Interlocking as to not overload a 75kva

[justin13me]

The primary 600V is fused at 90A; there is a load (per phase) of 60A or 173A on the 120/208 side (60x2.88 turns ratio xfmr).
There's actually 2 of these loads BUT only one is on at a time, 173A on then off and the other load/ room 173A on then off and so on.
Does a time clock count as an interlock?
By code I'm sure/ believe that this is allowed as long as it is physically impossible for both loads to be on at once. NEC and CEC feedback appreciated. Thanks.

 

[augie47]

As far as protecting the transformer, the 90 amp OCP protects your transformer from overload well within the 125% as allowed by Art 450.3. I see no requirement for any secondary interlock. With the 125% primary protection you can connect up to 6 secondary loads without concern for the transformer.

 

[petersonra]

The primary 600V is fused at 90A; there is a load (per phase) of 60A or 173A on the 120/208 side (60x2.88 turns ratio xfmr).
There's actually 2 of these loads BUT only one is on at a time, 173A on then off and the other load/ room 173A on then off and so on.
Does a time clock count as an interlock?
By code I'm sure/ believe that this is allowed as long as it is physically impossible for both loads to be on at once. NEC and CEC feedback appreciated. Thanks.

there is no provision in the code for sizing a xfmr.

you can make it as small or as large as you like regardless of the load.

you have to provide appropriate overcurrent protection.

if you use draw too much current the OCPD will open up.

 

[justin13me]

I'm trying to use two timers to turn on 2 loads from 1 transformer, one load at a time. One load turns on, stays on for (let's say) 20 hrs and then off and then half an hour later the other load turns on for same amount of time then off and half an hr later other load on etc. The loads on either time clock are actually 35 circuits w 70 electronic ballasts..

 

[justin13me]

So I guess the question now is how do I wire this?

 

[ptonsparky]

One cheap time clock controlling a DP interposing relay. The relay controls two contactors large enough for your loads. The NC contact controls Load 1, the NO controls Load 2. Loads will alternate as the time clock cycles.

I would use a small PLC with an HMI but I like to over complicate and give more options.

Replace the DP with a TDR(s)to give you the delay you want.

 

[justin13me]

One cheap time clock controlling a DP interposing relay. The relay controls two contactors large enough for your loads. The NC contact controls Load 1, the NO controls Load 2. Loads will alternate as the time clock cycles.

I would use a small PLC with an HMI but I like to over complicate and give more options.

Replace the DP with a TDR(s)to give you the delay you want.

I thought about the PLC option too, I don't think it's over complicated. I'm only (a little) familiar w Allen Bradley though and concerned it may be an expensive option. Timers and relay suggested sound good too. Thx

 

[ptonsparky]

AB is generally expensive although they have a new small one that is more cost effective. An 810 is what I have sitting in my basement waiting for me to learn how to program it.

CLICK from AD works well for most my small projects.

 

[ptonsparky]

Simple prog:
View attachment 17219

 

[Smart $]

I thought about the PLC option too, I don't think it's over complicated. I'm only (a little) familiar w Allen Bradley though and concerned it may be an expensive option. Timers and relay suggested sound good too. Thx

Depending on who you talk to, driving two relays with timers or a PLC does not interlock out the loads unless you use:

• a mechanical relay interlock, or
• electrically use a NC contact (auxiliary or otherwise)

...to completely disable the other relay while the one is in operation.

 

[ptonsparky]

Depending on who you talk to, driving two relays with timers or a PLC does not interlock out the loads unless you use:

• a mechanical relay interlock, or
• electrically use a NC contact (auxiliary or otherwise)

...to completely disable the other relay while the one is in operation.

Simple interlock in the logic works but if you must guarantee it, yes.

 

[Smart $]

Simple interlock in the logic works but if you must guarantee it, yes.

Programmable electronics have been known to lose their programming from time to time... then there's no interlock until reprogrammed. During that time, you may have an irreversible equipment catastrophe... or worse. That's perhaps an extremist point of view... but it is valid.

 

[ptonsparky]

Programmable electronics have been known to lose their programming from time to time... then there's no interlock until reprogrammed. During that time, you may have an irreversible equipment catastrophe... or worse. That's perhaps an extremist point of view... but it is valid.

I've also seen broken mechanical interlocks, welded contacts, etc. Keep putting in redundant checks until Iwire calls ENOUGH already!! An extremists point of view but valid.

 

[Smart $]

I've also seen broken mechanical interlocks, welded contacts, etc. Keep putting in redundant checks until Iwire calls ENOUGH already!! An extremists point of view but valid.

 

[iwire]

So I guess the question now is how do I wire this?

I am going to point out that the NEC does not require an interlock.

220.60 Noncoincident Loads. Where it is unlikely that two or more noncoincident loads will be in use simulta-neously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.

So if you don't expect the loads to run at one time ....

 

[Smart $]

I am going to point out that the NEC does not require an interlock.

So if you don't expect the loads to run at one time ....

Fair point. As I said earlier...

Depending on who you talk to...

Another possibility is to power via one OCPD. Let timer/programming handle load diversity... but if both loads are incidentally powered the OCPD will trip.

 

[iwire]

Fair point. As I said earlier...

And I do not think it maters who you talk to.

'Unlikely' has a different meaning then 'cannot'

NEC = minimum, we can always choose to go further.

 

[Smart $]

And I do not think it maters who you talk to.
...

We would not be having this discussion if it didn't matter who you talk to. Granted the requirement doesn't change, but its interpretation most certainly does. :happyyes:hmy:

 

[iwire]

We would not be having this discussion if it didn't matter who you talk to. Granted the requirement doesn't change, but its interpretation most certainly does. :happyyes:hmy:

Does unlikely have a diffrent meaning than cannot?

If so it should not mater who you talk to.

 

[JFletcher]

Does unlikely have a diffrent meaning than cannot?

Yes. Unlikely = not probable to happen. Cannot = not possible to happen.

ofc that is assuming failsafe engineering and proper use of the terms in the first place.

It is unlikely that my home will burn down today due to the failure of a FPE breaker.

It would be impossible for my home to burn down due to electrical failure if I did not have an electrical service. I realize there are some incredibly far-fetched scenarios where this may happen, but for all intents and purposes it is so far beyond the realm of possibility as to be 0.

eta: interlocks are not intrinsically failsafe. I dealt with interlocked equipment for many years, and while they worked, you were not allowed to turn off a breaker to say the ash screw to the bin (last eqpt in the long chain of eqpt than must run) and use its interlock to shut off the bucket elevator; if you were working on the bucket elevator, its breaker must be secured.

would a lead/lag system work for the OP?