# 300% rule



## ponyboy (Nov 18, 2012)

Nope. Measuring the current is the only way to determine a panels capacity with absolute certainty. A data logger is even better. 


Sent from my iPhone using Tapatalk


----------



## Dennis Alwon (May 9, 2009)

Samjon said:


> Hi Guys.
> When i was apprenticing my journeyman had said to me about a 300% rule of thumb when it came to seeing if a panel was near full capacity of branch loads.
> 
> ex.) 100 amp panel (100X3=300amp). Add up all the breaker amps. And if it totalled more than 300 than the panel is at full capacity, cant add any new circuits to panel.
> ...


Never heard of it and I would say that that would be a risky way to do it but it may work in some case.

I could have a 60 amp breaker and the load may be less than 40 amps or even less than that. 

I have feed a 200 amp single phase 40 circuit panel with a 125 amp breaker. It was full of 15 and 20 amp breakers

40 x 15 = 600 amps by your method--- 125 amps was and still is fine


----------



## backstay (Feb 3, 2011)

Urban legend?


----------



## MechanicalDVR (Dec 29, 2007)

_Alternate facts_ code edition.


----------



## B-Nabs (Jun 4, 2014)

MechanicalDVR said:


> _Alternate facts_ code edition.


Coming from you that is just too rich.


----------



## electricguy (Mar 22, 2007)

"He said his father (was an electrician, old school method) "
like in 1955


----------



## LGLS (Nov 10, 2007)

Mighta been applicable back then, definitely not now.


----------



## MechanicalDVR (Dec 29, 2007)

B-Nabs said:


> Coming from you that is just too rich.


I'll let that slide. :thumbsup:


----------



## MechanicalDVR (Dec 29, 2007)

electricguy said:


> "He said his father (was an electrician, old school method) "
> like in 1955


Or maybe 1935 based on a fuse box?


----------



## LGLS (Nov 10, 2007)

Does 3 watts per sq. ft. even work anymore?


----------



## MechanicalDVR (Dec 29, 2007)

LawnGuyLandSparky said:


> Does 3 watts per sq. ft. even work anymore?


Oddly enough that maybe real good with LEDs.

It's the 1.5 amps per receptacle that maybe low.


----------



## electricguy (Mar 22, 2007)

I have had customers tell me they added up the amps of each breaker and feel there panel is overloaded. Not sure who helped them count it up.


----------



## MechanicalDVR (Dec 29, 2007)

electricguy said:


> I have had customers tell me they added up the amps of each breaker and feel there panel is overloaded. Not sure who helped them count it up.


Sounds like an easy upgrade sale to me!


----------



## Semi-Ret Electrician (Nov 10, 2011)

Dennis Alwon said:


> Never heard of it and I would say that that would be a risky way to do it but it may work in some case.
> 
> I could have a 60 amp breaker and the load may be less than 40 amps or even less than that.
> 
> ...


True, how about a home workshop where only one person would be using only one tool at a time, but has dozens of breakers.


----------



## Samjon (Jan 19, 2017)

Dennis Alwon said:


> Never heard of it and I would say that that would be a risky way to do it but it may work in some case.
> 
> I could have a 60 amp breaker and the load may be less than 40 amps or even less than that.
> 
> ...


Thanks for your constructive feedback


----------

