AF vs AT

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loosegoose22

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I think I went through all posts regarding ampere frame(AF) vs ampere trip(AT) call outs for breakers on plans. I am still a bit confused on conductor sizing based on these. Plans say 250AF/100AT, am I to take this as meaning these are adjustable trip breakers? And if so do I base conductor sizing on 240.6(B) the conductor would be sized for 250A or can I size conductor for anywhere between a 100-250A setting? Also, can anyone send a link that fully explains this? Thanks in advance
 
I believe you base conductor size on AT rating. The AT rating is the amps the breaker trips. The AF rating has to do with how many amps the breaker can handle before it will explode.
 
The "AT" rating has nothing to do with conductor sizing, unless you want to someday increase the size of the circuit to whatever is the maximum trip rating (AT) available in that frame size (AF).

So if a spec says 250AF - 150AT, that means that they want a 250A FRAME SIZE, with a 150A trip element. SOMEDAY, you can theoretically swap out that breaker for one with a 200 or 250A trip element, and the 250AF means that it will be the same physical size so it will fit in the switchboard or panelboard. If you WANT to be able to do that, they you would want to plan ahead and put in over sized conductors now so that in the future, you don't have to re-pull. But if nobody needs to up-size the circuit, you would just size your conductors based on the load, then size the breaker based on the conductors.
 
So I guess what you're saying in my scenario of 250AF/100AT as the drawings indicate, this means at this time they expect the load to be max. 100A with min. #3cu conductors (75*) so the adjustment on the trip cannot exceed 100A. Does that sound right? Wouldn't there be a max of 250A on the trip adjustment or does that vary by MFG? Am I making sense here? Thanks again
 
loosegoose22 said:
So I guess what you're saying in my scenario of 250AF/100AT as the drawings indicate, this means at this time they expect the load to be max. 100A with min. #3cu conductors (75*) so the adjustment on the trip cannot exceed 100A. Does that sound right? Wouldn't there be a max of 250A on the trip adjustment or does that vary by MFG? Am I making sense here? Thanks again
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Probally not, depends on the breaker type but you would need to swap trip units or rating plugs in the future to get up to the 250A.
 
I guess what I'm saying is--on adjustable trip breakers would the one in my example adjust up to the 250A max? Or is the only significance of the 250AF just for sizing of the space in the switchgear for future use?
 
I often use the AF/AT notation with spare conduits so the feeder can be increased. For example. a 1200 amp service might have a 1600 AT / 1200 AF breaker, and 3 sets of 600 KCM wire in 4 conduits. That way the 4th set of 600 KCM wire doesn't have to be installed right now.

Another reason I might spec. this would be for breaker coordination. I might spec a 600 A frame for a 100 A breaker so I get the electronic trip settings.
 
steve66, Is there a typo in your post ( 1600 A/T 1200 A/F ) I'm learning here also and

that just messed me up. I'm thinking 1600 A/F 1200 A/T ?
 
My understanding is this:
Example: 250 AF breaker with a 100 amp trip unit, you would size the conductors at 100 amps. If you have a different 250 AF breaker with an adjustable trip rating (no replaceable trip unit), conductors must be sized according to the maximum possible current setting as the breaker is currently configured. Meaning if the adjustment allows up to 250 amps, even though it will be set at 100 amps, the conductors must be sized at 250.​

I hope that makes sense?
 
sii said:
My understanding is this:
Example: 250 AF breaker with a 100 amp trip unit, you would size the conductors at 100 amps. If you have a different 250 AF breaker with an adjustable trip rating (no replaceable trip unit), conductors must be sized according to the maximum possible current setting as the breaker is currently configured. Meaning if the adjustment allows up to 250 amps, even though it will be set at 100 amps, the conductors must be sized at 250.​

I hope that makes sense?
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Ok that's a relief!!!!!!!!! I think I might be doing things right. For a change.
 
Kinda sounds like an over kill sizing having a 250AT w/adjustable trip to size to conductors at 250A and dial down trip to 100A or is that what 240.6(B) and 240.6(C) really telling us?
 
loosegoose22 said:
Kinda sounds like an over kill sizing having a 250AT w/adjustable trip to size to conductors at 250A and dial down trip to 100A or is that what 240.6(B) and 240.6(C) really telling us?
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240.6 c) tells us that if the adjustable trip breaker is in a "restricted access" environment (locked/bolted enclosure or room), then you can go ahead and base the conductor sizing on the 100A load and set the breaker accordingly.
 
Jraef,

Read the link, very good explaination for adjustible trip units.Thanks
I could really see where 240.6(C) would apply in many cases where adjustable trips are installed.
 
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