Derating

[Sirdon61]

I’m trying to find the calculation for a pipe fill situation by using annex B It’s all switch legs. With there own neutral
I have 40 -10’s in a 2” pipe. With using the derating table I came up with 16amp. If my largest load is only 350 watt I should be able to put it on a 20 amp breaker or not?

 

[bwat]

What are you supplying power to? Not supplying receptacles, right? For example, 240.4(B) lists next size up as ok as long as

"The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug connected portable loads"

..plus the other requirements listed there.

 

[Sirdon61]

What are you supplying power to? Not supplying receptacles, right? For example, 240.4(B) lists next size up as ok as long as

"The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug connected portable loads"

..plus the other requirements listed there.

Lights only. Switch legs

 

[Dennis Alwon]

Why not just put it on a 15 amp overcurrent protective device but to answer the question I guess you could upsize the breaker

 

[infinity]

I'm curious why use Annex B?

 

[winnie]

I forget if the requirements for derating include a permission for simultaneous loading.

The OP has a pile of switch legs in a single conduit. The maximum load on a switch leg is 350W (no spec as to voltage, but assume 120V and 3A). How many of those switch legs are supplied by the _same_ OCPD?

If you have 5 switch legs(and thus 10 separate conductors) on a single circuit/OCPD then in my _opinion_ _for purpose of derating_ that should 'count' as 2 current carrying conductors. My reasoning is that in no case will you ever have the full circuit current flowing on all of the conductors at the same time; in the worst case for heat production all of the current would flow on a single switch leg.

However I don't remember if this is actually support by the code.

-Jon

 

[reast]

Winnie, that is an interesting idea. I was looking through and I couldn't find much to support it but here's what I have:
(2017)

310.15 B Informational Note: "Allowable ampacities result from consideration of one or more of the following ... (2) Coordination with circuit and system overcurrent protection"
This seems to say that if there is coordination (and I'm not completely sure what that is) with the overcurrent protection, that the ampacity adjustment can be changed. Informational notes are not code however...

310.15 B 3 a informational note: "See Annex B for adjustment factors for more than 3 current carrying conductors in a raceway or cable with load diversity"
This seems to indicate that there is some provision for altering the calculations for load diversity. Once again, informational notes are not code.

310.15 B 2 Ambient temperature correction factors: "Ampacities for ambient temperatures other than those shown on the ampacity tables...shall be permitted to be calculated using the following equation..."
This seems to indicate that there is an option for an alternate calculation, but only for temperatures outside what the chart has (extreme) and the equation has no provision for the calculated load, it only considers the ampacity of the wire. So no help there.

Annex B.3 Criteria modifications "Where values of load factor and Rho are known...and they are different from...a specific table...the ampacities...can be modified by...the use of figure 310.15 B 2 a
Which points back to our equation that has no provision for load, only the ampacity of the wire. Also it mentions duct banks, which we are not discussing.

In conclusion, it looks like the code is hinting at something, but it doesn't come out and give you permission to use the logic you are using. Although I would say that unless there is some sort of interlock that would 100% prevent all loads from operating, you have to consider all of them as carrying current.

But to answer the OP's question. Yes, you can put it on a 20A breaker