wiring another 12 amp draw through 15 amp receptacle

Because of the way my inverter is wired-Victron Multiplus 3000-I need to power two 1250watt heaters on my one non-inverter circuit which has two 15amp receptacles. This is for heating while in storage for the winter here in Alaska. The wiring is 12awg 105c. I plan on replacing the current 20amp breaker with a 30amp. As understand my wiring tables, the wiring should be fine for for the intended use. My question is whether the first receptacle can handle the 1250watt load as well as pass on the additional 1250watt load through the terminals on the back safely.

Thanks, Tator

It should be fine, but you can always use the screw terminals as a junction block and put both hot wires on the same screw and both neutral wires on the same screw.

Best practice us to insure terminations are rated for the max wire temperature. IME, terminations go as high as 75deg C.
So, unusual to allow 30A with 12AWG.

30 amp is usually 10 gauge wire. I would not use 12 gauge for 30 amp. Particularly if the boat is unattended during winter storage.

I've seen 15A receptacles that can do 15A passthrough, and 15A receptacles that can do 20A passthrough. You're essentially asking for 20A continuous if not a hair above. On land, code says continuous loads should be limited to 80% so 16A.



I'd go back to the beginning. Start at your source, and follow the path with a fine-toothed comb. EVERYTHING in the path should be ready to handle the continuous loads you're asking for.

I wouldn’t do it. 30amp breaker needs #10. 15 amp receptacles are heavily loaded continuously, unattended.
I would either, reduce wattage, or upsize wire and receptacle ratings. Those continuously loaded circuits are the ones that always seem to have trouble.
I also wouldn’t feel good running that much wattage on a single 30amp twistlock plug continuously.
I realize there might be limitations as to what’s available, but having the whole thing fail halfway through the winter would suck.
If I have to run two heaters on a30a service, I choose ones that move a lot of air, and can run at 750 watts or so. You stay cooler, but the air flow helps a lot.

Thank you for the replies. I have decided to go with another solution to my problem. I'm will be removing one of the breakers from the inverter side of my panel and moving it and the corresponding neutral over to the non-inverter sides to use for the other heater.

Concerning ampacity of 12awg 105c wire, my table shows that outside of engine spaces the wire is rated for 45 amps. Of course, terminations and such should be able to handle the load and temperature. This was my original question concerning the pass-through capabilities of a 15 amp receptacle.

Tator

What table are you using?

1994 ABYC Table V-A. Used it when rewiring our entire boat in 2004. My mentor and guru was a boat electrician in Port Townsend WA named Phil. Can't remember his last name at this time.

12 AWG wire can support up to 45A...if the run is very, very short, voltage drop isn't a concern, and it doesn't ever pass through an engine room.

To get to a 45A rating on a table, you'd probably have to be looking at 7 or 8 feet of total circuit length, an unbundled run, 10% voltage drop being okay, and never touching an engine room space.

Maybe the 10% drop is okay for a heater. I am skeptical that the wiring will never pass through an engine room. And, if have no idea if you are bundling the wires with others or not.

And, I really suspect you are miss reading the table for length. I can barely get out of my panel with 7-8ft.

Before looking up the length in the table, measure the run, including all of the bends, etc -- and then double it.

The tables use the circuit length, not the wire length. For AC this means the sum of the length of the black and white wires, combined. For DC this means the same or the red and black/yellow wires combined.

A 15ft unbundled run, which is a 30ft circuit running at any point through the engine room and required to supply 30 at no more than 10% drop would require 10 AWG.

If the wires are bundled, another aggressive derating needs to happen. If the circuit needs to have less than a 10% tolerance for voltage drop, another delegating needs to happen.

West Marine has this nice table with info about how to derate so you can verify what I write (always necessary) and find a solution given your actual situation.

https://www.westmarine.com/west-advisor/Marine-Wire-Size-And-Ampacity.html

The West Marine percentage drop (3% and 10%) tables specifically refer to 12 VDC, not AC.

Based upon psneeld's post below I decided to run some numbers through the ABYC calculator app.

Even controlling for a 10% drop, they do allow more current for AC than DC for the same conductor size for many parameter ranges.

I am at a loss as to why. But, I stand corrected and have edited this post to avoid misleading anyone.

For the curious, I have attached an AC ampacity table.

I have never met or read anyone that used the same tables for AC and DC.

