I'm working on a design for a generator control panel that requires both AC and DC circuits being in the same panel, raceway, cabling, etc. None of the control circuits are power limited. They will be tapped and fused from the respected power sources (9 of them!) Although 2005 NEC 300.3(C)(1) allows a mix of AC and DC circuits, is there any other issues I should concern myself with?
Not being an electrician but being an electrical engineer. I am not a PE.
If you are talking about modifying a piece of equipment, I don't think the NEC applies in this application.
If you are talking about a switchboard type application where you are monitoring multiple power sources, I do not know what standards applies. I will venture a GUESS that as long as the maximum voltage difference between adjacent conductors is less than 600 V, any suitable insulated conductor would be OK.
Aside from that, the general concerns I have dealt with in these applications are:
1) Wire insulation has be suitable for the highest voltage it could be in contact with, assuming ANY single fault insulation failure.
2) Sensitive signals should be shielded and or seperated from power wiring.
3) Anchor wiring so it does not rub thru insulation.
4) Make as easy as possible to service. Nylocks are your friend. Loose split and flat washers will fall into undesired locatings.
5) DOCUMENT CHANGES and make it easy to find the documents. SUITABLE LABELS are your friend.
When I assembled control panels for automated equipment, we typically used THHN stranded for all control and power wiring 16 AWG and larger. TFFN stranded was used for 18 AWG and smaller. All wiring I dealt with were typically 600 V and less.
Red was for AC wiring less than 300 V.
White was Neutral only.
Black was AC wiring between 600 and 300 V.
Blue was all low voltage DC wiring.
Blue with a white tracer was for the DC common IF IT WAS BONDED TO CHASSIS.
Green was for chassis / ground.
YELLOW was used for power from other systems that were not deenergized when the main disconnect was opened.
All hinged doors and removable covers had braid or Green bonding wires.
Fuses were grouped logically.
Leave approximately 10 % open space, terminals, DIN rails, ground terminals, etc. for expansion.
Label all wires. NEATNESS COUNTS !
Convience outlets and cabinet lighting, if appropriate, are nice.
Re: Mixing AC and DC circuits in same raceway
#197911 12/29/1006:57 PM12/29/1006:57 PM
Thanx for the response. The control panel will run and monitor 3 gensets, their DC control circuits and being a floating vessel, it has two separate shore connections. Plus throw in an inverter to boot. Not all of them will be running at one time.
The meters are programmable and will not only show what they are reading, but in color so at a glance any one can see if everything is ok even if they have no clue what it means. It will also operate a "christmas tree" to indicate the system status. The control panel will automate many of the current manual operations that are usually done wrong or incomplete.
So you best believe labeling, clean installation, maintenance friendly, and user friendly O&M manuals are not just the spec, it's my SOP.
I do plan to use red for AC and blue for DC. And as always, the white will only be for grounded conductor and green for ground. Another SOP of mine and a NEMA Standard. I will also be using #14 for the most part. Being a steel vessel and can be powered by shore power, proper and adiquate grounding and proper bonding is my top priority.
Being a floating vessel, it falls under Coast Guard and American Beareau of Shipping rules while the house on it falls under the NEC other building codes just to name a few. It has been intersting to say the least
I have been working with the meter manufacturer to insure a good quality signal
1) Is it all going to be single phase and less than 600 V? YES
2) Will the generators be paralleled to each other or shore power? NO
3) Will the switching between power supplies occur IN the console or this is more of a control and display unit? The switching will be in the cabinets. It will be done via mechanical and electrical interlocks.
4) Different DC voltages for inverter batteries and engine starting? Just the inverter AC loads will be in order to help manage its load via its meter and the "Christmas Tree". The DC circuits are just the 12volt start circuits from the 3 gensets which will also control a couple other functions along the way
5) LOCKABLE disconnects to prevent energizing exposed shore power connections? No. Each power source is mechanically and electrically interlocked so the buss will not back feed.
