Load creep over time after sizing a feeder and protecting it via the "next size up" breaker per 240.4(B)

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Matt_from_CA

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Suppose I've been asked to approve a drawing showing a feeder (3-ph, 480 V) fed from a given panel's spare 50 A breaker. There are no details yet on what the load will be; that will come at a later point, and be subsequently approved. I only know that it will further feed a sub-panel that has yet to have been finalized. Suppose I determine the ampacity of this feeder (taking the lower of the calculated ampacity at the middle of the wire and at the terminations)(considering appropriate ambient temperature derating, etc.) to be 44 A. I therefore approve connection to / use of the proposed 50 A breaker, as allowed per 240.4(B).

I now have a situation where someone in the future can specify that downstream load (sub-panel), say with a 50 A main breaker, and there is a chance that load on that sub-panel may creep up over time to 49 A. I've now exceeded the ampacity of the upstream feeder, and I would expect unacceptable temperature rise (somewhere on that conductor).

Is it right to expect and count on future engineers to determine and check upstream feeder ampacities to see if they are impinging on this "upsize wiggle room"? Do you as an engineer do it? I believe people are often only focused on what they are immediately connecting to and what they are installing. Perhaps this is reason to challenge that approach.

Even if some load information is known, there is always the chance that small incremental load can impinge on this "upsize wiggle room".

Perhaps there are enough layers of conservatism built into the NEC requirements below 800 A that this is a non-issue.
 
Cal_Poly_SLO_2011 said:
I now have a situation where someone in the future can specify that downstream load (sub-panel), say with a 50 A main breaker, and there is a chance that load on that sub-panel may creep up over time to 49 A
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Is the load continuous or non-continuous?

Welcome to the Forum.:)
 
Cal_Poly_SLO_2011 said:
I now have a situation where someone in the future can specify that downstream load (sub-panel), say with a 50 A main breaker, and there is a chance that load on that sub-panel may creep up over time to 49 A.
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Welcome to the forum.

Is that a continuous 49 amps as defined by the NEC?
 
I think worrying about what someone might do at some point in the future is not worth the effort. Its not a code requirement and you are not a seer.

personally, I would not try to save $5 on wire costs to use a slightly higher CB rating. But that is a design choice.
 
Cal_Poly_SLO_2011 said:
Suppose I've been asked to approve a drawing showing a feeder (3-ph, 480 V) fed from a given panel's spare 50 A breaker. There are no details yet on what the load will be; that will come at a later point, and be subsequently approved. I only know that it will further feed a sub-panel that has yet to have been finalized. Suppose I determine the ampacity of this feeder (taking the lower of the calculated ampacity at the middle of the wire and at the terminations)(considering appropriate ambient temperature derating, etc.) to be 44 A. I therefore approve connection to / use of the proposed 50 A breaker, as allowed per 240.4(B).

I now have a situation where someone in the future can specify that downstream load (sub-panel), say with a 50 A main breaker, and there is a chance that load on that sub-panel may creep up over time to 49 A. I've now exceeded the ampacity of the upstream feeder, and I would expect unacceptable temperature rise (somewhere on that conductor).

Is it right to expect and count on future engineers to determine and check upstream feeder ampacities to see if they are impinging on this "upsize wiggle room"? Do you as an engineer do it? I believe people are often only focused on what they are immediately connecting to and what they are installing. Perhaps this is reason to challenge that approach.

Even if some load information is known, there is always the chance that small incremental load can impinge on this "upsize wiggle room".

Perhaps there are enough layers of conservatism built into the NEC requirements below 800 A that this is a non-issue.
Click to expand...
your cable is protected. A 50 amp thermal-magnetic breaker (80% rated) will trip after a few minutes at 40 amps. the next size up rule is based on the result of the load calculation which will already have factored the continuous and non-coninuous nature of load(s).
 
In WA we have a state rule requiring an engraved label if the next size up rule is used, stating what the conductor ampacity is,this on a service
 
drktmplr12 said:
your cable is protected. A 50 amp thermal-magnetic breaker (80% rated) will trip after a few minutes at 40 amps. the next size up rule is based on the result of the load calculation which will already have factored the continuous and non-coninuous nature of load(s).
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That is not so. It will hold forever at 50 amps.
 
To be comprehensive, can I ask for thoughts for both continuous and non-continuous load? I pondered whether the 1.25x factor for continuous load is the safety margin here, but then I was thinking that might a 4-hour 50% duty cycle load qualify as non-continuous and still exceed that temp rating if on for 2 hours straight?

Edit: and thanks for the welcome!
 
Or just over engineer to provide margin. If dollars are king, then that can push you value engineer your design. I like to design from the 60C ratings even though it could be pushed to 75C. This reduces voltage drop and has additional margin for the future. But it does cost more.
 
tom baker said:
In WA we have a state rule requiring an engraved label if the next size up rule is used, stating what the conductor ampacity is,this on a service
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I really like that. Can you direct me to that specific rule so I can reference?
wwhitney said:
Yes. If you want to help out the future designer, you could label the panel appropriately.

Cheers, Wayne
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This seems like the responsible thing to do.
suemarkp said:
Or just over engineer to provide margin. If dollars are king, then that can push you value engineer your design. I like to design from the 60C ratings even though it could be pushed to 75C. This reduces voltage drop and has additional margin for the future. But it does cost more.
Click to expand...
Agreed.
petersonra said:
I think worrying about what someone might do at some point in the future is not worth the effort. Its not a code requirement and you are not a seer.

personally, I would not try to save $5 on wire costs to use a slightly higher CB rating. But that is a design choice.
Click to expand...
Nevertheless, this thought experiment has made more clear to me what must be considered when reviewing new load connections to existing systems.
 
Cal_Poly_SLO_2011 said:
Suppose I determine the ampacity of this feeder (taking the lower of the calculated ampacity at the middle of the wire and at the terminations)(considering appropriate ambient temperature derating, etc.) to be 44 A. I therefore approve connection to / use of the proposed 50 A breaker, as allowed per 240.4(B).
Click to expand...
240.4(B) doesn't allow a conductor with an ampacity of 44 to be protected by a 50A ocpd.
 
Cal_Poly_SLO_2011 said:
Oh man, that's embarrassing. I should have checked 240.6. Well, suppose it's 46 A. I think my point is still understood. I'll edit the original post. Edit: looks like I can edit this one, but not the original 🤷‍♂️
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Editing you original post after people have taken the time to reply wouldn't be etiquette.

Roger
 
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