Originally Posted By: mpinkley
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what do 20 and 30 amp breakers trip at?
what do 20 and 30 amp breakers trip at?
Originally Posted By: bbadger
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This is not a simple answer, the time and the load both figure into the answer.
A 20 amp breaker could carry 25 amps indefinitely and still pass a UL test.
The same breaker could also carry 100 amps for a few seconds.
If you really need the info you need to get it from the manufacture, many times this info is available on the companies web sites.
What you would be looking for is the Time-Current Characteristics.
I found one for a Dual-Element, Time-Delay 15 amp type S fuse.
It shows the 15 amp fuse will carry 25 amps for at least 100 seconds or forever.
It will carry 80 amps for one second.
The same 15 amp fuse will carry 250 amps for .01 seconds.
Follow this link to see the chart on a pdf.
http://www.bussmann.com/library/bifs/1032.pdf
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Bob Badger
Electrical Construction & Maintenance
Moderator at ECN
Originally Posted By: bbadger
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Here is the chart for the type ‘s’ dual element fuse.
Sorry it is so ruff from converting it to a jpeg.
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Bob Badger
Electrical Construction & Maintenance
Moderator at ECN
Originally Posted By: bking
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Interesting info-
I seem to remember from reading a prev. NEC book, that 14 gauge wire is rated for 20 amps but for purposes of the code it is 15 amps.
same with 12 gauge, good for 25 amps and limited to 20 amps by code.
Originally Posted By: phinsperger
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Also try going to this link and then go to section on trip units
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Originally Posted By: bbadger
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| bking wrote: |
| Interesting info-
I seem to remember from reading a prev. NEC book, that 14 gauge wire is rated for 20 amps but for purposes of the code it is 15 amps. same with 12 gauge, good for 25 amps and limited to 20 amps by code. |
That is correct.
Per Table 310.16
14 AWG is rated 20 to 25 amps.
12 AWG is rated 25 to 30 amps.
10 AWG is rated 30 to 40 amps.
The range comes from the different temperature ratings of the insulation and terminals.
That said, for most circuits 240.4(D) applies requiring 14, 12, & 10 AWG to be protected at 15, 20 & 30 amps respectively.
Exceptions to this that you may see are HVAC units and motors.
Bob
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Bob Badger
Electrical Construction & Maintenance
Moderator at ECN
Originally Posted By: bking
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Would you write up an outdoor heat pump unit on a 40 amp breaker with 10 gauge wire ?
I did the other day but it also had no ground either.
Originally Posted By: bbadger
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A 40 amp breaker feeding an HVAC unit wired with 10 AWG could very well be fine.
It is all in the units labeling.
If the tag shows;
Minimum circuit ampacity 30 amps. (Or Less)
Maximum overcurrent protection 40 (Or more)
It would be fine.
HVAC units are different then most circuits, as a rule of thumb you can wire for the minimum and protect at the maximum.
You might have an HVAC unit that has a minimum circuit ampacity of 20 amps with a maximum overcurrent protection of 40 amps.
I could wire that with 14 AWG ( 310.16 & 240.4(G) ) from a 40 amp breaker or fuse.
Bob
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Bob Badger
Electrical Construction & Maintenance
Moderator at ECN