Using a two-pole breaker to interrupt a single-leg circuit

In a system I'm cobbling together, there will be a 120VAC to 24VDC battery charger. The charger will sometimes be powered by plugging into the grid and sometimes be powered by plugging into a generator. The charger comes from the marine power industry, where they tend to be a bit cautious and mistrusting of the wiring jobs associated with the "shore power" supply. One of the things they recommend doing is using a breaker than will break both the hot and neutral leg of the shore power feed. This insures that the power is really and truly OFF when you flip the breaker (vs it only breaking the hot with the neutral still connected) and that it will catch overcurrents on both the hot and neutral legs (vs only protecting the hot). They seem to do this, basically, to protect against some moron on shore wiring the extension cable with the hot and neutral reversed. I see their point. While the thing I'm building right now is currently using an extension cable I've made plugged into a generator (or grid circuit) I've checked out personally, one of the purposes of this system is to be deployed into disaster areas to power radio operations. It's entirely possible some moron might hook this up to a bad cable or a bad generator at some point in the future, so I'd like to take a page from the marine industry and break both legs.

However, I'm not entirely sure how to facilitate this with the components I have to work with. Between the incoming AC power feed and the charger is a Square D QO load center to serve both as an incoming power disconnect and as overcurrent protection. To me, the simple answer is to use a 2-pole breaker. Since the two poles of the breaker are isolated from each other, I can run the hot through one and the neutral through the other. My concern is that I have no idea how this affects the breaker tripping behavior. Does a 2-pole 50A breaker trip at 50A on either leg? Does it trip at a sum of 50A on both legs? To do what I want to do, do I have to instead use two 1-pole breakers of the required rating (the load center I'm using would actually allow that keeping both legs fully isolated)?

I just don't have enough knowledge of how a 2-pole breaker works to figure this one out on my own. Thanks, guys.

Comments

  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Using a two-pole breaker to interrupt a single-leg circuit
    williaty wrote: »
    I just don't have enough knowledge of how a 2-pole breaker works to figure this one out on my own. Thanks, guys.
    Probably the easiest way to think about it is as two almost completely independent circuit breakers, each measuring current in its own wire, but mechanically connected inside, not just at the handle, so that when either one trips it also trips the other. The two sections can be on opposite phases, the same phase, or even one hot and one neutral without any effect on the current sensing.
    The one slight possible exception is that if the slow trip in the breaker is thermal, it might trip just a littler sooner if both legs are slightly over current compared to only one overcurrent and no load at all on the other.

    But using one side of breaker mounted in a panelboard can be a little tricky, since a standard split phase panel connects alternating breaker sections to opposite input wires. This is intended be the two sides of 240/120 volts, with the neutral wired to a bus bar instead. To get this kind of breaker to interrupt the neutral, you would have to wire up the neutral wire to one of those two phase busses so that it will connect to one side of one half of the double breaker. This may not be acceptable under the electrical code and will definitely confuse someone seeing it for the first time. That might lead to wiring errors or a hazardous mistake if somebody else works on the panel.
    A separately mounted double breaker could avoid this problem.
    SMA SB 3000, old BP panels.
  • williaty
    williaty Solar Expert Posts: 60 ✭✭
    Re: Using a two-pole breaker to interrupt a single-leg circuit

    The load center in question is so simple I don't think there would ever be any confusion. It's a Square D QO24L70. There's only two positions inside of it. Each has what would normally be the supply lug directly above the breaker it's connected to so there's a very straight-forwards "this goes in, then goes out" appearance to the layout.

