wire size: load vs OCPD

[art]

Greetings,

Is there a reason to size my feeder to the 15 A breaker and not the 2 A load at the subpanel?

 

[ptonsparky]

Yes. Have you tried researching it at all?

 

[Jraef]

And are you an electrician?

 

[art]

Can you point me to the right article please. I'm an engineer but, with limited NEC experience

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[Jraef]

No NEC article necessary (although there are several addressing this), it's more just common sense.

The 2A load is fine if there is no problem. But that's not what the breaker is for. The first breaker coming off of a panel or other distribution system is called a Feeder Circuit Protective Device who's main purpose only comes to play when there IS a problem and if the conductor is sized for a 2A load but protected by a 15A feeder breaker, the conductor could / would catch on fire before that breaker trips. Fire = bad.

So you always size the breaker to protect the conductor first, the load if it can. If it can't protect the load (as might be the case in your scenario), then you can add ANOTHER breaker or fuse (called the "Branch Circuit Protective Device) for the load.

 

[electrofelon]

For starters, look at 240.4. Generally, a conductor needs to be protected at its ampacity. There is a "next standard size up" rule for devices 800A and under that is commonly used, 240.4(B). There are taps that can be protected above their ratings, most common are feeder taps, see 240.21.

 

[augie47]

I'm basically on board with Jraef. IF you load requires a specific over-current protection you need to address that, perhaps with a supplemental fuse. The breaker provides short circuit protection and over-current protection for the conductor. The "normal" minimum based on standard sizes would be a 15 amp breaker and a #14 conductor.

 

[electrofelon]

For starters, look at 240.4. Generally, a conductor needs to be protected at its ampacity. There is a "next standard size up" rule for devices 800A and under that is commonly used, 240.4(B). There are taps that can be protected above their ratings, most common are feeder taps, see 240.21.

For completeness, I should mention that things are a bit different for motor loads, article 430.

 

[reast]

The basic procedure is this:
1. Find the maximum load to be served so you can supply it correctly. 210.19 A 1 - In your case the load is very low, so you don't have to worry about that.
2. Find a breaker that can supply the load 240.4 - In your case, you run up against the "normal" minimum breaker size of 15A that is readily available for most breaker panels Table 240.6 A. Although 3A is a standard size (240.6 A) but good luck finding that in a standard breaker. So technically you could install a 3 amp breaker using #18 wire rated for 5.6A (240.4 D 1) BUT...
3. Size the wire for the breaker, considering any derating you might have 310.15 B
4. Make sure there aren't other rules that supersede these like 310.106 A which sets the minimum wire size at #14 copper, unless specified elsewhere. And rules like motor loads mentioned by electrofelon, 430
5. As augie47 mentions, consider supplemental overcurrent protection for your particular device if needed. But this doesn't replace the other requirements. 240.10

So your stuck using #14 wire, but you can use a 3A breaker or fuse if you want (if you can find it for your application)

This is just the bare basics, of course.
(I hope I got all this right...)

 

[art]

Ok. Hang in there with me. I will be using. #6 for voltage drop as my feeder. So, I can protect it with an up to 50 amp breaker? A the subpanel, I'll need the fuse for the load...

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[suemarkp]

How long is this feeder that you need #6 for a 2A load? And if it really that long, you would want a small breaker on that feeder so no one gets the idea they can pull 50A from a 400' long feeder (or however long it is) to run something else from that panel. Also note that when conductors are oversized due to voltage drop, the equipment ground must also be proportionately oversized (i.e. you need a #6 ground if the ungrounded conductor is #6 on a 15, 20, or 30A circuit).

 

[reast]

Just so you know, designing for voltage drop is different than designing for safety. The code specifies the minimums needed for safety. The code only has an informational note on voltage drop 210.19 A . If you design for voltage drop, and it yields a more conservative installation than the safety minimum, then you are good to go. You can always go safer than code with the installation.

So you can put a 50 amp breaker on #6. You can also put a 3 amp breaker on #6 since it's more conservative (although a bit ridiculous). You can separately calculate the voltage drop to the Load to be Served since that's more of a design issue, and not a safety issue. You just need to make sure that you bring it up to the minimum safe installation if it falls below.

 

[art]

It's a 1500' run. I understand about the feeder breaker. Thanks all! Much appreciated.

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