continuous load

[Twoskinsoneman]

What is the idea behind a continous load needing to be derated? Is the NEC really stating that 20amps on a 12awg CU THWN wire for prolonged periods is unsafe? Who knows who will plug what into a receptacle and leave it on all the time! I know at work the two front desk girl each have a little space heater plugged in on the same circuit. Between the two heaters the circuit is pretty well maxed. Is this unsafe? Why place an OCPD in the circuit that only protects intermitent loads? If the wiring can't handle the constant load the rules should demand a bigger wire...

 

[360Youth]

Precautionary is the best reason I can come up with. For example, if you were wiring the office where you work and you knew that the secretaries would be pluging in heaters pushing the load limits, would you separate into different circuits even though one was substantial? My guess is you would unless cost was a major factor. Continuous loads take their toll over time, not just on the wire, but the breaker and devices as well. Yes many items last a long time, but I am guessing there is enough evidence to warrant such rules.

 

[bob]

What is the idea behind a continuous load needing to be derated? Is the NEC really stating that 20amps on a 12awg CU THWN wire for prolonged periods is unsafe? Why place an OCPD in the circuit that only protects intermittent loads? If the wiring can't handle the constant load the rules should demand a bigger wire...

Its not the wire that can not handle the load, its the 20 amp breaker that can
not handle the load. Actually #12 thhn can handle 30 amps. The breaker is rated at 20 amps non-continuous load.

 

[Twoskinsoneman]

Precautionary is the best reason I can come up with. For example, if you were wiring the office where you work and you knew that the secretaries would be pluging in heaters pushing the load limits, would you separate into different circuits even though one was substantial? My guess is you would unless cost was a major factor. Continuous loads take their toll over time, not just on the wire, but the breaker and devices as well. Yes many items last a long time, but I am guessing there is enough evidence to warrant such rules.

Would YOU wire a commercial application guessing what two secretaries MIGHT do 10 years after you wire the building?

 

[benaround]

The idea is that we must be able to supply power to branch circuits without

overheating the conductors. By following the 125% x the continious load to

figure wire size, overheating will be averted.

2005 NEC 210.19

 

[Twoskinsoneman]

The idea is that we must be able to supply power to branch circuits without

overheating the conductors. By following the 125% x the continious load to

figure wire size, overheating will be averted.

2005 NEC 210.19

That's cool and all...is anyone actually saying outright that a 12awg THWN CU wire and 20amp OCPD CANNOT sustain a constant 20 amp load.
And futher more the NEC does NOT protect against VERY realistic possiblity that a circuit could be loaded near max and fail causing a hazard?

 

[Dennis Alwon]

That's cool and all...is anyone actually saying outright that a 12awg THWN CU wire and 20amp OCPD CANNOT sustain a constant 20 amp load.
And futher more the NEC does NOT protect against VERY realistic possiblity that a circuit could be loaded near max and fail causing a hazard?

I suspect the heat would be so great that failure of the breaker would be inevitable. If the breaker is only designed for 80% continuous load then I would think they are saying it CANNOT sustain a constant 20 amp load. The breaker, mind you, not the wire.

 

[infinity]

I think that if there were a single breaker in an enclosure the 20 amp CB could carry 20 amps indefinitely. It when you start to add the heat from the surrounding CB's that you have a problem thus requiring the 80%. There is equipment rated for 100 % which has been designed with the heat factor in mind.

 

[winnie]

I am pretty certain that the reason for '80% continuous loading' is breaker rather than conductor limitations.

Many (if not most) of the situations that call for 80% loading (or 125% oversizing) have an exception that permits 100% loading if the _breaker_ and _enclosure_ is rated for such use.

-Jon

 

[peter d]

A 20 amp breaker can sustain a 20 amp load without tripping indefinitely, or it may trip with "x" amount of time. Circuit breakers just aren't calibrated perfectly enough to trip the instant they get loaded to their nominal amperage.

 

[Twoskinsoneman]

I am pretty certain that the reason for '80% continuous loading' is breaker rather than conductor limitations.

Many (if not most) of the situations that call for 80% loading (or 125% oversizing) have an exception that permits 100% loading if the _breaker_ and _enclosure_ is rated for such use.

-Jon

Help me put this all together with some code that allows continuous loads if the CB permits.

 

[tallguy]

A 20 amp breaker can sustain a 20 amp load without tripping indefinitely, or it may trip with "x" amount of time. Circuit breakers just aren't calibrated perfectly enough to trip the instant they get loaded to their nominal amperage.

Take a peek at the trip curves of a MCCB, and note that at the low end of the curve (in terms of multiples of rated current), the allowable trip range goes from 1.0 to 1.25 -- Seems like good enough reason to me to enforce the 125% rule.

See Square D QO here (starting page 19)

 

[tallguy]

Help me put this all together with some code that allows continuous loads if the CB permits.

210.19(A)(1) Exception #1
Ditto for 210.20 and 215.2

 

[Twoskinsoneman]

Its not the wire that can not handle the load, its the 20 amp breaker that can
not handle the load. Actually #12 thhn can handle 30 amps. The breaker is rated at 20 amps non-continuous load.

I still don't get it. If the OCPD is the issue why does it say the conductors need to be sized accordingly plus 125 percent?

210.19(A)(1)
(1) General Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125
percent of the continuous load.

 

[tallguy]

I still don't get it. If the OCPD is the issue why does it say the conductors need to be sized accordingly plus 125 percent?

If the OCPD might not trip (at all -- i.e. for an indefinite period of time) until it reaches 125% of its nominal trip value, then the conductor needs to be able to do the same.

 

[360Youth]

Would YOU wire a commercial application guessing what two secretaries MIGHT do 10 years after you wire the building?

I'm sorry you missed my point.

... if you were wiring the office where you work and you knew that the secretaries would be pluging in heaters pushing the load limits,...

The point is, if you knew there would be said heaters plugged in then I am sure you would accomodate for it the same as NEC is trying to accomodate for continuos loads wearing on each piece of equipment in the equation.

 

[raider1]

I still don't get it. If the OCPD is the issue why does it say the conductors need to be sized accordingly plus 125 percent?

That is a very good point. I have been wondering that same point ever since I saw the new exception that was added to 210.19(A) that allows a grounded conductor to be sized at 100% of the non-continious and continious loads.

If we are trying to keep the continious loading on a circuit to less 80% of the breakers ampacity then we already have provisions for that in 210.20(A), and 215.3.

Chris

 

[winnie]

As I understand things, the breaker gets upsized to protect the breaker...and then the conductor gets upsized so that it is still properly protected by the upsized breaker.

-Jon

 

[don_resqcapt19]

The breaker is actually designed and tested for 100% load at a 40?C ambient, but in free air, not in an enclosure. The reason for the maximum of 80% rating for continuous loads is to prevent nuisance trips when the breaker is in an enclosure and subjected to heat for the other breakers within that enclosure. The oversize wire that is required by the 125% rule acts as a heat sink to limit the heat build-up.
Don

 

[benaround]

Breakers protect conductors, the 125% added for the continious load has

no common turf with the 80% of the breaker thing, it's not added to see if

a certian breaker will handle 80%, it's added to detemine what size conductor

is needed.

Say you have a 1000w cont. load, @ 125% is 1250w = 10.4 amps, just

treat it like you would any other 10.4 amp circuit.