Not even close in my book.

lots of electricians here, lol. i think tator has a pretty good handle on his wiring, i've seen pictures of the project and it was done quite nicely.
but that 45 amp rating (in all of my books) is for one conductor in free air. if there's 3 in the bundle or raceway, that drops to less than 35. still more than i load my #12 wire. i use 20 amps as my baseline as my bundles are usually quite big, and they always seem to go through the machinery space.

Bmarler said:

but that 45 amp rating (in all of my books) is for one conductor in free air.

Click to expand...

I may have missed it in the thread, but what is the circuit length? What does that mean w.r.t. the voltage drop? And is that acceptable to the user for this application?

If it is a resistive heater, maybe losing more than 10% of the heat it could be putting out to the wiring in the engine room is okay for this install. Maybe it wouldn't be to another user. That's fine and fine.

If it is a compressor based heat pump, maybe not so good for anyone.

In any case, I'm curious about the circuit length. Otherwise I just can sort it in my head.

psneeld said:

I have never met or read anyone that used the same tables for AC and DC.

Not even close in my book.

Click to expand...

I don't understand why, but when I used the ABYC app it agrees with you. It does allow more AC current than DC current through the same conductor size for many parameter ranges.

I need stand to figure out why, but I stand corrected on this one. I've seemingly been oversized my wire.

Its not the ac vs dc. Its the circuit voltage. A 3 volt drop has much more impact on a 12v circuit, than 120v.

Dave,

What is confusing me is that when I used the ABYC calculator app, I controlled for voltage drop by percentage as I explored the parameter space.

In my mental model, skin effect doesn't matter enough to care for these frequency ranges and conductor diameters. (and would favor DC, anyway), so one should be able to use a base ampacity chart, then derate for allowable drop by circuit length, then derate for conductors by bundle size. But, the ABYC calculator didn't bear that out.

I am now wondering if the duty cycle of the AC has an impact that I'm not thinking about. I had assumed using an RMS voltage vs peak voltage would compensate for that well enough (though not exactly) but maybe not. It may be effect that the cooling time during the lower voltage periods more than makes up for heating time during the higher voltage periods. That isn't intuitive to me, but my model for heating copper wire with current is way less well formed than my mental models for current density and voltage drop.

I'm now really curious what's going on with the ABYC's recommendations here, because I don't understand them

STB: There are two forces at work. Insulation getting hot, and voltage disappearing at the load.
Both need to be satisfied.
Yes, on the skin depth on 60Hz, non-existent on DC, which uses the full conductor cross section. AC does not. 60 Hz SD is around 1/3”. So, you need >1” dia cable for that to be a worry.

The gauge table tells you that 45Amps will result in a 105 deg 12AWG conductor, given a certain ambient. Thus the ER limitation, and the bundling limitation.
In a vacuum spacecraft, yet another thermal challenge!

PS. Try using absolute V drop, rather than %. Or, try a solution for a 1 foot run. That will essentially ignore Vdrop

PPS: i usually use Southwire app. It tops out at 75 deg tho. I would not be designing any home or boat circuit to run over 30 deg C rise

diver dave said:

STB: There are two forces at work. Insulation getting hot, and voltage disappearing at the load.
Both need to be satisfied.
Yes, on the skin depth on 60Hz, non-existent on DC, which uses the full conductor cross section. AC does not. 60 Hz SD is around 1/3”. So, you need >1” dia cable for that to be a worry.

The gauge table tells you that 45Amps will result in a 105 deg 12AWG conductor, given a certain ambient. Thus the ER limitation, and the bundling limitation.
In a vacuum spacecraft, yet another thermal challenge!

PS. Try using absolute V drop, rather than %.

Click to expand...

I'm totally with you on all of this. But why were the results I was seeing in the calculator so very different between AC and DC when I controlled for wire temperature rating, wire length, current, and voltage drop as a percentage?

Certainly a 10% drop is more painful for more 12vdc applications than 120vac applications. But, the app only let me control for drop by percentage, so I can't get another type of data point for it. And, in any case, I'm not understanding what I am seeing.

Thanks!

Run the solution with absolute V drop , not %. They should be identical. Its exactly a 10:1 change, in %, from 120 to 12 v.

