Safe percentage utilized of 1200 amp service disconect.

Rivers

Member
Location
Tampa, FL
Occupation
Electrician/Maintenance Manager
I have always followed the rule of not loading a circuit or main breaker more then 80%.

Is there a code article to back this up?

Thank you
 

bwat

EE
Location
NC
Occupation
EE
Yep. It's a little more complicated than just "don't overload by more than 80%", but that concept is found is several places. A lot more circuit dependent, but without going into the weeds, that idea is there.

210.20(A) for example:

210.20 Overcurrent Protection (A) Continuous and Noncontinuous Loads. Where a branch
circuit supplies continuous loads or any combination of continuous
and noncontinuous loads, the rating of the overcurrent
device shall not be less than the noncontinuous load plus
125 percent of the continuous load.

The breaker being sized by 125% over the continuous load is the same as not loading the circuit with continuous loads beyond 80% of the breaker rating. 5/4 (125%) is inverse of 4/5 (80%).

A 100A CB loaded at 80% capacity means 80A of continuous loads. Similarly, if you take a 80A continuous load, and then multiply by 1.25 (125%) you get 100A for a CB size.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Basically the same as stated above.... A breaker can be loaded 100% if the breaker is rated for 100% otherwise you are dealing with continuous loads and non-continuous loads. For example, if all your loads are not continuous and you have a 200 amp overcurrent protective device and the calculated load is 200 amps then you are good to go. If the load were continuous or both continuous and non-continuous then you would have to deal with that

230.42 Minimum Size and Rating.
(A) General. Service-entrance conductors shall have an
ampacity of not less than the maximum load to be served.
Conductors shall be sized to carry 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 310.15. The maximum allowable
current of busways shall be that value for which the busway has
been listed or labeled.
(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 allowable 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.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm not sure what this means. Don't you simply perform a load calculation and that tells you the service size? If you want to add some room for future expansion you just add that percentage to the load calculation.
 

Rivers

Member
Location
Tampa, FL
Occupation
Electrician/Maintenance Manager
I took over a new building from the construction company. We have a 1200 amp service with a calculated load of 1130. I opened one of the service disconnects today to confirm the fuse size, I have 10 service entrance panels surrounding this 240 unit student housing building feeding various meter rooms and house panels. The panel is question, A phase has started to burn up! Probably a loose connection on the lug, beveled washer, the workmanship here in Tampa is ridiculous! I don't think the inspector even got out of his car!!

Anyway I am trying to reinforce my work ethics of not loading more then 80% or sizing up 125% of the calculated load. These are individual dwelling units, one, two and four bedroom units with full appliances, HVAC, water heaters etc...
 

kwired

Electron manager
Location
NE Nebraska
I took over a new building from the construction company. We have a 1200 amp service with a calculated load of 1130. I opened one of the service disconnects today to confirm the fuse size, I have 10 service entrance panels surrounding this 240 unit student housing building feeding various meter rooms and house panels. The panel is question, A phase has started to burn up! Probably a loose connection on the lug, beveled washer, the workmanship here in Tampa is ridiculous! I don't think the inspector even got out of his car!!

Anyway I am trying to reinforce my work ethics of not loading more then 80% or sizing up 125% of the calculated load. These are individual dwelling units, one, two and four bedroom units with full appliances, HVAC, water heaters etc...
That calculated load likely has demand factors included in calculations. Actual connected load may very well be over your 1200 amp service, but is likely you seldom or never see 1130 either, presuming those calculations were done correctly.

There is no continuous/non continuous load to deal with when you start getting into using demand factors in the calculations, the total calculation is minimum conductor ampacity and minimum overcurrent protection needed in the application.

1200 amp can still be supplied via panelboards, if you need more than that you need multiple mains (six disconnect rule comes into play) or possibly switchboard if you want a single self contained service gear setup.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I have always followed the rule of not loading a circuit or main breaker more then 80%.

Is there a code article to back this up?

Thank you
Not really unless you need to address continuous loads as mentioned above.
Your concern is understandable but you have to remember the NEC is MINIMUM. A calculated load of 1130 amps on a 1200 amp service meets NEC requirements. Taking future loads account your 80% "rule" is likely wise but not required.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Most of the time the loads calculated based on Article 220 are very much inflated. Many dwelling units draw less than 50% of the NEC calculated load.
 

Rivers

Member
Location
Tampa, FL
Occupation
Electrician/Maintenance Manager
I'm not sure what this means. Don't you simply perform a load calculation and that tells you the service size? If you want to add some room for future expansion you just add that percentage to the load calculation.
Read the thread!
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Read the thread!
I actually did read the thread and I would ask you to read it again because you've missed the point. The point being that the NEC load calculations are very generous with the actual load at any given point in time being nowhere near the calculated load. If your 1200 amp service has a calculated load of 1130 amps there is nothing to discuss unless you are planning for future expansion.
 

Rivers

Member
Location
Tampa, FL
Occupation
Electrician/Maintenance Manager
I actually did read the thread and I would ask you to read it again because you've missed the point. The point being that the NEC load calculations are very generous with the actual load at any given point in time being nowhere near the calculated load. If your 1200 amp service has a calculated load of 1130 amps there is nothing to discuss unless you are planning for future expansion.

Thank you, the damage on the lug is most likely a loose connection. When I submitted this to the electrical contractor that did the job, he asked if there were any unintended loads on this service. I have always worked a 80% rule or sized 125% of the calculated load. This would have meant a 1600 amp service. This is why I questioned if there were concrete articles in NEC. Thank you again.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Thank you, the damage on the lug is most likely a loose connection. When I submitted this to the electrical contractor that did the job, he asked if there were any unintended loads on this service. I have always worked a 80% rule or sized 125% of the calculated load. This would have meant a 1600 amp service. This is why I questioned if there were concrete articles in NEC. Thank you again.
You're welcome. Just realize that your 80/125% factors may be good design parameters they are not the required code minimum. It's hard to argue with the contractor who installed a code compliant 1200 amp service that he did something wrong.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I like to have a good margin in the motor control panels I design. I usually try to max out whatever frame size main breaker I end up with. It typically does not cost any more for me to have a 1200 Amp breaker even if i could get by with a 1000 A breaker. And you never know what a customer might decide needs to be added at the last minute.
 
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