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#3833 08/30/01 12:33 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
Does anyone know when the old DC supplies disappeared in America?

We had them in England in the older parts of many towns well into the 1950s, and I believe the last public DC supplies were converted in the 1960s.

That was before standardization at 240V, so the DC mains were anything from 200/400 to 250/500V.

#3834 08/30/01 05:17 PM
Joined: Oct 2000
Posts: 2,749

Look here: I believe subways are still operating on DC?

Joe Tedesco, NEC Consultant
#3835 08/31/01 12:12 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
Well, I was thinking of normal residential or light commercial service really, but thanks for the interesting link Joe.

I've always been fascinated by subway systems ever since I rode regularly on the London Underground as a kid. To date, he NYC system is the only other subway I've had a chance to ride on, so the info is quite fascinating.

#3836 08/31/01 06:50 PM
Joined: Feb 2001
Posts: 60
Yes the NYC subway is 600 volts DC. The car lighting is accomadated by wiring 5 120 volt fixtures in series. Same premis with some of the track and tunnel lighting. Most temporary lighting is simply tapped off the third rail and wired the same way.

As far as DC in use in other types of bldgs, I have seen dc in use for large motors in older manhattan high rises. But they are rapidly being replaced. The only place it commonly found is for elevator motors but there is an ac motor generator usually attached or close by.

#3837 09/01/01 04:51 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
The London Underground is also 600V DC, but using third & fourth rails. The outside third rail is positive as in NYC, but instead of using the running rails as a negative return we have a fourth rail mounted centrally between them.

In the early days of electrification there were some other arrangements used on certain lines, but these were all converted in the years following WWII.

#3838 09/01/01 06:03 AM
Joined: Oct 2000
Posts: 2,749
DC covered as follows:

1999 NEC Part H. Direct-Current Systems
250-160. General
Direct-current systems shall comply with Part H and other sections of Article 250 not
specifically intended for ac systems.
250-162. Direct-Current Circuits and Systems to Be Grounded
Direct-current circuits and systems shall be grounded as provided for in (a) and (b).
Two-Wire, Direct-Current Systems.
A two-wire, dc system supplying premises wiring and
operating at greater than 50 volts but not greater than 300 volts, shall be grounded.
Exception No. 1: A system equipped with a ground detector and supplying only industrial
equipment in limited areas shall not be required to be grounded.
Exception No. 2: A rectifier-derived dc system supplied from an ac system complying with
Section 250-20 shall not be required to be grounded.
Exception No. 3: Direct-current fire alarm circuits having a maximum current of 0.030
amperes as specified in Article 760, Part C, shall not be required to be grounded.
Three-Wire, Direct-Current Systems.
The neutral conductor of all 3-wire, dc systems supplying
premises wiring shall be grounded.
250-164. Point of Connection for Direct-Current Systems
Off-Premises Source.
Direct-current systems to be grounded and supplied from an off-premises
source shall have the grounding connection made at one or more supply stations. A grounding
connection shall not be made at individual services or at any point on the premises wiring.
On-Premises Source.
Where the dc system source is located on the premises, a grounding
connection shall be made at one of the following:
1. The source
2. The first system disconnection means or overcurrent device
3. By other means that accomplish equivalent system protection and that utilize equipment
listed and identified for the use
250-166. Size of Direct-Current Grounding Electrode Conductor
The size of the grounding electrode conductor for a dc system shall be as specified in (a)
through (e).
Not Smaller than the Neutral Conductor.
Where the dc system consists of a 3-wire balancer set or
a balancer winding with overcurrent protection as provided in Section 445-4(d), the
grounding electrode conductor shall not be smaller than the neutral conductor, and not smaller
than No. 8 copper or No. 6 aluminum.
Not Smaller than the Largest Conductor.
Where the dc system is other than as in (a), the grounding
electrode conductor shall not be smaller than the largest conductor supplied by the system,
and not smaller than No. 8 copper or No. 6 aluminum.
Connected to Made Electrodes.
Where connected to made electrodes as in Section 250-52(c) or
(d), that portion of the grounding electrode conductor that is the sole connection to the
grounding electrode shall not be required to be larger than No. 6 copper wire or No. 4
aluminum wire.
Connected to a Concrete-Encased Electrode.
Where connected to a concrete-encased electrode as
in Section 250-50(c), that portion of the grounding electrode conductor that is the sole
connection to the grounding electrode shall not be required to be larger than No. 4 copper
Connected to a Ground Ring.
Where connected to a ground ring as in Section 250-50(d), that
portion of the grounding electrode conductor that is the sole connection to the grounding
electrode shall not be required to be larger than the conductor used for the ground ring.
250-168. Direct-Current Bonding Jumper
For dc systems, the size of the bonding jumper shall not be smaller than the system grounding
conductor specified in Section 250-166.
250-169. Ungrounded Direct-Current Separately Derived Systems
Except as otherwise permitted in Section 250-34 for portable and vehicle mounted generators,
an ungrounded dc separately derived system supplied from a stand-alone power source (such
as an engine–generator set) shall have a grounding electrode conductor connected to an
electrode that complies with Part C to provide for grounding of metal enclosures, raceways,
cables, and exposed noncurrent-carrying metal parts of equipment. The grounding electrode
conductor connection shall be to the metal enclosure at any point on the separately derived
system from the source to the first system disconnecting means or overcurrent device, or it
shall be made at the source of a separately derived system that has no disconnecting means or
overcurrent devices.
The size of the grounding electrode conductor shall be in accordance with Section 250-166.

Joe Tedesco, NEC Consultant
#3839 07/01/02 10:31 PM
Joined: Apr 2002
Posts: 2,527
Found this *today* Noteworthy...

What a hoot!

Savings for DC Electric Customers Who Switch to AC Customers receiving direct current (DC) electric service from Con Edison pay more. Large commercial customers may be paying up to $4,600 annually in DC surcharges and additional money for DC kilowatt hour surcharges.

DC customers can reduce their energy bills and receive quality electric services by switching to alternating current (AC) power.

Switching is Easy There are several convenient ways to eliminate DC service. Customers can choose the one that’s best for them: Rectification- install a rectifier which would allow AC service to power DC equipment such as elevators, pumps and lighting;

Conversion- convert existing DC equipment to accept AC service; Replacement- replace existing DC equipment with AC-powered equipment.

Incentives Available DC customers who switch to AC service may be eligible to receive either new equipment or cash incentives from Con Edison.

For More Information For a brochure and an application click on Con Edison’s DC

Conversion Incentive Program, e-mail or call 212-460-3056.

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