hot wires

sueco
sueco Registered Users Posts: 22
I have 4' 4gauge wires from the 1500W inverter to the 12V battery bank. When I make coffee (900W for about 5 minutes), the wires get unbelievably hot. I would think that 90 Amps for 4 gauge at 4' is nothing - certainly, the wiring charts says so.

It's a stranded wire. My intuition told me that the solid wires would be best, but, of course, they are hard to work with, and I was assured that the stranded wire would be fine.

Thoughts? Should I be concerned? enough to change this?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: hot wires

    Yes you should be concerned: wires should not get hot to to the touch.
    Here's some of the reasons why they do:
    1). 900 Watts / 12 VDC = 75 Amps. 4 AWG maximum current for transmission: 60 Amps (Reference: http://www.powerstream.com/Wire_Size.htm)
    2). Longer wire, greater Voltage drop, more current.
    3). Heavier load, greater Voltage drop, more current.
    4). High resistance in connections, greater Voltage drop, more current.

    Basically, running high Amperage things off a 12 Volt system is asking for trouble.

    BTW, Rule #1 of off-grid systems is "don't use any electrical heating device".
    Rule #2 is "pay attention to Rule #1". :p
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: hot wires
    sueco wrote: »
    I have 4' 4gauge wires from the 1500W inverter to the 12V battery bank. When I make coffee (900W for about 5 minutes), the wires get unbelievably hot. I would think that 90 Amps for 4 gauge at 4' is nothing - certainly, the wiring charts says so.

    It's a stranded wire. My intuition told me that the solid wires would be best, but, of course, they are hard to work with, and I was assured that the stranded wire would be fine.

    Thoughts? Should I be concerned? enough to change this?
    I don't know the root cause of what you are seeing, but it's not that the wire is stranded rather than solid.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: hot wires
    Yes you should be concerned: wires should not get hot to to the touch.
    Here's some of the reasons why they do:
    1). 900 Watts / 12 VDC = 75 Amps. 4 AWG maximum current for transmission: 60 Amps (Reference: http://www.powerstream.com/Wire_Size.htm)
    2). Longer wire, greater Voltage drop, more current.
    3). Heavier load, greater Voltage drop, more current.
    4). High resistance in connections, greater Voltage drop, more current.
    Isn't another contributor that the current in the wire from his charge controller to the batteries is the load current plus the charging current? Assuming he's making coffee in the morning at the batteries' daily low point, the charging current could be substantial.
  • sueco
    sueco Registered Users Posts: 22
    Re: hot wires

    So, I need some help, here. I read the wire sizing for 4 gauge as being 125 Amps (upper limit), ignoring (??? because I'm ?educated? ?menopausal? :-) ) the concept of "power transmission". Why is that so different. For example, I have 4 gauge from battery to battery - those wires don't get hot. My thinking - it's all the same system - electrons swimming from battery to battery, up to the inverter, then out to my appliances. But, I hear you - and had already figured out something wasn't right, thus, this post: there is something wrong using the 4 gauge for the inverter, although it is fine for the batteries, even though it's the same system. But why?

    I have already purchased an additional solar panel (making 4 total), so I can move to 24 volt. That will lower the current to well under the 60 Amp limit. Still, I'd like an answer to the question (above).

    Perhaps there's a thread already about electric heating - I've already figured out that electric heating is inefficient at best, and stupid, in general.... However, I love coffee.... I'll start a separate thread, because this is also important.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: hot wires
    ggunn wrote: »
    Isn't another contributor that the current in the wire from his charge controller to the batteries is the load current plus the charging current? Assuming he's making coffee in the morning at the batteries' daily low point, the charging current could be substantial.

    This would depend on the actual physical wiring involved. Normally charge current is carried from the controller directly to the batteries on one set of wires, while another, heavier set supplies power from the batteries to the inverter. The charge current thus does not offset the load current in any way.

    And the calculation I did was based on nominal Voltage: battery Voltage in itself varies according to state of charge and load.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: hot wires

    The Ampacity for 4 AWG Copper wire should be 95 amps, if it is building wire THHN -- the standard quad-rated wire. AL wire would be 75 A.

