MauiPunter

2 x 25A => 50A circuit?

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I have two 25A circuits on my breaker panel that I would like to combine into a single 50A switch.  One is for FWD HEAD and the other is AFT HEAD, and I would just like to free one more slot on my panel by combining these into a single 50A switch.  Is this possible/advisable?  Any issues I need to be concerned with.  There is never a situation I can think of when I only turn on one and not the other.

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The breaker is to protect the wiring - not the end user. Is the wire going both ways sized to safely carry 50 amps? Protection of the end users is via fuse/breaker on that item. So probably no, it is not OK.

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23 minutes ago, Mitre cut said:

The 50A would only be needed if both were being used at the same time, how often does that happen? 

 

Not sure, but its good to have flexible options.

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3 minutes ago, longy said:

The breaker is to protect the wiring - not the end user. Is the wire going both ways sized to safely carry 50 amps? Protection of the end users is via fuse/breaker on that item. So probably no, it is not OK.

I have to assume the wiring is for 25A.

What is the difference between the specs on 25A wiring an 50A wiring?   How can I check whats installed?  Should cable be labeled?

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I noticed its labeled 10 AWG.  I assume that's 30A cable?   Seems like I should leave the switch at 25A, possibly upping it to 30A.  But, can I combine the two heads onto one circuit?  Is there a reason they should be separate?  If both toilets flush at the same time, does it double the amperage on the wire?

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  Say you have a 20 amp draw and 30' of wiring (measured out & back).  20 amp would require 12 gauge AWG at 10% voltage loss (OK for motors). I always go by the 3% voltage loss tables, as input voltages on a boat are often low to start with, so 20 amp needs 6 g .

  10 g for your use is good sizing, can safely carry up to 30 amps at 10% voltage loss.

Wire gauge size in low voltages is very connected to total circuit length due to voltage loss

10 g wire has an ampacity of 60 amps, so technically your 10 g wiring could be protected by a 50 amp breaker. Not much safty zone, tho

Can you measure the actual amp draw of your heads?

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Its drawing right around 18A when I flush one toilet.

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6 minutes ago, MauiPunter said:

Its drawing right around 18A when I flush one toilet.

Damn

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1 minute ago, Raz'r said:

Damn

Is that good or bad?   Its a macerating freshwater head.

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Just now, MauiPunter said:

Is that good or bad?

Impressive load is all...

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  There are 40 amp breakers available thru Blue sea, if not most companies.Should be OK as long as batt voltage stays reasonable.

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Just now, longy said:

  There are 40 amp breakers available thru Blue sea, if not most companies.Should be OK as long as batt voltage stays reasonable.

Ok.  So, you picked 40A because 18A x 2 = 36A and 40A is the next size up?

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Yes. Lowest possible amp rating since you're so close to max ampacity rating (for new, healthy wire) on an old wiring loom.

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Thank you.   I appreciate the guidance.

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Hi, I just replied to your later post on generator size.

Everything that Longy says above is right.

I'd add that if you're going to load your cable to a current approaching the cable's rating, make sure that both cables are not bundled with other cables, which is asking a lot on a boat.

The cable needs to be able to get rid of the heat, the heat being the critical factor in determining current rating.

If the load is only the head, then it's a transitory load and heat won't build up.

Your macerator will momentarily pull 6 times it's normal current on start-up but, again, not long enough to cause any grief.

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I would not double the size of the breaker, primarily because it's not necessary. You probably have thermal breakers and they require time to trip. Because your heads only run for a very short period of time per-flush, they never generate much heat in the breaker. Combine that with the odds of flushing both heads simultaneously being approximately 0% and you're much better off just running the two existing circuit legs onto one of the existing breakers.

If you get plagued by a tripping breaker, then consider alternatives.

 

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I wouldn't do this, not enough safety overhead should a terminal corrode or loosen, etc. What I did to free up panel space after replacing all my bulbs with LEDs was to combine a bunch of lighting circuits onto just a couple of breakers. The LEDs draw ~20% of the incandescents they replaced but the wire gauges stayed the same so this was a safe route.

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By many standards 18 amps on a 20 amp breaker is already too much anyway, but for short time periods it won't cause a problem depending on the breaker.  A load like that probably should be on a 25 or 30  (18A x 125%) depending on the wire used.  Do the current ones ever trip?

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Comments on comments: Istream has the best work around if you can do it. MD hopes there is no morning rush to the heads and that if the breaker does trip the crew can reset it & re-flush on their own. HG is correct on presence of start up surge, amount is unknown, age of motor & age of wiring will increase surge.

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1 hour ago, longy said:

Comments on comments: Istream has the best work around if you can do it. MD hopes there is no morning rush to the heads and that if the breaker does trip the crew can reset it & re-flush on their own. HG is correct on presence of start up surge, amount is unknown, age of motor & age of wiring will increase surge.

Wiring and equipment is 3yrs old.

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6 hours ago, Moonduster said:

Could you name three of those "many standards"?

