240.6(c) and the accuracy and precision of adjustable-trip breakers

[JuliaTruchsess]

Say I have a load I know is going to be 278A max. After temperature and fill derating, I find that 4/0 can handle it, and 240.6(c) will let me use it with a 400A breaker if the breaker is restricted-access and "dialed-in" to 278A. But really, is it possible to get that fine an adjustment on an electronic-trip breaker with a single-turn screwdriver adjustment, especially given the inherent tolerances of trip curves? If I go to an easy-to-find spot on the dial like 300A (75%) then I have to use 250MCM. I like the logic behind 240.6(c) but it doesn't seem all that practical in real life.

 

[petersonra]

It doesn't work the way that I think you think it does. There are two different issues. The first issue is the ampacity of the conductors. You have to figure out what your actual load is and sometimes you have to add 25% to it or to at least a portion of it. This tells you what the minimum conductor ampacity will be, which tells you what size wire to use.

The second issue is what rating of overcurrent protection is required for those conductors. Up to eight hundred amps of ampacity you are allowed to go to the next standard size of overcurrent protection to protect the conductors.

 

[JuliaTruchsess]

It doesn't work the way that I think you think it does. There are two different issues. The first issue is the ampacity of the conductors. You have to figure out what your actual load is

My actual load is 278A, as stated.

and sometimes you have to add 25% to it or to at least a portion of it. This tells you what the minimum conductor ampacity will be, which tells you what size wire to use.

The 125% test is to ensure there's enough metal attached to the termination to keep it at or below its rated temperature, and it's a separate issue from conductor ampacity.

The second issue is what rating of overcurrent protection is required for those conductors. Up to eight hundred amps of ampacity you are allowed to go to the next standard size of overcurrent protection to protect the conductors.

Suggest you read 240.6(c)...

 

[packersparky]

Say I have a load I know is going to be 278A max. After temperature and fill derating, I find that 4/0 can handle it, and 240.6(c) will let me use it with a 400A breaker if the breaker is restricted-access and "dialed-in" to 278A. But really, is it possible to get that fine an adjustment on an electronic-trip breaker with a single-turn screwdriver adjustment, especially given the inherent tolerances of trip curves? If I go to an easy-to-find spot on the dial like 300A (75%) then I have to use 250MCM. I like the logic behind 240.6(c) but it doesn't seem all that practical in real life.

4/0 copper is only rated for 230 amperes at 75 degree C . You would need at least 300 kcmil copper or 500 kcmil aluminum for a 278 ampere load.

 

[JuliaTruchsess]

4/0 copper is only rated for 230 amperes at 75 degree C . You would need at least 300 kcmil copper or 500 kcmil aluminum for a 278 ampere load.

I'm running two of them in parallel :roll:

My question was about breaker trip settings, not about how much current 4/0 can handle.

 

[augie47]

I don't see a lot of adjustable trips but my "real life" experience is the same as yours. Dialing to a definite spot on the breaker seems to be the best you can do.

 

[petersonra]

My actual load is 278A, as stated.

Ok.

The 125% test is to ensure there's enough metal attached to the termination to keep it at or below its rated temperature, and it's a separate issue from conductor ampacity.

There is no 125% "test". There is a requirement that for some loads or portion thereof that the ampacity of the conductors be increased by 25% over the calculated load.

Suggest you read 240.6(c)...

This would appear to allow one to use an adjustable trip CB as being rated at whatever the trip setting is set at. It is a permission. not a requirement.

 

[steve66]

If you have an electronic breaker, its not usually a turn dial - its usually small rotary switches.

So if you have a 300A breaker, you will probably get your choice of 0.8 * 300A = 240A , or 0.9 * 300A = 270A, or 1.0* 300A = 300A.

Some might have a .85 or a .95 setting, but you probably won't get to set it at 378 amps.

More typical would be to just size the wire for 300A, and use a 300A breaker. Especially on anything that small. Do you really need an electronic trip breaker? That will probably add a lot of expense compared to a basic thermal magnetic breaker without adjustable settings.

 

[drktmplr12]

There is no 125% "test". There is a requirement that for some loads or portion thereof that the ampacity of the conductors be increased by 25% over the calculated load.

motor circuit, for example.

 

[kingpb]

Say I have a load I know is going to be 278A max. After temperature and fill derating, I find that 4/0 can handle it, and 240.6(c) will let me use it with a 400A breaker if the breaker is restricted-access and "dialed-in" to 278A. But really, is it possible to get that fine an adjustment on an electronic-trip breaker with a single-turn screwdriver adjustment, especially given the inherent tolerances of trip curves? If I go to an easy-to-find spot on the dial like 300A (75%) then I have to use 250MCM. I like the logic behind 240.6(c) but it doesn't seem all that practical in real life.

