My friend who lives on the Chesapeake bay is running electrical wires
to his dock for some new boat lift motors (two). Each motor draws
about 12.3 amps under load and the run from the service panel is going
to be about 150-175 feet. I know that because of the distance,
voltage drop is a problem.
He does not want to install a sub-panel near the dock. Instead, he
plans to run two 15 amp circuits, one for each motor. He plans to run
the wiring in the grey, PVC-type conduit (schedule 40?) and bury it.
Questions (for those folks who are up on the NEC):
1) If he is using conduit, does he still have to use direct bury (UF)
cable? Or can he use NMB in the conduit?
2) Does each cable have to be in its own conduit? (eg. Each length of
UF or NMB in it's own conduit). Or can they share a conduit of
3) What diameter conduit? Given a run of 150'+, probably looking at 8
4) Since 8 awg is required, is single strand wire an option (better
I assume these are 120V circuits. I would run individual THWN (or
equivalent) conductors in the conduit rather than multiwire cables, and
connect the two circuits to opposite sides of a 2-pole breaker and let
them share the neutral wire and ground wire. That will save you the
cost of 2 wires, and if you ever run both motors at the same time they
will work better. If these are 240V motors, they can only share a
Are these going to be direct-wired, or connected with a cord and plug?
If 120V and cord-and-plug I think you'll need GFCI's unless you use some
kind of weird outlets, like twist-loks. Even with twist-lok outlets (so
nothing can plug into the outlets except these motors) the GFCI might be
a good idea. I don't have my code book handy to see what it says about
docks and marinas.
I really don't think you'll need 8 awg wire for this, but I haven't
NEC Article 555
Marinas and Boatyards
555.1 ........ Private , noncommercial docking facilities constructed
or occupied for the use ofthe owner or residents of the associated
single family dwelling are not covered b this article.
Now I would run #10 single strand wire on the pvc. type NMB and UF
aren't supposed to be run in conduit . I would use GFI protection ,
I am a little fuzzy on my NEC right at the moment, but I would think that 12
guage wire and a 15 amp breaker would be adaquate. 8 is way over kill, 10
guage would be a safe bet. Run two hots off a two pole breaker, one neutral
and a ground wire, four wires total. 1/2" conduit would be fine.
A sub-panel would be the way to do it.
2 circuits, with EGC is 6 wires, 5 if you share EGC. You only need 4
wires for a sub-panel.
Not only that, but then the wiring to the device can be 12ga, and fit in
the available connections.
Not to mention it saves 365 feet when (not if) the breaker trips, or
there is an electrical problem.
No, you use THHN individual wires in a conduit, not a cable.
You can run more than one circuit in a conduit. Each circuit would be 3
wires, with EGC (black, white, green), you would have 6 wires for 2
a SWAG says Inch and qtr.
Not unless you have an insane desire to make your job harder than it
needs to be, in a word, no.
IMHO, run conduit, and qty 4 #8 (or #6) stranded THHN (black, red,
white, and green) out to the dock, and install a 4 place 30amp subpanel.
The ability to have an light or outlet for battery charger, or something
in the future, not to mention saving an long conductor. Then there is
also a single on/off.
Don't use UF in conduit or NMB or at all. #8 wire isn't needed either. A
1/2" PVC with 4-#10 THHN/THWN conductors is sufficient. Two hots (black and
a red or blue), one neutral (white), one equipment ground (green). The
neutral can be shared, as can the equipment ground. The volt drop for a
12.3 amp load at 175' is 5.4 volts (4.5%) or 114.6 volts at the load.
Since no feeder is involved, the total volt drop is still less than (NEC
recommended) 5% total. At 150', volt drop is 4.5 volts (3.8%) with 115.4
volts at the load. 3/4" PVC isn't much more expensive and will make for a
much easier pull. Be sure to use GFCI protected receptacles and
weatherproof "in-use" covers for cord connected motors.
Not installing a sub panel may not be a choice he has. He would do well
to check with his AHJ in advance. If the AHJ treats the dock as a
structure; and if he would need a permit to build it they probably will;
then it can only have one source of supply.
225.30 Number of Supplies.
Where more than one building or other structure is on the same property
and under single management, each additional building or other structure
served that is on the load side of the service disconnecting means shall
be supplied by one feeder or branch circuit unless permitted in
225.30(A) through (E). For the purpose of this section, a multiwire
branch circuit shall be considered a single circuit.
(A) Special Conditions. Additional feeders or branch circuits shall be
permitted to supply the following:
(1) Fire pumps
(2) Emergency systems
(3) Legally required standby systems
(4) Optional standby systems
(5) Parallel power production systems
(B) Special Occupancies. By special permission, additional feeders or
branch circuits shall be permitted for the following:
(1) Multiple-occupancy buildings where there is no space available for
supply equipment accessible to all occupants, or
(2) A single building or other structure sufficiently large to make two
or more supplies necessary.
(C) Capacity Requirements. Additional feeders or branch circuits shall
be permitted where the capacity requirements are in excess of 2000
amperes at a supply voltage of 600 volts or less.
(D) Different Characteristics. Additional feeders or branch circuits
shall be permitted for different voltages, frequencies, or phases or for
different uses, such as control of outside lighting from multiple locations.
(E) Documented Switching Procedures. Additional feeders or branch
circuits shall be permitted to supply installations under single
management where documented safe switching procedures are established
and maintained for disconnection. (copyright 2002 the National Fire
If there are no other loads on the dock then he can supply the two lifts
from a single multiwire branch circuit. That is the type of circuit
that firstname.lastname@example.org was explaining to you were the neutral and
the EGC of the circuit are shared between the loads. The much more
likely state of affairs is that your friend will want a receptacle or
lights or both on the dock. To serve those loads he will need to
install a feeder supplied panel at or on the dock and supply the
individual loads from that panel.
The first thing to do if he'll need an inspection is to speak with one
of the inspectors to see what will float. I suspect he'll need a subpanel.
For my dock I elected to put a waterproof subpanel at the pier and then
feed the dedicated outlets for my hoists and boat feeds from it.
A good guideline is to have no more than 3% voltage drop under load.
For the situation that you put forth (assuming 120 volt devices) 3% in
3.6 volts. At 12.3 amps this is a maximum (two way) resistance of
0.2927 ohms. #8 copper has a resistance of 0.0008 ohms/ft. 175 feet
(x2 conductors) is 0.28 ohms.
You could run a #8 feed to the subpanel (I'd use #6 so I would have some
head room for other loads.) Then from the subpanel I'd run 20 and
circuits for the two lifts. With dedicated loads GFCIs aren't required
and I wouldn't run them.
If you put in convenience outlets I'd use GFCIs though.
I haven't found GFCIs that hold up well in saltwater environments so I
prefer to design around them.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.