NEC wire size / breaker matching

electron

electron

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I'm about to run a dedicated circuit for a 240V device (no motors or other weird startup loads, just charging a battery), and while the device only calls for a 40A breaker (and 8 AWG), I'd like to run 6 AWG, so I can upgrade to a 50A breaker in the future.
 
However, the device requires a 40A breaker, and NEC dictates that you follow the manufacturer's instructions when it comes to properly sizing the circuit.  So the question is, does anyone know if I can run a 40A  breaker on 6 AWG wiring, from a code/NEC perspective?
 
 
There should be no problem with oversizing the wire.  For long runs it is often done to reduce voltage drop to acceptable levels.
 
I completely forgot about distance sometimes requiring a larger wire anyways, good point, thanks!
 
It's too bad the manufacturer of the device insists on a 40A breaker, or I would go ahead with a 50A breaker as well.
 
That's not uncommon - it wouldn't matter if you ran #2AWG - you have to make sure the device doesn't get more current than it's designed for - basically trip the breaker before a fire breaks out.
 
What you can find though and it's quite annoying, is that some devices have limits to the size wire the connectors can handle - I don't suspect it'll be a problem in a breaker, but I've seen situations where a terminating device is limited to a specific gauge so you have no choice but to pigtail.
 
I just did the same thing wiring up my generator inlet last week. A 30A inlet/breaker, but used #8. Why? Well... I couldn't get a 3' run of 10/3, but there was a 5' run of 8/3 already cut that I picked up for less than the 10/3 would have cost! And it's perfectly legal to size up the wire. Odd part (to me anyway since I'm no full-time sparky) the 30A breaker is rated for a max wire size of #8 - figured it would be good down to #6 for any really long runs.
 
Oh, over-sized grounds is common too.
 
Yeah, longer runs or aluminum wire you would up the size.  Shouldn't be an issue.  The NEC are minimum requirements, it's ok to exceed them and do something safer.  Assuming you can get the wire into the breaker terminal... :)
 
So would a 40A breaker be able to accept 6AWG wire?  I know my device won't take 6AWG, so I'll have to pigtail that one, but don't want to pigtail the breaker of course.  The plan is to install a 14-50R outlet, and get a 14-50P cord set at Lowes to act as the disconnect.  The manufacturer requiring me to use a 40A breaker is really screwing up my plans ;)
 
I would get a piece of wire and take it with you when you go breaker shopping.  You don't mention what type of device it is or where you are mounting it, but a breaker can be used as a local disconnect.  So you could do the 50A breaker in your panel to 6ga to a small breaker box at the end, then add your 40amp breaker there and from there wire to your device with 8ga or whatever.   Just in the future don't replace that 40a with another 50a cause you will get unpredictable results as far as which breaker will blow.  You would be better off removing the breaker and do a junction when that time comes.
 
I have done that before with some advice from a power engineer/friend, but working with that gauge wire is not easy in a small box.  I would make sure the kids aren't around, cause there's gonna be some f'bombs dropped... ;)
 
wuench said:
You don't mention what type of device it is or where you are mounting it, but a breaker can be used as a local disconnect.  So you could do the 50A breaker in your panel to 6ga to a small breaker box at the end, then add your 40amp breaker there and from there wire to your device with 8ga or whatever.   Just in the future don't replace that 40a with another 50a cause you will get unpredictable results as far as which breaker will blow.  You would be better off removing the breaker and do a junction when that time comes.
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This is how my Air Conditioner is currently wired; breaker is too high so they just used the correctly rated fuse at the disconnect.
 
I looked at the disconnect boxes Lowes sells, but didn't see any 40A units (they were 50/60A iirc).  As for the device, it's a charging station for an electric car (really just a fancy overpriced electric plug, as the charger itself is in the car).
 
The box doesn't have to be specifically designed as a disconnect box.  If you don't mind it being a bit bigger just get a small "main lug" (rather than main breaker) sub panel box and drop in a 40A breaker.  You do need to be sure the input lugs are rated for the wire size you will be feeding it with as it may not go that small.  Here is an example:
 
http://www.homedepot.com/p/Square-D-by-Schneider-Electric-Homeline-70-Amp-2-Space-4-Circuit-Indoor-Surface-Mount-Main-Lugs-Load-Center-with-Cover-HOM24L70SCP/100202333#.UdS_JKy0TVE
 
Or as they did with my air conditioner, is they used a separate disconnect that took fuses and installed the fuses with the correct rating.
 
The only caveat to oversizing the wire is that you are supposed to stay oversized downstream.  For example, if you run 12 guage wire, you should use 20amp receptacles, even if you put a 15amp breaker in the box.  That is why a 15 amp receptacle doesn't accept 12 gauge wire.  In this example, you wouldn't want a future person to open the panel and think that the entire circuit is rated for 20 amps because there is 12g wire at the panel and put in a 20 amp breaker.
 
Lou Apo said:
The only caveat to oversizing the wire is that you are supposed to stay oversized downstream.  For example, if you run 12 guage wire, you should use 20amp receptacles, even if you put a 15amp breaker in the box.  That is why a 15 amp receptacle doesn't accept 12 gauge wire.  In this example, you wouldn't want a future person to open the panel and think that the entire circuit is rated for 20 amps because there is 12g wire at the panel and put in a 20 amp breaker.
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That's a reasonable line of thinking (IMHO) but I don't know that it's actually a rule, or perhaps just a local one.  Here we aren't allowed to use anything smaller than 12 AWG wire but are allowed to use 15A breakers and 15A outlets.  But that size wire won't fit in the quick connect holes so the screws must be used - a much more reliable connection anyway. 
 
I have heard of people using a smaller wire pigtail at the source breaker to make the sizing obvious to future workers. 
 
If a breaker is used as a disconnect at the unit then there is no problem with a breaker sized for the larger wire run at the source panel.  The disconnect breaker could be the smaller rating and the short run to the unit can be the smaller size.  No confusion later on either...
 
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