I think you will find that the dial adjustments have discrete set points; i.e. they are not continuously adjustable. Thus you will be restricted to the multiple/percentage of current selected. The curve selected will be within the TCC band tolerances as provided by the manufacturer.

 

[electrofelon]

So they way I read 240.6(C) is that I can use that and still use 240.4(B) (next size up). Does everyone agree on that? For example, the OP could set the setting at 300A (assuming 278A conductors).

 

[packersparky]

So they way I read 240.6(C) is that I can use that and still use 240.4(B) (next size up). Does everyone agree on that? For example, the OP could set the setting at 300A (assuming 278A conductors).

Agreed. Additionally, the OP stated that parallel 4/0 conductors are being used. No need to use 240.4(B) in that case. I don't understand the need for an adjustable trip circuit breaker in this instance.

 

[electrofelon]

The 125% test is to ensure there's enough metal attached to the termination to keep it at or below its rated temperature, and it's a separate issue from conductor ampacity.

Perhaps you are thinking of the fact that terminations are (usually/mostly) rated for 75 degrees and conductors 90 degrees, thus the terminations are the "weakest link"? (110.14(C))

 

[topgone]

Say I have a load I know is going to be 278A max. After temperature and fill derating, I find that 4/0 can handle it, and 240.6(c) will let me use it with a 400A breaker if the breaker is restricted-access and "dialed-in" to 278A. But really, is it possible to get that fine an adjustment on an electronic-trip breaker with a single-turn screwdriver adjustment, especially given the inherent tolerances of trip curves? If I go to an easy-to-find spot on the dial like 300A (75%) then I have to use 250MCM. I like the logic behind 240.6(c) but it doesn't seem all that practical in real life.

No. You can't possibly adjust an adjustable-trip, electronic trip breaker to 278 A. But you you could dial it up to the nearest amps, see below: It's either you choose 0.7 (280A) or 0.65 (260 A)

 

[electrofelon]

Agreed. Additionally, the OP stated that parallel 4/0 conductors are being used. No need to use 240.4(B) in that case. I don't understand the need for an adjustable trip circuit breaker in this instance.

OP did say there was temp and fill derating....

 

[packersparky]

OP did say there was temp and fill derating....

2 conductors per phase x 3 phases= 6 current carrying conductors. (2) 4/0 conductors per phase = 520 amperes.
520 x .80 = 418 amperes per paralleled run if all in same raceway.

IMO. No issues unless there is an ambient temperature correction, but we were not given any specifics.

 

[electrofelon]

2 conductors per phase x 3 phases= 6 current carrying conductors. (2) 4/0 conductors per phase = 520 amperes.
520 x .80 = 418 amperes per paralleled run if all in same raceway.

IMO. No issues unless there is an ambient temperature correction, but we were not given any specifics.

Yeah I agree it does seem fishy....maybe the neutral is a CCC making 8 total and it runs thru a sauna or something Or more likely perhaps there are other circuits in the raceway so a lot of CCC....

 

[JoeStillman]

Why dial it down at all? You've got over 400A of ampacity with the parallel 4/0's.

 

[kingpb]

Yeah I agree it does seem fishy....maybe the neutral is a CCC making 8 total and it runs thru a sauna or something Or more likely perhaps there are other circuits in the raceway so a lot of CCC....

More specifics weren't given because she wasn't asking a question about conductors; only about trip settings. The discussion about conductor size, type and rating is inconsequential to the OP.

 

[JuliaTruchsess]

Thanks to all who've replied! I've come to the same conclusion first mentioned by [MENTION=72722]electrofelon[/MENTION] - set the adjustable trip to the next higher standard breaker size; there's no need to dial it exactly to the load current as I was initially thinking, since the NEC always considers "one-size-up" to be adequate protection.

Here's a more complete example:

• 150A continuous load with 16 current-carrying conductors in the raceway, fill derating = 50%.
• 125% of 150A is 187A so I need a 200A breaker, if 80%-rated.
• 300MCM is required to carry the effective 300A derated load.
• Termination heatsinking of 187A is fulfilled for 75C terminations by 300MCM, since its ampacity at 75C is 240A.
• The derated ampacity of 90C 300MCM is 160A. NEC requires that the conductor be protected by the next standard size up, which is 175A.

Therefore the adjustable trip needs to be set at 175A to protect the conductor. If not using an adjustable trip breaker, you'd have to increase the conductor size, because a 200A breaker is not the next size up from 160A.