DC voltage wiring in home

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nicknorth

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Has anyone here done any DC wiring in a home with panel and breakers for lighting? Now that renewable energy along with LED lighting is getting way more popular I would think its a matter of time before people start doing something like this.
 
nicknorth said:
Has anyone here done any DC wiring in a home with panel and breakers for lighting? Now that renewable energy along with LED lighting is getting way more popular I would think its a matter of time before people start doing something like this.
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It may be coming, but there are several problems along the way to be considered:

1. Switches and circuit breakers for DC, especially at moderate power, are larger and more expensive than AC breakers for the same current. This is because it is harder to interrupt a DC arc when opening.
2. Low voltage DC requires larger wiring (both higher current and lower tolerable voltage drop) which costs more and may be harder to run. High voltage DC presents as whole new set of hazards, including a lack of low priced reliable non-contact voltage sensors.
3. Most current dimmers and solid state controls will not work with DC.
4. You will still probably want to provide AC for motors unless each motor gets an associated VFD or pulse controller, so you would have two different wiring systems within the house which might need to be kept separate, with some difficulty.
5. You would have to standardize the supply voltage if you want to avoid lots of different models of each luminaire. Changing DC voltage requires a DC-to-DC convertor which adds cost and new points of failure.

If answers are developed for all of these, we may see it someday.

In renewable energy forums (solar PV in particular) people keep proposing a DC wiring network serving loads to avoid the need for an inverter, but few if any people end up trying it.
 
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Yes, out at my cabin. Battery bank is 24V. I used QO CBs and panel. Works great.

Also used QO CBs and panel for the inverter fed 120VAC.

ice
 
It can be done but very expensive as Gold Digger outlined. It gets real expensive if using low voltage of 12, 24, or 48 volts. You could very easily find yourself using 1/0 or larger for a 20 amp circuit. At minimum 6 AWG unless your conductor length is really short.

I will give you a real life example. Let's say you are operating at 12 volts and want a 20 amp circuit that can handle up to 16 amps continuous load. Your initial thought would be 12 AWG just like you would use for a 120 VAC circuit in your home. If you did that, the circuit is only good for 10 feet with a voltage drop of 4.2 %. However your load device is 75 feet and would require a minimum of a #2 AWG to achieve 5% voltage drop. Ouch! Keep in mind 16 amps @ 12 volts is only 190 watts.

To put it into perspective a 20 amp, 120 VAC circuit using 12 AWG is good out to 100 feet with just 4% voltage drop delivering 1920 watts or 10 times more power than the low voltage DC circuit.Last thing anyone wants to do is move to low voltage circuits as it is way too expensive and inefficient which is bad on the wallet and Mother Earth. Low voltage is Green, your suppliers green, but not green for the environment. So don't get sucked into the idea of RE is green as it is anything but green.
 
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GoldDigger said:
In renewable energy forums (solar PV in particular) people keep proposing a DC wiring network serving loads to avoid the need for an inverter, but few if any people end up trying it.
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Actually, it's pretty common in off-grid installations. They tend to be much smaller houses with shorter runs feeding lower current devices, though, so conductor size is not that big of a problem.
 
RV's and campers have used 12 volt lighting and lower wattage equipment for years, but users still prefer to connect to AC source whenever possible so they can run higher wattage appliances, or have more lighting, the LED's we have today take another step toward being able to use the lower voltage for general lighting though.
 
kwired said:
RV's and campers have used 12 volt lighting and lower wattage equipment for years, but users still prefer to connect to AC source whenever possible so they can run higher wattage appliances, or have more lighting, the LED's we have today take another step toward being able to use the lower voltage for general lighting though.
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RVs and campers don't need runs longer than a few feet. That's not the case in a home, which complicates LV DC design matters somewhat.
 
K8MHZ said:
How do you know the QO's will trip at their rating with 24 VDC?
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I see two questions here, which one are you asking?
1. How do you know that the QOs will have the same current sensitivity (thermal and magnetic) for 24V DC that they do for AC?
2. How do you know that when they trip they will successfully interrupt the current?

(PS: I would guess that in addition to the maximum nominal current rating, the interrupting rating for DC will be substantially different than the value for AC. So you also need to do a separate available-fault-current calculation for your DC source. That said, likely DC sources will be current limited to a far higher degree than a POCO transformer or even a generator.)
 
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GoldDigger said:
I see two questions here, which one are you asking?
1. How do you know that the QOs will have the same current sensitivity (thermal and magnetic) for 24V DC that they do for AC?
2. How do you know that when they trip they will successfully interrupt the current?

(PS: I would guess that in addition to the maximum nominal current rating, the interrupting rating for DC will be substantially different than the value for AC. So you also need to do a separate available-fault-current calculation for your DC source. That said, likely DC sources will be current limited to a far higher degree than a POCO transformer or even a generator.)
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Because they are listed for use on DC up to 48v.
 
Here in the UK, I have done a number of installations at 12 or 24 volts, for both off grid homes and for backup purposes in premises that normally use utility power.

Provided that loads are are modest, wire sizes are manageable.

For lighting I normally use 2.5mm wire (similar to #14) on 10 amp fuses
For low power outlets I normally use 4.0mm (similar to #12) on 16 amp fuses

Heavier loads need calculations for voltage drop.

Whilst standard "twin with earth" cable (very similar to Romex) could be used, I prefer single wires in plastic conduit as being easier to install.
Single wires in conduit also permits of less common coloured wire being used, for 12 volts DC with a negative ground I normally use white for the grounded negative, and red for the positive, with orange or yellow for switch wires. White wire is not much used in the UK (special order item) and the presence of white and red should alert anyone working on the installation to the fact that it is unusual in some way.

