NEC and 80% Utilization

[zvezda08]

Hello. Can someone tell me exactly where in the NEC it states that one can only load their main breaker for incoming service up to 80%?
Thanks in advance.

 

[kwired]

Hello. Can someone tell me exactly where in the NEC it states that one can only load their main breaker for incoming service up to 80%?
Thanks in advance.

It doesn't directly say that anywhere that I am aware of.

However one can sort of come to this conclusion as being what one does in practice because of rules that require conductors to be sized @ 125% of load in a lot of circumstances as well as overcurrent protection to also be 125% of the load in those same circumstances - mostly for "continuous loads".

 

[Dennis Alwon]

Hello. Can someone tell me exactly where in the NEC it states that one can only load their main breaker for incoming service up to 80%?
Thanks in advance.

Unless the loads are continuous you can load the breaker 100%

230.42 Minimum Size and Ampacity.

(A) General.
Service-entrance conductors shall have an ampacity of not less than the maximum load to be served. Conductors shall be sized not less than the largest of 230.42(A)(1) or (A)(2). Loads shall be determined in accordance with Part III, IV, or V of Article 220, as applicable. Ampacity shall be determined from 310.14 and shall comply with 110.14(C). The maximum current of busways shall be that value for which the busway has been listed or labeled.
Informational Note:
For information on busways, see UL 857, Standard for Safety for Busways.

• (1)
  Where the service-entrance conductors supply continuous loads or any combination of noncontinuous and continuous loads, the minimum service-entrance conductor size shall have an ampacity not less than the sum of the noncontinuous loads plus 125 percent of continuous loads.
• • 
    Exception No. 1:
    Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the sum of the continuous and noncontinuous load.
    Exception No. 2:
    The sum of the noncontinuous load and the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100 percent of their rating shall be permitted.
    
    
  • (2)
    The minimum service-entrance conductor size shall have an ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.

 

[jaggedben]

This is one of those oversimplifications that some people think is a rule but it isn't.

 

[Strathead]

You must also be aware of the rating of the equipment. A typical "400A" feed through 120/240 meter socket is actually rated 320A.

 

[PD1972]

You must also be aware of the rating of the equipment. A typical "400A" feed through 120/240 meter socket is actually rated 320A.

I don't think it's entirely accurate to say that it is rated for 320A. 400A meter sockets are rated for:

1. 320A with only continuous loads
2. 400A with a mixture of non-continuous loads at 100% and continuous loads at 125%
3. 400A with only non-continuous loads

Most buildings end up being a mixture of non-continuous and continuous loads (case 2 above). If you're calculating the service size based on article 220, conductors per 230.42(A), and your calculated load comes out to 399A, you may use a 400A meter socket and a 400A main circuit breaker.

 

[Strathead]

Our Utility Company states that they are rated for a calculated load of 320 amps. If your calculated load exceeds that they require us to install a CT meter.

 

[jaggedben]

Our Utility Company states that they are rated for a calculated load of 320 amps. If your calculated load exceeds that they require us to install a CT meter.

Then I might apply an 80% factor to all the non-continous loads, 100% for the continuous load, and give them that number. Or are they very clear they mean NEC calculated load?

 

[Strathead]

Then I might apply an 80% factor to all the non-continous loads, 100% for the continuous load, and give them that number. Or are they very clear they mean NEC calculated load?

When we fill out the request form they ask for the load. By definition this would be either the calculated load, or the load from a 30 day study. Telling them anything else would be false. that is all I am basing this on.

 

[kwired]

When we fill out the request form they ask for the load. By definition this would be either the calculated load, or the load from a 30 day study. Telling them anything else would be false. that is all I am basing this on.

NEC calculated load can have demand factors involved. Result is often on the conservative side unless it is a fairly specific load and not much else being served.

30 day study - question may be do they look mostly at average demand or do they look pretty hard at peak demand? How long the peak occurs can also be significant.

 

[Strathead]

NEC calculated load can have demand factors involved. Result is often on the conservative side unless it is a fairly specific load and not much else being served.

30 day study - question may be do they look mostly at average demand or do they look pretty hard at peak demand? How long the peak occurs can also be significant.

My point was they the Utility company ask for the information then require the meter accordingly. Lately every discussion has turned in to a study down to the molecular level of electrical theory. The field isn't that way.