Its a calculus problem to determine what difference there is on ac vs dc loss in a conductor. I do believe that 120V dc will have very slightly less loss than 120v ac. But i dont think that diff affects the tables.
If the waveform was a low duty cycle pulse, that would be very measurable.

diver dave said:

Run the solution with absolute V drop , not %. They should be identical. Its exactly a 10:1 change; from 120 to 12 v.

Click to expand...

Sadly, their calculator just gives me a choice if 3% or 10%, no ability to tune that more.

Can. You help me to understand why voltage dropped by the wire is fixed not a percentage of the voltage across the wire?

Conductors have no idea what voltage is applied. V=IxR

So, given a fixed length (fixed resistance), and fixed current, the Vdrop is a constant. Whether it is a 12v dc system or 238kV ac powerline. System voltage is not in the equation.

diver dave said:

Conductors have no idea what voltage is applied. V=IxR

So, given a fixed length (fixed resistance), and fixed current, the Vdrop is a constant. Whether it is a 12v dc system or 238kV ac powerline. System voltage is not in the equation.

Click to expand...

Oh. Indeed. I was specifying the voltage across the circuit to the calculator's interface and the current through the circuit to the calculator's interface. I was not specifying the voltage across the wire, which obviously changes with the load drawing the current.

Yes, this all makes sense now. Thank you!

Thanks for all the info. Much of it is over my head.

Quoting Nigel Calder "Onboard a boat, voltage drop is not such an issue with AC circuits because of the higher voltage..." I used a 3% voltage drop on all my new DC runs and took into consideration the length of the runs going and returning. All wire both AC and DC is 105c rating. I Paid no attention to the length on AC runs in considering drop or ampacity.

Tator

In regards to running two 12.5a heaters on 30 amp shore power, I've been doing it for 16 winters here in Alaska with no problem. I check both ends of the cord every year as well as the receptacle on both ends as well as the wiring involved. All look new. Ambient temperature might have something to do with that.

Tator

Tator said:

Thanks for all the info. Much of it is over my head.

Quoting Nigel Calder "Onboard a boat, voltage drop is not such an issue with AC circuits because of the higher voltage..." I used a 3% voltage drop on all my new DC runs and took into consideration the length of the runs going and returning. All wire both AC and DC is 105c rating. I Paid no attention to the length on AC runs in considering drop or ampacity.

Tator

Click to expand...

Indeed. I guess someone just pointed me at that 12v 10% table and told me to use it for AC and it sort of aligned with what I'd used in a house and it made sense to derate the ampacity table for voltage, so I never questioned it.

Tonight, as a bunch of people noted, I didn't think it through very well. Drawing it out would have made it clear in 2 seconds.

I'm in a very different time zone than normal. I'd like to think I'd have done better more awake!

In any case, I'm thankful for your post and for psneeld and diver dave for straightening me out!

Tator said:

In regards to running two 12.5a heaters on 30 amp shore power, I've been doing it for 16 winters here in Alaska with no problem. I check both ends of the cord every year as well as the receptacle on both ends as well as the wiring involved. All look new. Ambient temperature might have something to do with that.



Tator

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Your shore power cord is not 12ga wire I am going to believe. Its all about the amps not the volts. Im just tossing this out there I do not know where you are for considering how to power your heaters as of yet. There is a certin voltage drop that is the same across the bord if you lose 2v on a 12v circut you still will loose 2v on a 120v circut. Going up like that yes the amps go lower and your percentage of voltage drop goes lower goving you a better threshold. Hence 120v has small loss with a 30' run compared to a 30' run at 12v. The voltage loss is less harmfull at higher voltages because of the allowable threshold to power such device. Volts carry the Amps.

Atrached just as a example to show the differenceof voltage drop.

Tator said:

In regards to running two 12.5a heaters on 30 amp shore power, I've been doing it for 16 winters here in Alaska with no problem. I check both ends of the cord every year as well as the receptacle on both ends as well as the wiring involved. All look new. Ambient temperature might have something to do with that.

Tator

Click to expand...

You might be right regarding the ambient temperature. I’ve lived aboard for thirty years or so, and know what can happen when you run at the limits of a thirty amp service on a continuous basis. My temps aren’t as low as yours though.
It seems like when all the equipment is new, it lasts for quite awhile. (Years) then when you least expect it, you melt an end.

Marinehowto.com has an excellent article on shore power plugs and loading.