For lighting I would normally use 12 volt CFLs of 11 watts each for a good level of lighting, and 12 volt 3 watt LED lamps for smaller lighting requirements.
Up to about 6 of the 11 watt lamps may be placed on one circuit without excessive voltage drop, unless the circuits be unusually long.

Small power outlets are usually standard USA type 120 volt 15 amp outlets, these are virtually unknown in the UK for line voltage use, and therefore ideal for 12 volt use.

Appliances likely to be used from such outlets include desk fans, table lamps, radios, and small laptop PCs or televisions.

12 volt fridges or freezers would be on dedicated circuits, usually 6.0mm cable on a 20 amp fuse.

The last job I did was for a large off grid farmhouse, this was 3 wire DC at 12-0-12 volts so as to give 24 volts for heavy loading appliances and 12 volts for most lighting.
Such a system has much to commend it, but tends to be a bit complex.
 
K8MHZ said:
How do you know the QO's will trip at their rating with 24 VDC?
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That question has had some reply, but why would a current responding device care about voltage? Now GFCI or AFCI units need a specific voltage to drive some of the logic components but not a straight thermal magnetic device. The issues of voltage with such devices is max voltage from an insulation breakdown point of view or arc extinguishing abilities of the contacts when they open - which will be higher for DC current than AC current @ same voltage level anyway.

K8MHZ said:
RVs and campers don't need runs longer than a few feet. That's not the case in a home, which complicates LV DC design matters somewhat.
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I agree runs would typically be shorter, but not always limited to short enough distances that voltage drop would never be a concern.

iceworm said:
Catalog says they are rated for 48VDC and 5KA. Hopefully they aren't lying.

ice
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Why do think they would be lying?

fmtjfw said:
Because they are listed for use on DC up to 48v.
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don't forget - up to 5kA
 
kwired said:
That question has had some reply, but why would a current responding device care about voltage? Now GFCI or AFCI units need a specific voltage to drive some of the logic components but not a straight thermal magnetic device. The issues of voltage with such devices is max voltage from an insulation breakdown point of view or arc extinguishing abilities of the contacts when they open - which will be higher for DC current than AC current @ same voltage level anyway.
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I did not know they had a DC rating. My concern was more along the lines of the contacts in the breaker not being able to work without fusing shut. I see that is addressed by the fact that only 48 VDC is allowed on a 120 VAC breaker, which makes sense to me.
 
I had a leg up. I ran in to SQD QO panels in DC service thirty years ago, when living in Southeast Ak. There is a large fishing fleet and a lot of the medium sized boats (30' - 60') use QO panels on their 24V systems. Easy jump for me to select one for my cabin DC service. (But I did check the ratings first :thumbsup:)

If I had not used QO, I likely would have gone with 32VDC ATO fuses and screw terminal fuse blocks. Although, that would have been a pain terminating solid #14 or #12 CU - I used romex. I never thought this one clear through.

And I did connect +12, 0V, -12 (ungrounded). I'm still not sure this was a great idea - it takes 2pole CBs for the 24V loads.

ice
 
iceworm said:
I had a leg up. I ran in to SQD QO panels in DC service thirty years ago, when living in Southeast Ak. There is a large fishing fleet and a lot of the medium sized boats (30' - 60') use QO panels on their 24V systems. Easy jump for me to select one for my cabin DC service. (But I did check the ratings first :thumbsup:)

If I had not used QO, I likely would have gone with 32VDC ATO fuses and screw terminal fuse blocks. Although, that would have been a pain terminating solid #14 or #12 CU - I used romex. I never thought this one clear through.

And I did connect +12, 0V, -12 (ungrounded). I'm still not sure this was a great idea - it takes 2pole CBs for the 24V loads.

ice
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Not saying it wouldn't be acceptable to use there, but 90.2(B)(1) says the NEC doesn't apply to watercraft, and I'm not sure if there is a code that would apply to watercraft. Chances are there are standards that do apply but are not as well known and enforced as NEC is for other applications.
 
In our aircraft labs, we have 120V 400Hz supplies and 28 VDC power supplies. These 28VDC ones can be huge -- 400A or more. One lab in particular uses QO panels for the 28 VDC distribution. It is has 30A single pole breakers feeding non-locking 120V 30A receptacle because we don't seem to use those receptacles anywhere for anything else. Kind of unwieldy though to have a 30A cord for a device using 5 amps. But the runs can be long and you need the larger wires.

I think the 12-0-12 system has a lot of merit and mirrors the 120V multiwire branch circuit scheme. Finding 12V devices is easy. 24V is less common, but there are some maritime and aircraft items in this range. Automotive items are really things that will operate on 12 to 14V, as the system is typically 12.5V on just the battery and 14.4V when the alternator is running. It would be nice to see some standardization on the 12/24/28 VDC systems with common receptacles configurations and sizes.
 
kwired said:
Not saying it wouldn't be acceptable to use there, but 90.2(B)(1) says the NEC doesn't apply to watercraft, ....
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Yes (to the bolded statement) that is true.

Q: What exactly would be (or not be) acceptable to use exactly where? I can't tell if you are talking about QO panels in boats, or QO panels in dwellings on DC service. I'm not clear why the NEC not applying to watercraft has anything to do with the discussion. Clear me up here.

ice
 
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