I'll take that as sloppy wording and not technically correct, my apologies. The 80%/125% rules tend to apply to the wire, and since these are coordinated systems then require sizing the OCPD accordingly. These cases don't necessarily apply here but:

NFPA 70 210.3 and 210.19 Could imply 125% (this also ends up in other sections of the code, 690 for example)

NFPA 79 mentions 125% for the circuit conductors many times.  This standard isn't very broad and is a little ambiguous, but it is in there.

You could infer 80% from manufacturers derating factors: (diversity factors, more than 4 devices then you are less than 80% for an Eaton brand UL 489 breaker anyway) and at least the common Bluesea breakers are manufactured under the UL 489 standard.  There is also temperature derating that could apply.

UL Labeling standard:  "Unless otherwise marked for continuous use at 100 percent of its current rating, a circuit breaker is intended for use at no more than 80 percent of its rated current where in normal operation the load will continue for three hours or more."  (hope the head isn't on for more than 3 hours!)

For the fun of it I tried the Bluesea circuit wizard, and for an 18 amp load at 20 feet, with 800CCA it recommends 10AWG and a 32 Amp breaker.

Again, a lot of this is based on continuous loads and isn't the standard  in the case of a boat head.. 

Honest question here:  when would you select a breaker this close to the load current?  I could see special cases where you are trying to protect the device but in my experience this ends up resulting in nuisance trips and eventually wearing out the breaker.  The breaker in this case is generally there to protect the wire against overload and against short circuits.  on a boat anyway 12AWG is good for around 30 amps according to ABYC, so a 25 or 30 amp breaker seems like the best fit.  In this specific case would a 30 amp breaker not provide better service than a 20?  I suppose if the 20's are working fine then go with them, and sure put both heads on 1 breaker.  Its a rather pedantic topic but I'd be interested in a long term follow up. 

 

 

Edit:  thinking about the breakers got me distracted.  The "right" answer is to follow the instructions from the manufacture of the head, no?

Here is an Example from Raritan, 18 Amps

 

 

Clipboard03.jpg

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the post with all the details of different "standards" is spot on.  It can be confusing - i would follow the head manufacture's guide lines.  

Yes you would be ok if you wired the 2 together with only one head being used at a time.  That is a good temporary solution as you are limited by your panel.  Long term you really need to upgrade your panel.  I would personally keep the one on the panel and put the other breaker in another spot perhaps behind the panel or in another area clearly marked as to what is is.

 

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On 9/13/2018 at 2:39 PM, longy said:

The breaker is to protect the wiring - not the end user. Is the wire going both ways sized to safely carry 50 amps? Protection of the end users is via fuse/breaker on that item. So probably no, it is not OK.

That is half true. If the wire is the correctly sized for 25 amps or rated higher the load item(head) will cook if its overloaded. That could possibly occur if someone took a larger than design shit. The correct wire would overheat loose its cover, possibly start a fire and otherwise turn to shit. The oversized wire would handle the excess load from the head, except the breaker wouldn't trip until it hit 50 amps. If somebody took a 50 amp shit then the wiring in the boat is the least of your worries.

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This will be the new standard by which we measure shits.    "Yesterday I had a 5A shit"...  "Aww, that's nothing, I had a 10A shit today".

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On 9/13/2018 at 1:17 AM, MauiPunter said:

I noticed its labeled 10 AWG.  I assume that's 30A cable?   Seems like I should leave the switch at 25A, possibly upping it to 30A.  But, can I combine the two heads onto one circuit?  Is there a reason they should be separate?  If both toilets flush at the same time, does it double the amperage on the wire?

Let's just go with worst case.  One of the wires break.  Now you have a very real fire hazard as your single wire is not large enough to safely carry 50 amps for periods of time - especially since 50 amps sounds like shore power that would run pretty continuously.

Pulling new wire should be pretty easy - you've already got the old stuff to act as a lead to pull the other larger wire through.

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5 hours ago, Grrr... said:

Let's just go with worst case.  One of the wires break.  Now you have a very real fire hazard as your single wire is not large enough to safely carry 50 amps for periods of time - especially since 50 amps sounds like shore power that would run pretty continuously.

Pulling new wire should be pretty easy - you've already got the old stuff to act as a lead to pull the other larger wire through.

The toilets are DC not AC.  Each wire from the DC distribution panel is 10AWG to each head.  Each head draws 18A when flushing.  If both toilets are flushing at the same exact time, that's 36A total at the DC panel.  However, does that mean each wire carries the full 36A or just the individual draw of 18A, and only hits 36A at the panel when combined at the breaker?  A 40A breaker should handle that scenario.  The reality is that just about never will both toilets flush for 10 seconds exactly the same time.  If I leave the breaker alone at 25A, and just let it break in the off chance two people flush at the same exact time, then it can be rectified easily without concern about damage or fire hazard.   So, I am most likely going to combine, and free up the space, and just deal with the off chance two people flush.  I could probably safely upgrade to 30A and give myself a little buffer as the start amperage is higher than the sustained amperage, so, it spikes to 18A on startup of the macerator/pump, but then drops to 15A after a couple seconds.

