Loading a Main Breaker

[jeepinrob]

I know you can only load a branch circuit breaker 80%, but can I load a Main breaker/F.S.S. 100%

Thanks

 

[wirenut1980]

Re: Loading a Main Breaker

Don't have a code book with me, but I think 408.36 applies in the NEC

 

[petersonra]

Re: Loading a Main Breaker

Isn't the 80% number mostly for calculations and figuring requirements?

After its installed nothing prevents you from using the full capacity of the circuit.

 

[ira]

Re: Loading a Main Breaker

Breaker loading is limited to 80% unless it is listed as a 100% rated breaker. This is due to the heating at the terminals, which corresponds to the 80% continuous load limit for breakers.
For equipment rated less than 100 Amperes wire sizing is based on the 60 degree C rating of table 310-16 unless the terminals are listed for a 75 degree C rating(for higher temperature).

 

[jtester]

Re: Loading a Main Breaker

I believe that the 80% applies to continuous loads. I don't think that there is anything wrong with loading a breaker to more than 80% if the load is not continuous. That would apply to a branch breaker as well as a panel main breaker.

Jim T.

 

[wirenut1980]

Re: Loading a Main Breaker

To go along with what Jim is talking about, in the 1999 NEC under 348-16(d) it talks about OCPD for continuous loads in panelboards.

Continuous Load. The total load on any overcurrent device located in a panelboard shall not exceed 80 percent of its rating where, in normal operation, the load will continue
for three hours or more. Exception: An assembly, including the overcurrent device,
shall be permitted to be used for continuous operation at 100 percent of its rating where it is listed for this purpose.

 

[charlie b]

Re: Loading a Main Breaker

Originally posted by wirenut1980: Don't have a code book with me, but I think 408.36 applies in the NEC

Sorry, but that is in the ?construction specifications? area. I don?t think article 408 will help. It just talks about the ratings of the panelboard, and about protecting the panelboard. The issue here is protecting branch circuits. That is in 210.

 

[charlie b]

Re: Loading a Main Breaker

Originally posted by jtester: I believe that the 80% applies to continuous loads. I don't think that there is anything wrong with loading a breaker to more than 80% if the load is not continuous. That would apply to a branch breaker as well as a panel main breaker.
Jim T.

I agree. 210.20(A).

 

[rr]

Re: Loading a Main Breaker

Originally posted by charlie b:

Originally posted by jtester: I believe that the 80% applies to continuous loads. I don't think that there is anything wrong with loading a breaker to more than 80% if the load is not continuous. That would apply to a branch breaker as well as a panel main breaker.
Jim T.

I agree. 210.20(A).

I agree that it is allowed by Code. But is it a good design practice? IMO, no it's not.

I use the 80% rule on all of my designs. Why? Because we have no idea what the customer load is going to be if equipment is changed, moved, or modified once the engineering has been completed. It's better to have a little capacity left on the branch circuit rather than have a tripping breaker 6 months later.

Just my 2 cents...

 

[iwire]

Re: Loading a Main Breaker

Originally posted by rr:
I use the 80% rule on all of my designs. Why? Because we have no idea what the customer load is going to be if equipment is changed, moved, or modified once the engineering has been completed.

Thats great if the customer wants to pay for that, if we are taking about adding a few more 20 amp branch circuits the cost is not a big deal.

If we are talking about a service the cost could be great and the benefit small.

Say you do you NEC load calculations and find the NEC allows a 1000 amp service. Now you apply your '80% rule' and the service jumps up to 1250 amps which is not a standard size. The next standard size is 1600 amps. Now we have a 1600 amp service where a 1000 would have been fine and the real kicker is it is highly likely the utility company will consider the load to be 500 or 600 amps.

The NEC load calcs and the real load are always far apart.

IMO good engineering looks at each installation individually, a one size fits all approach may work but it also will cost the customer money they do not want to spend at this time.

Just my opinion, Bob

 

[rr]

Re: Loading a Main Breaker

Originally posted by iwire:

Say you do you NEC load calculations and find the NEC allows a 1000 amp service. Now you apply your '80% rule' and the service jumps up to 1250 amps which is not a standard size. The next standard size is 1600 amps. Now we have a 1600 amp service where a 1000 would have been fine and the real kicker is it is highly likely the utility company will consider the load to be 500 or 600 amps.

School construction is performed this way. Practically 100% of the time, schools will expand for future relocateable classrooms, ballfield lighting and other additional equipment. The schools don't have the budget for everything on their wish list, but they sure want the capacity there for future additions.

IMO good engineering looks at each installation individually, a one size fits all approach may work but it also will cost the customer money they do not want to spend at this time.

True and it's a common problem trying to explain this to a customer. For example, if they have plans for future additions or equipment, I always try to explain to them that it's cheaper to have it done during the initial construction phase. Downtime as a result of additional/modified branch circuits, panelboards and transformers might be costly. And let's not forget about the labor of cutting, drilling, coring, patching and painting that's involved. Yikes!

 

[jtester]

Re: Loading a Main Breaker

rr

I agree completely that future growth is an important part of any design. I encourage everyone to consider it, however that is a separate point here. The correct answer to the question is that you can load a breaker to 100%. 'preciate your thoughts.

Jim T

 

[john m. caloggero]

Re: Loading a Main Breaker

It is important to mention that 110.3(B) has to enter the picture. The listed breaker is allowed to carry the full amount of current for which the breaker is rated. PROVIDED THE LOAD IS NOT A CONTINUOUS LOAD. See UL directory.