?
If it's a dwelling unit, 310.15(B)(7) applies

#### hhsting

##### Senior Member
Not applicable in this case. You have a 122 amp load so your conductor must be rated at at least 122 amps 215.2

If it's a dwelling unit, 310.15(B)(7) applies

The cable is feeding 208/120V three phase panelboard.

I am confused by 310.15(b)(7) as to how it applies? Would it apply even if its 5 stories building with dwelling units? Also how would it apply to three phase?

#### augie47

##### Moderator
Staff member

For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of 2 ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permit‐ ted to be sized in accordance with 310.15(B)(7)(1) through (3).

sounds like it is a dwelling unit in a multi-family dwelling so at that point it meets the requirements BUT, now that you added info (Sting SOP) that it is a 3 phase feeder it does not meet the requirement .... so 2/0 !

#### don_resqcapt19

##### Moderator
Staff member
The cable is feeding 208/120V three phase panelboard.

I am confused by 310.15(b)(7) as to how it applies? Would it apply even if its 5 stories building with dwelling units? Also how would it apply to three phase?
Please try reading all of the words in the section that you have cited. The code language in that section very clearly answers your questions.

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