What is equivalent in copper to 350mcm aluminum?

Question

I am in the process of building my barndominium. My power company has set a pedestal at my pole with the meter and also two 200 amp (400 amp service) breakers included. The length of my run requires me to use a 350mcm direct bury aluminum wire. The problem I'm running into is that wire is too large for the lugs on my main breaker and my electric pedestal breaker.

I've been told to do an underground box at the pedestal and use polaris taps to downsize to the correct copper wire size for about 5' and then tie into the pedestal breaker. Same as in the house on the incoming side which I would have to install a gutter and tap them the same as at the pedestal.

On another note, I've been told to buy reducer lugs and crimp them to wire and go that route. The problem with that is trying to work with that big of a wire in the enclosure. If I'm going the polaris tap route from my aluminum to copper then what is equivalent in copper from the 350mcm aluminum?

Answer 1

250 MCM copper (it's actually rated 5A higher, but is the smallest size of at least equivalent current rating.)

215A/60°C 255A/75°C

.vs.

210A/60°C 250A/75°C for 350 MCM aluminum.

Answer 2

You don't need to enlarge the entire run for voltage drop.

That is to say, sections of the run are severable for voltage drop calculation purposes.

Avoid blunders in doing voltage drop calcs.

The first blunder is using the amps of the breaker trip. You're not allowed to plan to max out a service - e.g. if you're building Bitcoin miners and plan a total of 400A @ 240V, you really need to provision 125% of that or 500A service. And if you're doing a residential load calculation, that is already accounted for in the demand factors. Actually, use the Calculated Load or 80% of breaker trip, whichever is lesser, and this is based on Canada rules.

Don't forget the Load Calculation is a practical worst case scenario: imagine if panels only caught fire once every 10 years on average, that would NOT be acceptable! So Load Calculations are set even higher so that doesn't happen. That means having actual amp draw equal to your Load Calculation will be a very unusual and transient event.

Second, the actual voltage drop you experience will be proportional to actual amps right now, - if you set wire to 3% @ 80% load, then at 40% load (itself quite uncommon) you'll have 1.5% drop. 3% is a wire salesman's suggestion, and not a hard and fast rule except in Canada.

Multi-segment cable and voltage drop: calculate separately.

Let's say for weird reasons, you have two existing segments of 30' of 4/0 wire and 20 feet of 250 kcmil (all aluminum of course). Lets see how that computes.

• For the 30' run of 4/0 at 160A, we have 0.47% voltage drop.
• For the 20' run of 250 at 160A, we have 0.26% voltage drop.
• These total 0.73% drop.

So, the remainder of our cable must hit our voltage drop target minus 0.73%.

Let's say we have a 300' total run (250' to add) and we're OK with a 4% voltage drop. So we ask the calculator for 250 feet, 160 amps and 3.27% drop (4% - 0.73%). And it spits out 300 kcmil at 2.88%. 3.61% drop.

However, here's a teaching moment. If you tell the calculator you'll tolerate a little more drop, surprise! Out target was 3.27% drop and 250 kcmil works at 3.30% drop. That's only 0.03% off our spec - perfectly tolerable... and the voltage drop calculator didn't bother to mention that! Because it is literally run by wire salesmen. So yeah, it's worth checking target + another fraction of a % to see if a perfectly acceptable value is just around the corner.

So your jumper, being only 6 feet, will be negligible voltage drop, and I would just use the largest aluminum that will fit on the lugs and call it a day.

Demand factors work even better if you bring a single cable over.

You might have one of your two 200A panels peaking. It could happen. But in that moment, you are vanishingly unlikely to have both of them peaking.

With two separate long feeders, the one that is peaking will experience "spec" voltage drop. The other one will not, and its wire capacity is largely wasted. (no, you can't bridge them at the house).

However, with one large e.g. 600 kcmil feeder, yes, one 200A panel is peaking, but since the other one is not, the feeder is not either, so it is not up against spec voltage drop.

How do you make that work? You get a different meter pan that accepts single 600 kcmil lugs, or swap lugs if the pan supports it. Then you use a 400A disconnect out there, if that's even required.

Answer 3

The length of my run requires me to use a 350mcm direct bury aluminum wire.

No, it doesn't. Well, it probably doesn't, as some jurisdictions may have different requirements. But generally speaking:

Size

• 200A as entire service requires wire capable of 166A, 2/0 copper or 4/0 aluminum. You don't get to do that because your service is 400A.
• 200A as a part of your service requires wire capable of 200A, 3/0 copper or 250 MCM aluminum.

Looking at some samples (found some Square D and Siemens pretty easily), the maximum is typically, but not always 300 MCM. 300 MCM allows an upsize even on aluminum. Why upsize? The only real reason is voltage drop. That depends on distance and on expected usage and on voltage drop allowed. 300' at 160A is a lot worse than 150' at 125A. And 3% will require larger wire than 5%. So a lot depends on the specifics. As I understand it, the NEC does not force any upsize for voltage drop, but some locations do depending on the specifics.

Wire Type

There is no such thing as direct bury aluminum wire. There is direct bury aluminum cable. There is also individual copper or aluminum wires for use in conduit, and direct bury copper cable. With rare exceptions, you either want to use direct bury aluminum cable or aluminum wires in conduit, because copper is a lot more expensive. Which makes the most sense depends on the type of ground you have to run the wires/cable/conduit through. Keep in mind that direct bury cable generally has to be deeper than conduit, and conduit depth depends on the type of conduit. If you have a flat open area then trenching is easy. If you have hills and trees and rocks it is not so easy. If you are going to hire someone to do the trenching then let them figure out what is most cost-effective. If you are going to do it yourself then you need to look at costs of rental equipment, value of your time and other factors. But nothing requires "direct bury aluminum" per se.