+ and -Inverter DC cables both through breaker box?

Bigwooo
Bigwooo Solar Expert Posts: 62 ✭✭
in Advanced Solar Electric Technical Forum #1
I have a trace DC250 disconnect breaker box with two- 250 amp breakers. My breaker box is mounted on the wall above my AGM batteries and inverters, so the 4/0 DC wires go from the batteries up to the 250 amp breakers in the breaker box, then back down to the inverters, about 5.5 feet.

Is it necessary to run the negative cables up through the breaker box along with the positive cables, or can I have the positive cables go to the breakers and back down to the inverters while the negative cables go directly from the batteries to the inverters.

I'm curious if the positive and negative cables have to be paired for the entire distance and of the same length. If I take the negative directly to the inverters, the negative wire will be about 30 inches while the positive cables will be about 5.5 feet. Is that a no no?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: + and -Inverter DC cables both through breaker box?

    AC requires wires to be paired (if the wire follow different routes--any metal between the AC wires can pick up inductive currents). Also, wires run in pairs have lower inductance if they are keep together.

    DC do not require paired runs...

    However, for any wiring, it is better not to make a big circle with any "loop" of wire. It acts like an antenna and can either broadcast noise (hash from the inverter) or pick up noise (like a nearby lightning strike and cause damage to the connected devices).

    In your case, where the wire path lengths are so different... I would probably just keep the pairs together (start and destination) where it makes sense--then for the + that takes off to the breaker and back; keep those close together; up to the breaker and back down (keep the "area between the wires" small). Should work fine (assuming all of the other issues with current, wire gauge, etc. are all correctly done).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • nigtomdaw
    nigtomdaw Solar Expert Posts: 705 ✭✭
    #3
    Re: + and -Inverter DC cables both through breaker box?

    If you install the -ve earth bonding block designed for the Dc250 you can keep the wire runs a similar length have a DC shunt fitted also for a battery monitor and keep the +ve and -ve wires similar length and close together which helps reduce inductance as Bill has already stated .
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #4
    Re: + and -Inverter DC cables both through breaker box?

    There are two good reasons to route the negative along with the positive cables through the breaker box.

    The first is keeping the two lines close together which reduces the effective inductance of the cabling to the inverter resulting is less possible ringing on surge loads (better for your inverter). It also cancel some of potential interference from the cables as mentioned in earlier post.

    Second, if you don't route the negative through the box and if the box is steel you may find the cover on the box buzzing from time to time. This is due to the strong magnetic field created which will shake any loose metal on the box. With the negative inside with the positive cable it will cancel much of this magnetic field.
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  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    #5
    Re: + and -Inverter DC cables both through breaker box?
    RCinFLA wrote: »
    Second, if you don't route the negative through the box and if the box is steel you may find the cover on the box buzzing from time to time. This is due to the strong magnetic field created which will shake any loose metal on the box. With the negative inside with the positive cable it will cancel much of this magnetic field.

    If there is an AC component on the DC cables (there will be) and the two conductors aren't run through the same opening for a metal box, it's possible that inductive heating will occur.

    Always, always, always keep all wires for a circuit together.
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