Motor & signal cables, trench sharing 2

[bentov]

So, I have 480V 3 phase motors, 120V single phase motors, 120V control circuits, 4-20mA signal circuits, maybe phone & ethernet cables, etc., need to install underground runs for all. Is there a physical trench separation recommendation when all are in PVC?

 

[dpc]

There's no NEC restriction, but a recommendation would be to keep the signal circuits (phone, 4-20ma, Ethernet) as far away as possible from the power circuits. A 12-inch minimum separation would be good, if you can manage that.

The 480V and 120V circuits can all go in the same conduit (provided you use 600V insulation for everything), but it's generally best to separate the 480 V from the 120 V if practical.

Use shielded cable and twisted pairs for all the signal circuits.

Of course, you could run fiber, then all your separation issues go away.

 

[buzzp]

I agree with DPC, this is best practice, although not always practical.
This has been discussed several times in multiple threads. The search function should find some more discussion.

 

[mc5w]

Except for Colorado. Colorado has an antiquated law that prohibits installing 2 different voltages in the same conduit. Primarily based on several factors that may or may not be valid for your system:

1. A 480 volt ungrounded system can have up to 2,000 volts of static charge or capacitively induced AC voltage on it. This law seems to predate 277Y480 volts solidly grounded or resistance grounded.

2. Some makes of circuit breakers, such as Federal Pacific would develope arthritis after a few years and refuse to trip. In some cases turning off a breaker for outlet replacement would restore normal operation. Then, the customer would discover that they have 30 amps of load on a 20 amp circuit. An upgrade to 2 circuits put more money in my pocket! All of these manufacturers have gone out of business or have been tossed out of the United States by the insurance companies.

3. Back in the olden days ALL insulation systems were flammable.

4. SquareD is one of the few makes of circuit breakers where 120 volt circuit breakers have an unofficial 277 volt rating even if that is NOT stamped on the circuit breaker. 200 amp class J time delay fuses in the 277Y480 volts feed also help.

5. Class J fuses had not been invented yet.

I have no objection to putting a 20 amp 120 volt circuit and a 45 amp 480 volt circuit off of 277Y480 solidly grounded in the same conduit. In fact, a forklift truck driver sheared of one such power feed - both circuit breakers tripped and two of the class J fuses in the feeder blew.

 

[buzzp]

I just wanted to add something about comment number 4 concerning the higher 'rated' than whats stamped on the breaker. If it is a UL breaker, only the ratings stamped on the breaker are applicable from UL's perspective. Even if SquareD's CB has passed at the rating from UL, it is not rated unless its stamped. I can't imagine a company having a higher rated device (more market) and not stamping it on the device. I imagine some inspectors would have fun with this one. Interesting.

 

[mc5w]

The unofficial rating is more of a hidden safety factor (paranoia) against what can go wrong. Some (cheap) circuit breakers are only useful in a residential environment because they have no safety factor against power cross with a higher voltage. UL is not concerned with a hidden safety factor against power crosses. An unofficial rating is one for which the manufacturer has NOT applied when obtaining UL listing but rather that the circuit breaker can survive a power cross when the higher voltage circuit also has reasonable overcurrent protection.

SquareD's QYU circuit breakers have both a 120 volt rating and a 277 volt rating. They are the same design concept as the QO circuit breakers except for mounting method and that both ends of a QYU circuit breaker can have box lugs.

The same goes for 48 volt DC rated QO circuit breakers. Only certain sizes of circuit breakers have this listing because at higher ratings you really do need an I-line circuit breaker because typically a high ampere panelboard bus is needed.

 

[buzzp]

I guess in any case, you better not use this particular SquareD breaker in applications above what is stamped on the device. Bound to be asking for trouble, at least from any inspectors point of view.
Maybe your not saying to do this, merely pointing out this particular breaker is more robust than most others with similar, official ratings.

 

[mc5w]

But when there is a power cross you have 2 circuit breakers in series and one of them IS rated for the higher voltage. If there is a class J fuse in the higher voltage then things are a little better. This is just a matter that the lower voltage circuit breaker has above average ruggedness so that you can put 2 different voltages in the same conduit and only have to worry a certain amount as to what happens with forklift drivers and backhoe operators.

Also, certain SquareD QO circuit breakers are rated for use on 240 volt corner grounded systems which is almost as hard on circuit breakers as 277 volts. The higher official rating is just a matter of stronger plastics and so forth.