 

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Look up the breaker "trip curve."  If it is anything like the UL489 breakers you see in industrial environments (many Bluesea breakers under this standard) then 36 amps can be drawn for 120 seconds on a 25 amp breaker.  In other words, you need two shits, at the same time, that are both 18 amp shits, that both take 2 minutes to flush to trip the breaker.  The wiring will be fine since each is pulling an 18 amp shit. 

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Size 10 wiring can handle 36 amps without a problem.  But the issue is what happens when one of those wires breaks and shorts to ground.  In that case the wire may light up before the breaker trips.  Just sayin'.  The rules are there for a reason.  In all likelihood you'll be fine, but you'll never get me to recommend doing it.

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Hmm. How long is the flush / macerate cycle? What are the odds of synchrony/overlap?

Use a pair of relays so that when macerating one load, the other head won't flush, separating the peak current loads. This is perhaps simpler than just training the users not to synchronize flushes, or gestate on 50-amp dumps until the very last second.

Randii

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Well, short of potty training your guests, putting in a no flush relay to leave one or both stranded, specifying the size and amp load of their respective shit size...

...you could hire a pro to install a new panel w extra breakers and/or pull any undersized wiring.

Dont ask the old panel/wiring to handle the upgrade loads it wasn't designed for. If it sounds and looks like a short cut, it might start to smell like one...at the moment you least expect it.

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3 hours ago, fufkin said:

Well, short of potty training your guests, putting in a no flush relay to leave one or both stranded, specifying the size and amp load of their respective shit size...

...you could hire a pro to install a new panel w extra breakers and/or pull any undersized wiring.

Dont ask the old panel/wiring to handle the upgrade loads it wasn't designed for. If it sounds and looks like a short cut, it might start to smell like one...at the moment you least expect it.

The wiring is all new between toilets <=> panel <=> batteries.  10 AWG is apparently over sized for the 18A draw of each toilet.  The 25A breaker is on the edge if both toilets flush.  Seems like it should be ok.  

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Not an electrician, but it seems to me that the purpose of the breaker is to protect the wiring from overload, right? So putting on a bigger breaker on the same wiring seems like exactly the wrong answer.

It does seem to me (again, not an electrician) that you could wire both heads into the same 25A breaker on the panel. The wiring to each individual head should be fine since the draw to each would be 18A. On the (probably rare) occasion that both heads were flushed at once, the wiring shouldn't overload but the breaker might trip.

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I agree no safety issue writing both to the same 25A breaker.  Maybe it will trip if both are flushed at once and maybe it won't.  If it's a problem, you can always:

1. Use a larger breaker up to the point where it does become a safety issue by exceeding the ampacity of the wire.

2. Use a sub-panel.

3. Use a larger breaker combined with a new run of larger wire.

4. Use a pair of fuse blocks to each head just after a larger breaker, with the short breaker to fuses wiring sufficient for the larger breaker, but the existing 10g wire kept, as it will be sufficiently protected by 25A fuses.  This should be cheaper, smaller, and easier to install than a new sub-panel.

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All shit aside. What is the shittiest consequence of putting two loads on the same line? The breaker will trip while both shitters are being flushed. This would obviously cause both to clog with shit. Cleaning up two loads of shit  is worse than one. 

The wire won't melt. The breaker will trip. You will be cleaning up 2  shitty shitters. Keep in mind that simply having one typical 18 amp shit would allow for a simultaneous  7 amp piss. Considering that you are on a yacht, there is higher chance of someone taking a beer piss. Based on the data that could be as high as 10 amps. It will trip the breaker. So the limiting factor may be beer consumption not amperage of shit.

In that case you would be cleaning up one shitty head and a cabin sole of beer piss. Depending on type of beer that could be a challenge. Data indicates that watery lager(Heiny) is similar to regular piss. The greater the hops(IPA) the shittier the smell. Stout is approximately the same as a diesel spill in the bilge. 

Its a tough decision. The shit will be on you if it all goes shitty.  

 

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You keep claiming that 10AWG wire is sufficient for the loads, but you don't indicate the length of the wire so it's not clear that's the case.

Ampacity for 10AWG wire is around 30A. This is why you should not install a breaker rated for 2x your 18A requirement. If you do, you run the risk of starting a fire if you develop a short on one of the two legs.

10AWG wire conducting 18A @ 13V and allowing for a 5% voltage drop will run 10.6m, round trip or 17.5' from breaker to macerator. Allowing for 10% drop, you can double that distance. It's probably ok but warrants some approximate measurements based on the actual wiring routing from breaker to load.

Combining the two loads onto a single breaker and using a breaker in the 20-25A range should work fine and be safe.

 

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9 hours ago, Moonduster said:

Combining the two loads onto a single breaker and using a breaker in the 20-25A range should work fine and be safe.

 

That is the plan.  Going to keep the 25A breaker for now.   I had already decided after your last post that upgrading the breaker to 40A is not proper.

Fyi, The length is around 10', 20' round trip.

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