400A 3 phase wire size question

[whtRuss14n]

a buddy of mine is building a new shop, he's got a 400 amp, 3 phase service coming in and he's looking at wire.

He found a bunch of 500 kcmil aluminum (XHHW) and wants to use that. The thing is, he looked up the rating and it's good for 350 amps.
His reasoning is that since it's a shop, the load won't be constant at 400 amps all the time, so the 500 kcmil should be fine.
He says it's rated for 350 amps, and that's "close enough" since it'll never be running at full capacity for long.

What do you guys think? Is he on the right track or is this a bad idea?
Also, is the wire size for the neutral and ground conductors the same as the main phase wires, or are they sized differently?
Hit me with your advice and insights guys!

 

[Andre Tardif]

Does he plan on getting the project permitted and inspected? If permits and inspections are not required for some reason, I suggest it still be done to code, as code is the minimum safe installation.

If so, then 500 kCmil will not do 350 AMPS; it is only good up to 310 AMPS for aluminum conductors. I know that the 90-degree column of Table 310.16
shows XHHW, but the terminals of the breakers and panels will only be rated to the 75-degree column. So he has to use that column.
He can still use the 500 kcmil conductors if he runs two conductors of 500 kcmil for each phase.
That means 6 separate conductors of 500 kcmil if he is going to go three-phase plus two more conductors of a smaller diameter for the neutral and the Grounding Electrode Conductor. I don't know what size for the neutral, and I am too lazy to figure it out.

There are a heck of a lot of other considerations when doing a service like this, also. Will the utility's conductors be underground or in free air, etc?

Andy.

 

[lightbender]

My take
He's pushing it. 500 kcmil aluminum is only good for 350 A continuous, so for a 400 A service it's undersized per code. Also, the neutral and ground don't need to match the phase conductors; the neutral is usually smaller unless it carries the full unbalanced load, and the ground is sized based on OCPD rating

 

[PGB1]

A Consideration:
Should there ever be a fire (or other damage) by ANY cause, the insurance adjuster will inspect the entire building. If he or she finds the 500 mcm al on a 400 amp overcurrent device, the company has an instant out of paying- no matter if it had anything to do with the damage or not.

And:
If the insurance company finds that the job was not "green tagged": No Payment (Permit & Inspection(s) = Green Tag)

One small item can render the coverage void, whether or not it had to do with the claim

Reference:
I worked (and still sometimes do work) as an impartial, neutral expert for civil and criminal courts and arbitration companies concerning insurance disputes. Any reason not to pay is validated & upheld.

Example: Lightning caused a fire. Insurance adjuster found a non-permitted electrical project. Electrical had nothing to do with the fire. Claim denied. Court Upheld

Example: Flood caused damage. Insurance adjuster found undersized wire on a circuit. (No electrical damage). Claim denied. Court upheld.

Example: Car hit building & damaged it. Adjuster found plumbing that was not installed under a permit. Plumbing had nothing to do with damage. Claim denied. Court upheld.

Should anyone or anyone else's property be injured or damaged, you can be held criminally liable (as well as tort liability).
Should the injured require medical attention, you will have to pay. (Think of long term care for injury causing disability.)

More: Fire Department & Public Safety charges will be borne by you if the insurance company gets out of paying.

All of the above have happened over and over. There are dozens of more scenarios to cite.

Please have the job permitted and inspected for your own sake.

Paul

 

[hiddenarena]

That's a common question. The issue with his reasoning is that electrical code is based on the maximum possible load a circuit could ever see, not the average or what's expected.

For a 400 amp service, the wire needs to be rated for a full 400 amps, no matter how much load the shop is typically drawing. The fact that he found a wire rated for 350 amps means it's not the right size for a 400 amp service, and using it would be a violation of the NEC. It could also pose a safety risk because if a fault or surge were to cause a brief, but high, current, the wire could overheat and cause a fire.

So... using the right wire size for all parts of the circuit is not just about performance but about safety and adhering to industry standards.

 

[Andre Tardif]

You could use the 500 kcmil conductors on a 300-AMP panel. The panel can be rated greater than 300, though, if that is important to you.

Andy.

 

[PGB1]

What HiddenArena mentioned is something to absolutely pay attention to. Unforeseen over-amperage is a common problem.

In my work (mostly industrial), I've seen many times the over-current alarm will trigger or a fuse will open because a lug worked loose or corrosion caused resistance, thus an amperage increase.

Were the over-current protection device (fuse, breaker, etc) larger than the capacity of the wire, the wiring would have burned.

Suppose that were to happen in a house or other building with wooden framing. Yikes!