Demand Calculations?

[Solar Noob]

I have a question about how demand/load calcs work when examining breaker schedules. If the schedule is not at all labeled, how do you calculate the demand on the panel itself, especially if one of the loads is a main breaker for a subpanel? Based purely off of the amperage of the breakers labeled, the demand is way over the buss rating, however I understand that a) not all breakers are sized for the actual amperage draw of whatever is attached to it and b) the sum of all breakers rule applies for subpanels.

Can anyone explain how to manually calculate the actual demand of the panel, or does anyone have a good demand/load calculator they use that is reliable?

 

[retirede]

Article 220 load calc?

 

[mayanees]

...or 220.87 Determining Existing Loads

 

[petersonra]

I have a question about how demand/load calcs work when examining breaker schedules. If the schedule is not at all labeled, how do you calculate the demand on the panel itself, especially if one of the loads is a main breaker for a subpanel? Based purely off of the amperage of the breakers labeled, the demand is way over the buss rating, however I understand that a) not all breakers are sized for the actual amperage draw of whatever is attached to it and b) the sum of all breakers rule applies for subpanels.

Can anyone explain how to manually calculate the actual demand of the panel, or does anyone have a good demand/load calculator they use that is reliable?

article 220 tells you exactly how to manually do load calculations.

 

[drktmplr12]

I have a question about how demand/load calcs work when examining breaker schedules. If the schedule is not at all labeled, how do you calculate the demand on the panel itself, especially if one of the loads is a main breaker for a subpanel? Based purely off of the amperage of the breakers labeled, the demand is way over the buss rating, however I understand that a) not all breakers are sized for the actual amperage draw of whatever is attached to it and b) the sum of all breakers rule applies for subpanels.

Can anyone explain how to manually calculate the actual demand of the panel, or does anyone have a good demand/load calculator they use that is reliable?

you penalize yourself by using breaker sizes to calculate the demand load. in some cases, you may have capacity to add what you need. if you don't you are stuck applying any of the other methods in 220.

if there is a utility service meter for the panel, you can ask the utility for the demand data, otherwise, you need to put a meter on it yourself and collect data for 30 days and use 220.87.

or you can find all the receptacles, motors, appliances, heaters, HVAC, etc connected.

also, what are you adding? if it's a really small load, you should be able to convince the AHJ that your 1 amp load isn't going to put the panel over.

 

[jaggedben]

..

also, what are you adding? if it's a really small load, you should be able to convince the AHJ that your 1 amp load isn't going to put the panel over.

Just guessing from their handle, they're probably trying to show an AHJ they can legitimately downsize the main breaker to allow more headroom for the solar 120% rule.