    HOT is a relative term. It could be warm at this current level. ggunn had a good observation about current being higher when battery charge is low. And if the batteries are straining to meet the demand, that, too increases the current.

    Also, please check for loose connections, as they add to the drop, and the heat of a loose, higher resistance connection could be conducted along the largish 4 GA wire, and so on.

    I use an electric coffee maker, and a toaster. BUT, for making the morning brew, a propane stove, or even single burner would probably be much more efficient, as was noted above.

    Good Luck with the upgrade to 24 V. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: hot wires
    sueco wrote: »
    So, I need some help, here. I read the wire sizing for 4 gauge as being 125 Amps (upper limit), ignoring (??? because I'm ?educated? ?menopausal? :-) ) the concept of "power transmission". Why is that so different. For example, I have 4 gauge from battery to battery - those wires don't get hot. My thinking - it's all the same system - electrons swimming from battery to battery, up to the inverter, then out to my appliances. But, I hear you - and had already figured out something wasn't right, thus, this post: there is something wrong using the 4 gauge for the inverter, although it is fine for the batteries, even though it's the same system. But why?

    The current handling capacity of wire is a nightmare to understand. It's not a simple linear thing. Much depends on the application and the wire's ability to dissipate heat: put it in conduit and its rating goes down. The chart I quote actually gives conservative numbers, and in some instances the same size wire can handle more current. But it always assumes a steady Voltage and fixed resistance. Those don't exist in the real world. Power transmission is the type of application here, and you have to remember that the length of wire is really the positive and negative cable lengths added together because that is the complete circuit and therefor the total length of wire. Under "ideal" conditions that 4 AWG would handle 92 Amps (by my secret formula which I never reveal). However, the more Amps it handles the hotter it gets. When wire gets hotter, guess what? It's ability to carry current goes down.
    I have already purchased an additional solar panel (making 4 total), so I can move to 24 volt. That will lower the current to well under the 60 Amp limit. Still, I'd like an answer to the question (above).

    Perhaps there's a thread already about electric heating - I've already figured out that electric heating is inefficient at best, and stupid, in general.... However, I love coffee.... I'll start a separate thread, because this is also important.

    Search for "coffee maker" and you'll find some threads with suggestions for non-electric makers. I think Icarus recommends one made by Coleman. Propane is your friend in heating off-grid. :D
  • sueco
    sueco Registered Users Posts: 22
    Re: hot wires

    I brew (electric) coffee in the middle of sunny days (we have enough of those), and then reheat (propane - easy to do) the next morning. Thus, the high demand is in the peak charging. Perhaps you could say that I was smart to figure that out. However, it doesn't work any other way? :-)

    Thanks for your help - the coffee maker issue is going on another post.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: hot wires
    sueco wrote: »
    I brew (electric) coffee in the middle of sunny days (we have enough of those), and then reheat (propane - easy to do) the next morning. Thus, the high demand is in the peak charging. Perhaps you could say that I was smart to figure that out. However, it doesn't work any other way? :-)

    Thanks for your help - the coffee maker issue is going on another post.
    Consider this:
    http://www.toddycafe.com/about/

    You process a pound at a time with cold water overnight, which makes a concentrate. Dilute and heat in the microwave, or dilute with boiling water. It's how I've made coffee for years, and it's really good. Better than drip and MUCH better than perked.
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: hot wires

    You likely have a voltage slump on the battery with that heavy a load. You may still have enough voltage to prevent the inverter low voltage alarm from tripping but the lower voltage will cause even more current to be drawn from battery to maintain the 900 watt load.

    You probably have about 11.6 vdc at inverter terminals depending on battery size and condition. With inverter efficiency and voltage slump you are pulling about 97 amps.

    You will have about 19 watts total heat generated in the two 4' #4 wires. That is about 9.5 watts each. They will get fairly warm.

    You may have extra heating due to insufficient contact at the clamp terminal points. If this is the case the wire will be hotter near the connection ends and the terminals will be hot.

    The NEC wire amperage maximum is based on heating per foot and the ability of wire to dissipate the heat based on insulation and any conduit that may restrict air flow. It is roughly around 2 watts per foot but is modified based on insulation and conduit which effects the ability to dissipate heat. For low voltage wiring the maximum amperage should be lower to avoid too much voltage drop.