Service wire size with 83% rule

[Little Ames]

You would use 150 amps not 140..

Thank you. I guess I will be telling the client she needs to upgrade. Thank you for your response.

 

[infinity]

Can you confirm that you take the size of the overcurrent protection device, and not the load demand calc on a panel to apply the 83% rule. I have an existing 150 amp panel with a 2/0 AL. wire. Appliances were added and the new load demand on the service is 140 amps. So do you apply it to 150 x .83= 124.5 amps or 140 x .83= 116.2 amps. I am trying to identify If I can keep the 150 amp panel with a 2/0 AL. wire or I need to upgrade to 175 amp 3/0 AL wire

There are two things involved, the size of the service required and the size of the conductors. A load calculation tells you the minimum service size. The 83% rule tells you the minimum conductor size for that service.

With a 140 amp calculated load the minimum service size of 150 amps (Table 240.6(A) for standard size OCPD's). Multiply that by 83% and you'll get the minimum conductor size for a 150 amp service.

 

[frofro19]

Thank you. I guess I will be telling the client she needs to upgrade. Thank you for your response.

Why would you need to upgrade?

 

[kwired]

Can you confirm that you take the size of the overcurrent protection device, and not the load demand calc on a panel to apply the 83% rule. I have an existing 150 amp panel with a 2/0 AL. wire. Appliances were added and the new load demand on the service is 140 amps. So do you apply it to 150 x .83= 124.5 amps or 140 x .83= 116.2 amps. I am trying to identify If I can keep the 150 amp panel with a 2/0 AL. wire or I need to upgrade to 175 amp 3/0 AL wire

You need at least 140 amps of capacity (presuming you did calculations correctly) you likely not finding a 140 amp breaker so you will be going with minimum of 150 amp breaker. If you do so you can use that 83% factor on 150 amps to select a conductor. Should you decide to go with 175 or 200 amp breaker then you apply that 83% to 175 or 200 when selecting a conductor - whether the calculated load is 140, 45, 125, or 173