Potential failures from poor conductor sizing practices?

[sammiol720]

Hello,

I am an EE doing design work for an EV charging project. We have a 400A panel and the EC would like to use 750 MCM ALU (THWN) conductors for the feeder.

I'm aware this is compliant with 215.2(A)(1) since the conductors are rated for our load, but I worry because I've seen enough projects where property owners are squeezing as much as they can out of their panels to accommodate growing EV drivers (in select areas of course). My concern is that we are using a 400A MCB and a future EC will be tempted to keep adding chargers beyond the 385A that we are rating the conductors for. If EV adoption really does pick up like some anticipate, and these level 2 chargers are being used daily, how likely and when is it likely that there will be damage if the panel is overloaded but not to the point that the breaker will trip?

Am I being paranoid? I want to be understanding that increasing the rating to 400A across the length of the run can really run up material costs and I want to design practically, but safety trumps costs so I appreciate the insight.

Thanks!

 

[jim dungar]

Electricians that cheat, cut corners, and ignore the code, will be a concern regardless what you do.

 

[petersonra]

You are being paranoid. There is no good reason to worry one bit about what someone might do in the future, because they might do just about anything. You have to trust they will do the right thing.

 

[don_resqcapt19]

Any load that the 400 amp OCPD does not clear is not a real world issue on a conductor rated at 385 amps.

 

[sammiol720]

You are being paranoid. There is no good reason to worry one bit about what someone might do in the future, because they might do just about anything. You have to trust they will do the right thing.

That's good advice, thank you! On the flip side, I would still like to think of best practices when designing. If I know that adequate infrastructure for EV charging is always limited and panels are constantly being stressed, wouldn't it be poor practice to rate the conductors below the max OCP?

 

[sammiol720]

Any load that the 400 amp OCPD does not clear is not a real world issue on a conductor rated at 385 amps.

These are all chargers so I am considering them all as continuous loads. Isn't there a greater risk of overheating before tripping the breaker because of this? If I cap at 80% which would be 308A, they could go well above that before the breaker trips which only compounds because the conductor is rated below the OCP.

 

[petersonra]

Any load that the 400 amp OCPD does not clear is not a real world issue on a conductor rated at 385 amps.

Likely it would hold indefinitely at 450 amps anyway.

 

[petersonra]

These are all chargers so I am considering them all as continuous loads. Isn't there a greater risk of overheating before tripping the breaker because of this? If I cap at 80% which would be 308A, they could go well above that before the breaker trips which only compounds because the conductor is rated below the OCP.

It is not like the conductor is going to fail if it is run 10 or 20 % above its nec ampacity. The nec ampacity numbers are very conservative.

 

[electrofelon]

, wouldn't it be poor practice to rate the conductors below the max OCP?

Not at all, almost everyone uses the next size up rule

 

[electrofelon]

I want to be understanding that increasing the rating to 400A across the length of the run can really run up material costs and I want to design practically, but safety trumps costs so I appreciate the insight.

Thanks!

750 is really a horrible choice considering Amps per amount of conductor. Consider one could use parallel 300's, and even if in the same raceway and with the derating, get more amps (probably 416 in this case) with less conductors and about the same cost.

 

[infinity]

I would just use 600 kcmil copper at 420 amps and you'll sleep better at night.

 

[raberding]

we "always" spec parallel #3/0 cu for 400 amps

 

[petersonra]

I would just use 600 kcmil copper at 420 amps and you'll sleep better at night.

Why would you sleep better knowing you made a bad design choice that cost a fair amount of extra money for no benefit?

 

[infinity]

Why would you sleep better knowing you made a bad design choice that cost a fair amount of extra money for no benefit?

It does have a benefit for the designer who is worried about using 750 aluminum with a less than 400 amp ampacity. We have had many jobs where the design specifies no next size up rule. If someone is willing to pay for the extra headroom give them what they want. Personally I think that the 750s are fine.

 

[Knightryder12]

I would do 2 sets of 250 al before 1 set of 750. Could not imagine working with 750, even if it is AL.

 

[David Castor]

If you're the designer, you can use the size you feel comfortable with. No matter what your design is, there is always a cheaper way to do it. The NEC is not a design guide.

 

[jaggedben]

These are all chargers so I am considering them all as continuous loads. Isn't there a greater risk of overheating before tripping the breaker because of this? If I cap at 80% which would be 308A, they could go well above that before the breaker trips which only compounds because the conductor is rated below the OCP.

If you sized both your conductors and OCPD to 125% of the continuous load, there is not a greater risk of overheating before tripping the breaker. That 125% factor is required essentially to allow for more heat dissipation before nuisance tripping. If you have an NEC compliant installation and someone adds more load, they are more likely to trip the breaker than damage the conductors or any other part of the installation.

If you are concerned that the client will want to add more load than what you are designing for, and that this will encourage someone unscrupulous to mess with the installation, making it non-compliant then discuss designing for more future load. At the client's expense, of course.

 

[junkhound]

+1 on parallel 3/0 Cu suggestion.

Unless yer GC has a big spool of 750 Al he is trying to use up <G>

 

[petersonra]

+1 on parallel 3/0 Cu suggestion.

Unless yer GC has a big spool of 750 Al he is trying to use up <G>

There are a lot of reasons why design decisions are made.

I am a fan of 1/0 copper. You get the most ampacity per $ in parallel runs. although sometimes it does not work out as well as other times.

 

[electrofelon]

I would do 2 sets of 250 al before 1 set of 750. Could not imagine working with 750, even if it is AL.

I have only used each once. 750 AL isn't horrible, but it does suck. 750 CU is completely awful. I think I am still sore from the having to wrestle that and that was 6 years ago