continuous load 125% 3 hour rule (80%)

[brother]

I am just curious as what was the history of the 'continuous load' rule. How did they come up with the 125% and 3 hours for needed to add the 125%, or allowing only wiring to be at 80% loading under 3 hrs. Why not 4 hours, or 2 hours etc..?

 

[Dennis Alwon]

One can only assume that some testing was done on the breakers that determined 3 hours as the cut off for damage to the breaker.

 

[infinity]

The definition with the 3 hour parameter for a continuous load first appears in Article 100 of the 1965 NEC. In the 1962 NEC it said this under branch circuits:

210-23. Maximum Load.
(b) Other Loads. The total load shall not exceed the branch circuit rating, and shall not exceed 80 per cent of the rating where in normal operation the load will continue for long periods such as store lighting and similar loads.

The commentary in the 1962 Handbook stated:

Where the load of a branch circuit is expected to continue for long periods, such as in stores, hotel lobbies, and places of public gathering, such loads shall not exceed 80 per cent of the circuit rating. The term long periods is considered to be approximately 6 hr or more.

 

[augie47]

When I view some of the "numbers"in the NEC, that being one, I picture the CMP throwing darts at a board

 

[electrofelon]

When I view some of the "numbers"in the NEC, that being one, I picture the CMP throwing darts at a board

I always thought the three hour period to be way too long. It's pretty obvious that the maximum heat is going to build up long before 3 hours. In say a panelboard with a bunch of breakers running near 100%.

 

[jim dungar]

One can only assume that some testing was done on the breakers that determined 3 hours as the cut off for damage to the breaker.

As far as I have seen majority of the factors, like this, were chosen almost 100 years ago and have never been substantiated by actual tests.

 

[Tulsa Electrician]

I find it pretty cool it went for six to three.

 

[Tulsa Electrician]

StackPath

Not directly related to you question however pretty cool read.
Old like me however kinda cool

 

[electrofelon]

I find it pretty cool it went for six to three.

It must have been hot not cool

 

[retirede]

I find it pretty cool it went for six to three.

It was “approximately 6 or more” which is pretty much meaningless.

 

[Little Bill]

When I view some of the "numbers"in the NEC, that being one, I picture the CMP throwing darts at a board

You mean like two ground rods vs 25 ohms?

 

[brother]

The definition with the 3 hour parameter for a continuous load first appears in Article 100 of the 1965 NEC. In the 1962 NEC it said this under branch circuits:


The commentary in the 1962 Handbook stated:

The definition with the 3 hour parameter for a continuous load first appears in Article 100 of the 1965 NEC. In the 1962 NEC it said this under branch circuits:


The commentary in the 1962 Handbook stated:

Thanks or the history. So 6 hrs use to be the rule, wow. I wonder were they seeing people overload circuits back then and this caused issues. So it may have prompted the change to ensure the minimum safety. Sometimes I wonder did they have water heaters and electric space heating equipment have those issues back then as well. Even though most water heaters have a thermostat and it unlikely it would run past the 3 hours at max load, we have to treat it as a continuous load. I suppose if someone did run hot water too frequently with a large family, then i suppose it is possible to have it heating trying to keep up for more than 3 hours.

 

[electrofelon]

I think that there should be no 125% continuous load adder and it should be a design issue.

 

[Tulsa Electrician]

Three, six, nine whatever.
How about just a long time.
Pre 62

 

[Tulsa Electrician]

I think that there should be no 125% continuous load adder and it should be a design issue.

Back in the day I would think most work was done by design.
Now days it seems like the installer is also the designer.

My have things changed over the years.

 

[Strathead]

I read through and thought I would see this theory before this. A standard breaker tolerance is 20%. The 125% rule is waived when the breaker is a 100% rated breaker. I posit that the 125% rule is to prevent a breaker from tripping if it is built on the low end of the allowed tolerance. Nothing more.