Problems sizing main over current protection and conductor sizing

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Minimum size main over current protection device in the MDP for 838.5Amps of load. I get 1000 amp OCP? Am i correct? I got this because you go up to the next standard size or not required to go up? It is asking minimum. If I go up then my conductors have to be equal to or over this breaker size. My service entrance conductor options are 2 sets 500kcmil, 2 sets of 400kcmil, and 3 sets 300kcmil. The three sets of 300kcmil is the only set that would put me up over 800amps at 855amps. Either the question is screwed up or it is ok to round down you main OCP size? Please help
 

Smart $

Esteemed Member
Location
Ohio
Minimum size main over current protection device in the MDP for 838.5Amps of load. I get 1000 amp OCP? Am i correct? I got this because you go up to the next standard size or not required to go up? It is asking minimum. If I go up then my conductors have to be equal to or over this breaker size. My service entrance conductor options are 2 sets 500kcmil, 2 sets of 400kcmil, and 3 sets 300kcmil. The three sets of 300kcmil is the only set that would put me up over 800amps at 855amps. Either the question is screwed up or it is ok to round down you main OCP size? Please help
Minimum OCPD rating is 838.5A. You are not required to go up but you can go up to the next standard rating if you are using a fuse or non-adjustable breaker (1000A).

Over 800A the conductor ampacity must equal or exceed the rating of the OCPD. The 2 sets of 500's or 400's are out of contention (<838A). The 3 sets of 350's would require an adjustable breaker.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Do you know the nature of the load ? Motors or HVAC equipment would be approached differently than other loads.
 
Please help

Please help

Minimum OCPD rating is 838.5A. You are not required to go up but you can go up to the next standard rating if you are using a fuse or non-adjustable breaker (1000A).

Over 800A the conductor ampacity must equal or exceed the rating of the OCPD. The 2 sets of 500's or 400's are out of contention (<838A). The 3 sets of 350's would require an adjustable breaker.

So my options of answers for my minimum main OCPD is 600 amp, 750 amp, 800 amp, or 1000 amp? With a total load of 838.5 amps. Is the answer 800amp or 1000amp main breaker size? If it is 1000amp then my options for sets of wire for the feeder conductors is not right.........Please help
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
or... you relating the question has a flaw.. can you state the question exactly as worded.....
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Take a look at 215.2, 230.23, 230.31, 230.42, 230.90 and 240.4. If there are exceptions, read them too.
 

Smart $

Esteemed Member
Location
Ohio
So my options of answers for my minimum main OCPD is 600 amp, 750 amp, 800 amp, or 1000 amp? With a total load of 838.5 amps. Is the answer 800amp or 1000amp main breaker size? If it is 1000amp then my options for sets of wire for the feeder conductors is not right.........Please help
If those are your choices, then it has to be 1000A... but none of the three options for conductors are compliant.
 

bob

Senior Member
Location
Alabama
Minimum size main over current protection device in the MDP for 838.5Amps of load. I get 1000 amp OCP? Am i correct? I got this because you go up to the next standard size or not required to go up? It is asking minimum. If I go up then my conductors have to be equal to or over this breaker size. My service entrance conductor options are 2 sets 500kcmil, 2 sets of 400kcmil, and 3 sets 300kcmil. The three sets of 300kcmil is the only set that would put me up over 800amps at 855amps. Either the question is screwed up or it is ok to round down you main OCP size? Please help

If the calculated load is 838 amps and you are going to use a 1000 amp breaker, then the minimum
conductor is 3-400 kcm per phase rated at 335 amps x 3 = 1005 amps. 3 #350 kcm = 3 x 310 amps = 930 amps
which does not meet the code requirements. These ratings are assuming no derating is required. As Don said,
there may be some allowed methods to reduce the load. Is this an industrial customer?
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If the calculated load is 838 amps and you are going to use a 1000 amp breaker, then the minimum
conductor is 3-400 kcm per phase rated at 335 amps x 3 = 1005 amps. 3 #350 kcm = 3 x 310 amps = 930 amps
which does not meet the code requirements. These ratings are assuming no derating is required. As Don said,
there may be some allowed methods to reduce the load. Is this an industrial customer?

It sounds like an exam question to me.
 
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