6) Suitable enclosures / gasketing to withstand marine environment? You betcha!
7) CT and isolation transformers to isolate different systems from inadvertant cross connections? Not sure what you mean
8) Communication connections to shore power, generator, and battery bank within reach to aid in trouble shooting and system reconfigurations. Not sure what you mean. The O&M manual will cover all the trouble shooting. Its not a complicated system and the automation will take of and eliminate all the mundane steps of the existing systems
Sounds like a fun project. Good luck with it all. It has so far. I enjoy doing automation and industrial wiring. Just starting pricing it out and hope to move FWD on them ASAP while the barges are parked in an relatively easy location for the work for the winter. I have three systems to do and the old systems needs to come out. Electrically speaking, these barges are a hoot to work on.
"Live Awesome!" - Kevin Carosa
Re: Mixing AC and DC circuits in same raceway
#197917 12/31/1001:39 AM12/31/1001:39 AM
7) CT and isolation transformers to isolate different systems from inadvertant cross connections?
8) Communication connections to shore power, generator, and battery bank within reach to aid in trouble shooting and system reconfigurations.
If you are monitoring either current draw or using power meters, 5 Amp CT's are much preferred to running differential mV shunt voltages. Plus they are already isolated. 1:1 voltage isolation transformers also minimize the chance of a significant oops during installation / maintenance / troubleshooting.
Communications just means you have easy and safe communications to whomever is connecting / disconnecting shore power, if someone is working on the generator sets, if you are troubleshooting something, etc. Nothing worse than trying to hold a phone handset while handling test leads in energized cabinets, or shouting readings to someone else across a room whil trying to find what failed.
Are the battery banks lead acid or some other chemistry that requires ventilation or special heating? Do you have a separate alarm panel or remote indications for damage control items such as bilge alarms, low fuel alarms, fire / smoke / heat sensors near the generator sets, generator master alarms, air flow sensors for the battery room, failure of heat trace lines, etc.
The old Navy days are coming back.
Re: Mixing AC and DC circuits in same raceway
#197918 12/31/1003:18 AM12/31/1003:18 AM
In makes more sense to me now. I will be using 5 amp CT's per the meter manufacturer's requirements if you are talking about the control panel meters. As for Isolation transformers, the two shore conections are different different configuration so without a big hammer, theres no cross connection issues. Each shore power connection and its genset(s) counterpart and mechanically and electrically interlocked so they will not but head one another.
There is no communication for shore power connections since it is a one person operation if it happens at all. Currently, all three barges in question are tied to a dock which is a rarity already but only one has one of its shore power connected. Even if two or more people are playing with the shore power connections and the if the gensets were running (normally not when docking), and even if the yahoo on the dock plugs in the shore power with the pedestal breaker on (a violation and against SOP), at the very worst, it will only disengage the genset power from its bus only if the transfer switch is already switched to "shore power" (another SOP no-no). Some one will have to go out of their way to screw something up which is out of the scope of the system.
The existing battery bank which not part on the project is a 48 volt wet cell battery bank. They are in enclosed, stout battery boxes that mechanically ventilated to the exterior while charging. They are not heated since the barges are only used in the summer time. The solar array keeps the batteries topped off which prevents them from freezing. It doesn't get that cold in my neck of the woods. The angle of the arrays minimizes snow build up on them. and worst comes to worse, the inverter will start the little genset to protect the batteries automatically.
Are you by chance familiar with American Bureau of Shipping standards?
Is each shore power connection / genset a stand alone system? There's two, 16kW gensets. They and one of the shore connections provide 120/240 volts for the large electrical loads. They are currently mechanically interlocked so that only one can be powering the main bus. The inverter system provides power for lights and general purpose recepts, refrigeraters, etc. There's a 6kW 120v genset that is slaved and controlled by the inverter. The second shore power connection is for the inverter system in place of the genset.
Do you have three independant systems? In a sense, they are two independent system