    So it sounds like if I need a 50A rated breaker for each leg, using a 50A 2-pole is a reasonable thing to do based on what you've said. I really wasn't sure how they worked inside and if the thermal issue of only one wire seeing current was likely to make it function as though it were much higher rated than its label.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Using a two-pole breaker to interrupt a single-leg circuit

    "Only one wire" does not see the current: both do. Current through the circuit is the same on both wires.
    A 240 VAC "double" breaker will trip with over-current on either side, as it has to protect against either leg becoming shorted not just the current as a whole exceeding the rating.
  • svNorthStar
    svNorthStar Solar Expert Posts: 47
    Re: Using a two-pole breaker to interrupt a single-leg circuit

    williaty... yes you can do as you describe. We live on a boat and our incoming 110 V ac from a marina for instance requires both legs be broken. ABYC....the wiring code for the marine industry requires either all 110V breakers be double pole, or if a sub panel, you can have a dpdt on the feed to the sub panel and then SP in the sub panel.

    You would be amazed at the number of "hot marinas" we encounter. Basically, someone with a wiring problem makes the entire dock hot. The electrolysis in some marinas will eat the zincs off your boat in a month, instead of the 6+ months you normally get.

    We usually check our ac upon arriving at a marina. We have found many with reverse polarity and worse. We also drop a DVM with a long lead on it from both the ground and neutral at the dock pedestal. Many times we will end up using shore power very intermitttently, or in the case of Cartagena not at all.

    The only caution would be what inet said, it may be a little confusing in a panel box to the next guy. Stand alone breaker mounted no problem. We always plan for the worst and celebrate better when it actually happens.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Using a two-pole breaker to interrupt a single-leg circuit

    I don't know about your marine system... But (few/some/all?) large ships use "floating" 120/240/etc. AC voltage to prevent return current from flowing through the hull and causing corrosion and other issues. So, they would use a double pole breaker for safety if either leg ever gets shorted.

    They would also put a set of small indicator lights from line A to line B, Line A to ground, and Line B to ground.

    The first indicates the circuit is hot, the second two each glow at 1/2 brightness if the output is still floating with respect to ground. If one line gets shorted, then one of those Line to ground lights glows bright, and the other turns off--indicating a fault somewhere in the system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Using a two-pole breaker to interrupt a single-leg circuit

    One of the tools that everyone should have: http://www.canadiantire.ca/AST/browse/6/Tools/2/Electrical/ElectricalTesters/PRDOVR~0520027P/Gardner-Bender+GFI-3501+GFCI+and+Receptacle+Tester.jsp?locale=en

    You may have to do a bit of adapting to suit your particular application as they are mean to plug in to a standard outlet and the shore power may be high-Amp (or even 240 VAC) but there's nothing like checking the wiring before you hook to it to be sure it's safe. Heck I'd even test the Voltage first. You just never know what someone else has done. :roll:
  • svNorthStar
    svNorthStar Solar Expert Posts: 47
    Re: Using a two-pole breaker to interrupt a single-leg circuit

    Yep, correct on both counts. I forgot to mention, that per ABYC, you can use SP breakers for you AC circuits IF, and only if the master AC breaker is a DPDT and as BB noted there is a reverse polarity indicator on the panel that lights if there is voltage on any leg other than the hot leg. Here is a link to a pretty example of the problem and the differences of wiring between boats and houses. You can get away with strapping ground and neutral in houses, bad in boats.

    https://homeport.uscg.mil/cgi-bin/st/portal/uscg_docs/MyCG/Editorial/20120326/Neutral-Ground%20Bond,%20Exchange%20Article,%2012-2005.pdf?id=58e8bde5ede509bccd505aea26528f0e5c9bdc18&user_id=be1ba304989cdf8d1bc6794a5af75c39

    The outlet tester cariboocoot shows, we have 4 on board just in case and use them everywhere we plugin.
  • williaty
    williaty Solar Expert Posts: 60 ✭✭
    Re: Using a two-pole breaker to interrupt a single-leg circuit

    I actually found a male plug end at Lowes that does the circuit check. Glows green for correct wiring, red for switched hot/neutral or anything wrong with the ground. It's going on this project, but it never hurts to have all your ducks in a line in case someone doesn't notice a red light some day.