DC circuit in metallic conduit, unwanted induction

[BretHeilig]

Hi,

I've always been told that when running a solar DC output circuit (typically ~7A @ ~400V), if any conductor passes through a metallic conduit, that another conductor on the same circuit with current flowing the opposite direction must also be in the same conduit (ex - the positive and the negative); that if this is not done, there will be undesirable induction on the conduit.

I know induction can only occur when there is a change in current flow. On most inverters, this happens on a programmed basis, and will happen about every 15 minutes, and the change is usually very small (< 2%) though it can sometimes be significant (for example if a cloud passes overhead). The direction of current flow never changes (as it does in AC).

The problem is, this requirement can be a serious pain sometimes, and I find myself questioning whether it is a real issue, or just superstition. Does anyone have any insight?

 

[iwire]

The problem is, this requirement can be a serious pain sometimes, and I find myself questioning whether it is a real issue, or just superstition. Does anyone have any insight?

I do not think induction an issue with DC however 300.3(B) of the NEC requires all the conductors of a circuit to be contained in the same raceway or cable. No exception is granted for DC circuits.

 

[charlie b]

I can address this point:

The direction of current flow never changes (as it does in AC).

True, but the magnitude of the current can change in a DC system. Any current, including a DC current, will create a magnetic field around itself. That will not induce a current to flow in nearby conductive materials, such as a conduit, unless there is a change in the magnetic field. The change need not be in direction, such as you get in an AC system. A change in magnitude will also induce a current in nearby conductive materials. I don't know anything about the equipment you are addressing, so I don't know if the magnitude of the induced current would be high enough to cause a significant problem.

 

[BretHeilig]

Thanks for the input gents. To clarify two things:

- The conductors are always together when they're going on a bona fide raceway run. The specific problem I'm having is short jumps between rows of PV modules. In each case, the total conduit length is about three feet.

- The changes in magnitude have essentially two types. One is caused by the sun moving through the sky, which of course happens very slowly. One inverter cycle the wire is carrying 6.1A, the next cycle maybe 6.06A, and so on. The other type is when the sun goes behind a cloud. Then the change can be quite sharp. 6A --> .5A, and vice versa.

But, just the same, it seems as though the best thing to do is always keep conductors together.

 

[steve66]

Thanks for the input gents. To clarify two things:

- The conductors are always together when they're going on a bona fide raceway run. The specific problem I'm having is short jumps between rows of PV modules. In each case, the total conduit length is about three feet.

- The changes in magnitude have essentially two types. One is caused by the sun moving through the sky, which of course happens very slowly. One inverter cycle the wire is carrying 6.1A, the next cycle maybe 6.06A, and so on. The other type is when the sun goes behind a cloud. Then the change can be quite sharp. 6A --> .5A, and vice versa.

But, just the same, it seems as though the best thing to do is always keep conductors together.

Those currents are very small compared to the currents that one would normally think of causing inductive heating problems. And you're rate of change is also going to be very small compared to a 60 Hz. AC that makes a full swing from a positive peak to a negative peak (and back again) 60 times per second.

I wouldn't worry at all about induction.

But on the other hand, I would worry about following the NEC, and I'll take Iwire's word for it that its not allowed.

 

[dicklaxt]

I was always instructed to always have a return path in DC to cancel out whatever may buildup,,,,,,so I always did High current flow possibilities should be routed in PVC even protective kick pipes on grounding leads,error to overkill is a good thought on this subject IMO,may be wrong but I will keep designing things that way.

dick

 

[broadgage]

There will be no inductive effects at all with steady DC, and insignificant effects with slowly changing values of DC.
The sun going behind a cloud takes several seconds and is a very slow change compared to AC line fequencies.

As regards the NEC requireing all conductors of a circuit to be in the same cable or raceway, this is needless on DC, but should still be followed.

However I dont see how in practice code can be followed with any common PV installation.
All common PV installations use single cables either weatherproof types, or installed in conduit, to connect the modules in series and to connect the ends of the series string to the rest of the installation.
Such installations are now very popular, and I have not heard that the normal way of wireing them is a violation.
Could the whole PV array be considered an appliance ? or a listed and approved assembly ? the internal wireing of which is outside the remit of the NEC.
And what about large battery strings, they unavoidably use single cables or other conductors to link the cells in series, and to connect the battery to the load.

 

[iwire]

However I dont see how in practice code can be followed with any common PV installation.

This rule allows it within the array, once you start heading out of the array it is back to our normal rules.

690.31(B) Single-Conductor Cable. Single-conductor cable type
USE-2, and single-conductor cable listed and labeled as
photovoltaic (PV) wire shall be permitted in exposed outdoor
locations in photovoltaic source circuits for photovoltaic
module interconnections within the photovoltaic array.