Derate

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bennie

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When de rating the ampacity of a No.#10 to one half, due to conduit fill, is the over current device sized to the de rated ampacity or to the ampacity as indicated on 310.16 tables?

When increasing the wire size, to compensate for continuous load, do you increase the over current device for the increased ampacity of the wire size?
 
Re: Derate

If, by adjustment factors of 310.15(B)(2), the Ampacity of the conductor is between two of the standard OCPD of 240.6, the conductor may be protected by the next size larger where it meets the requirements of 240.4.

For a 10 AWG of the 75?C range of T310.16 which is derated by 50%, 35A * 50% = 17.5 Amps. The conductor may be protected by a 20A OCPD but, the load on the conductor cannot exceed 17.5 Amps.

For a 10 AWG of the 90?C range of T310.16 which is dreated by 50%, 40A * 50% = 20 Amps. The conductor OCPD cannot exceed 20Amps and the load cannot exceed 20 Amps.

Be sure to observe 240.4(D) as noted at the bottom of T310.16 for 14 AWG, 12 AWG and 10 AWG.

The load of a conductor with adjusted ampacity, after applying any adjustment factors of 310.15(B)(2), cannot exceed the adjusted ampacity of the conductor.
See 240.4(E) and (G)for specific applications.

Continous load should not affect conductor ampacity from the ampacity tables or after the Adjusted ampacity.
See 210.19(A) for conductor ampacity and 210.20(A)for OCPD.
 
Re: Derate

While the word "derating" is most commonly used, the correct word is "adjustment". If you look at 310.15(B) you will find that the conductor ampacity is the Table value as modified by 310.15(B)(1) thru (6). The numbers shown in the Tables are the ampacities only under the conditions specified. If the conditions are different, the ampacity is adjusted per (1) thru (6). The overcurrent device must be based on the adjusted ampacity as that is the actual ampacity of the wire under the installation conditions.
Don
 
Re: Derate

Bennie, I'd bet the farm that you already know this, so I'll let you see if I do.
Yes to the first part of the first question, second part is moot.
No to the second question.
 
Re: Derate

Is everyone stating; that when No#10 is required for a 15 amp continuous load, due to conduit fill. Do you put a 20 amp or 30 amp breaker on the No.#10?
 
Re: Derate

240.4 says conductors "shall be protected against overcurrent in accordance with their ampacities specified in 310.15"

310.15 tells us to use the tables to determine conductor ratings but Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a).

So after using Table 310.15(B)(2)(a) the conductor rating is reduced and you will have to use a smaller OCPD.

In real life the load is the load so we have to run larger conductors in order to provide the required current, the breaker size remains the same.

If you are feeding 20 amp duplex outlets from a raceway with 12 current carrying conductors you will may need 8 awg to do so, but you certainly can not put a 40 amp breaker as the OCPD for the 20 amp duplex outlet.
 
Re: Derate

Not sure when Bennie 'drops the other shoe', but when he does, it must be about the wording of 210.19(A)(1) and 210.20(A).

It is curious that he specifically address's 15A continous load and a specific maximum conductor ampacity when adjusment factors are involved.

Drop the other shoe, Bennie.

gwz2

[ October 18, 2003, 08:55 PM: Message edited by: gwz2 ]
 
Re: Derate

Bennie,
If you have 40 current carrying #10s in a conduit the ampacity of the #10 is now 16 amps. 16 amps is the maximum permitted load and if the 10s do not serve multiple receptacles the maximum code permitted OCPD is 20 amps. If the raceway is in a 36?C ambient the ampacity drops to 14.56 amps.
Don
 
Re: Derate

For me multiwire is often the difference between 84 current carrying conductors or 42 current carrying conductors. :)

It makes a big difference in conductor and raceway sizes. ;)
 
Re: Derate

I don't believe you can go to the next size fuse for branch circuits unless you meet all three conditions under 240.4(B) 1 throught 3
 
Re: Derate

IMO 240.4(B) has a lot of uses.

Say you are running a branch circuit to a fixed load that needs 54 amps of conductor and OCP minimum.

You could run this in 6 awg NM and still put a 60 amp OCPD. :)

If it was not for 240.4(B) you would have to run 4 awg for this load. :(
 
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