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Glaesemann · Dec 6, 2006, 05:56 PM

Hi All!

I currently have an old 40 amp service in my pre-war condo. We are allowed to run BX securely attached for a new service.

The distance is 250 feet. I am looking to get a 100 amp service. What size BX cable should be used for this? Do they make BX large enough for a 250 foot 100 amp service?

I am also wondering what wire it should be, copper, aluminum?

Any help before I speak with the electrician and board is greatly appreciated.

Thank you!

Tim

tkrussell · Dec 8, 2006, 06:07 AM

You need to consult with your electrician, as this is an odd situation.

A 100 amp panel will need to assume the maximum load is 80 amps, and a 100 amp feeder at 250 feet needs to deliver no less than 3% voltage drop.

This will need either #3/0 copper or 250 MCM aluminum wire.

Glaesemann · Dec 8, 2006, 06:37 AM

I was just trying to prepare before I spoke with my electrician and my condo board.

My electrician quoted 2/3 BX.

$2000 in BX alone!!

Yikes!

Job starts today. No choice I guess.

Thank you.

Tim

tkrussell · Dec 8, 2006, 12:01 PM

Yes I know that copper is expensive, aluminum for feeders is much less.

So someone is running 250 feet of #2/3 copper cable for a 100 amp serivce?

I will be very interested in the voltage drop.

Mind you, if the electrician measures the voltage when he is done, with no or little load, the voltage reading will probably be OK, the voltage drop will become evident as more load is applied.

Glaesemann · Dec 8, 2006, 12:09 PM

He played this fact down. He said it only mattered if you had "voltage sensitive" items on the load.

I told him I DO have voltage sensitive items on the load. I am an audiophile and would love to see as little voltage drop as possible.

He assured me it was only an issue with sensitive computer equipment.

I am glad you mentioned this. I am going to ask some more questions.

He didn't come today after all this, because they had an emergency job.

Would conduit and 2/0 be the correct wire method to avoid voltage drop?

Thank you!

Tim

tkrussell · Dec 8, 2006, 01:55 PM

If #2 copper is 250 feet long, with a reasonable load of 50 amps, the voltage drop will be 4.9 volts, or 4.8%.

Maximum recommended voltage drop for a feeder is 3% to the distribution point, or the breaker panel.

#2/0 will offer 2.5 volt drop (2.1%) for the same load. An 80 amp load will result in a 3.25% Vd.

This is acceptable if the branch circuit wiring is not too long, as the final Vd at the outlet should not be more than 5%.

If this was my job I would be selling you #2/0 Aluminum in PVC conduit (if PVC is allowed in your area) The Vd is still a bit high for 80 amps, but here is where what are the chances of a full load being applied. For a 50 amp load the Vd is perfect,2%.

What does the local electrical inspector say about this?

Glaesemann · Oct 17, 2009, 01:28 PM

Resurrecting this thread because we are finally go ahead with the upgrade and right back where we were 3 years ago. :-)

It looks like we are going with two 2/0 for the 200 foot run. My question is can we still use our 100 amp (2x100 amp) loadcenter?

Thanks all.

Tim

tkrussell · Oct 18, 2009, 03:28 AM

Sure can, just will need to handle the large wire size at the terminals, which may not be large enough to take the 2/0. Either change out the lugs, or reduce the wire size with short pieces of #2.

Glaesemann · Oct 18, 2009, 05:17 PM

Great!

This is a two step install. Meaning the electrician is completing an upgrade of the kitchen wiring and also the loadcenter. He will break the feed in the basement (one floor below) and install the 2/0 from the break (junction) to the loadcenter. The 2/0 will splice to the current #8 at the break junction box. Is it okay to have 2/0 from the box going to a junction with #8 that runs for 200 feet to the service?

My last question, I promise!

Thank you!

Tim

tkrussell · Oct 19, 2009, 04:09 AM

You can post as many questions as you like.

What is the #8 for?

Tev · Oct 19, 2009, 05:51 PM

#8 is not large enough for a 100 amp service. You need the 2/0 running the full distance. It blows my mind that the "electrician" would even consider using #8 for this. It was fine for your old 40 amp service but it's not even close to what is needed for 100 amps. I'd find a new electrician if I were you.

donf · Oct 20, 2009, 08:07 AM

Please check T310.15. For a 100 A. Main Panel Feed, you can use #4/3 AWG cable.

Any cable smaller than a 4/3 cable would violate the NEC code.

You will also need to check with the LAHJ for condo installations to find out who is allowed to do work on Condo electrical systems. (correction to NEC reference.) Then I would ask to see the license for the electrician that is scheduled to do this work.

Given the history you have chronicled here, I'm very suspect of your electrician's qualifications!

tkrussell · Oct 20, 2009, 11:39 AM

Don, the issue is voltage drop due to a 250 foot length. The #4 is a minimum feeder size, and is irrelevant to the topic.

Also, would you please direct me to the Condo code in the NEC?