engineer argument about xfmr fusing secondary

[wyreman]

We are doing a lot of building here and we are commissioning a new 160ton chiller. There is only 277/480 at the pad and there was a 1000VA for three convenience outlets and a few lights. Well that 1000VA xfmr got overloaded and stopped so they replace it with a 3000VA xfmr. There is no secondary fusing and the engineer doesn't want it.

To me the old 1000VA xfmr was like a fusible link.

My view is that the 20A CB is protecting the little wire going in the xfmr but there should be a big wire leaving the xfmr. If not, there should be a fusible link.

The new 120v secondary can carry all the amps the 277v primary will supply. 277v @ 20A = 5540VA
5540VA / 120V = 46A
The devices and the wire are rated for 20A.

The distribution is split evenly left and right leaving the xfmr.
I say he needs two [2] 20A fuses, one to the left and one to the right.
How can I prove this?
Here's some fotos.

 

[raider1]

The secondary conductors of the transformer must be protected from overcurrent in accordance with one of the methods provided in 240.21(C).

Chris

 

[david luchini]

A couple of questions:

Is the 3000VA xfmr connected as a 120V secondary or a 120/240V secondary? The pictures show a 277-120/240V transformer. I suppose it can be connected as either 120/240, or 120 only. If it is 120/240, then you will require protection of the secondary conductors.

If it is 120V only on the secondary, then you can do without the secondary OCPD only for certain conditions. First, the primary OCPD must meet the primary only protection in T450.3(B). That is 125% of the rated primary current. The primary has a rated current of 10.8A, so the max primary only OCPD would be 15A. By using the 20A OCPD on the primary, you would need a secondary OCPD.

The second is for the secondary conductors to be protected by the primary OCPD, the rating of the primary OCPD cannot exceed the ampacity of the secondary conductors multiplied by the transformer secondary-to-primary ratio. So if the primary OCPD was reduced to 15A (as required by 450.3(B)) the secondary conductors would need to have an ampacity of at least 35Amps. (35x120/277=15.1) So the secondary conductors would have to be #8 (or #10 if you can use the 75 deg rating.)

So it sounds like your existing installation without secondary OCPD would violate the code.

 

[wyreman]

ground rod should be connected to the neutral at the little xfmr.

ground rod should be connected to the neutral at the little xfmr.

My boss thinks a ground rod should be connected to the neutral at the little xfmr.
I can't believe this is true because there should only be one neutral/ground connection and that is at the service and this definitely is not the service.
But for sure the equipment bonding shown in the photos could be improved, improve that flex and check for a green wire...

While we are at it
No one believes me that a green wire is not required in an accessible surface raceway system of EMT in the case in question.
Some say it is in the California Electrical Code which we are under.

Others say - it is just because the engineer says so - whether or not he spec'd it in the construction documents.

So, a green wire is required in [California jurisdiction] EMT if it is not spec'd in the prints?
thanx

 

[raider1]

The transformer needs to be grounded and bonded in accordance with 250.30. This would include connecting the secondary of the transformer to the grounding electrode system.

Chris

 

[wyreman]

Does that mean it's own ground rod?

 

[raider1]

Does that mean it's own ground rod?

No, that means that the secondary must be connected to the building grounding electrode system.

250.30(A)(7) gives you the requirement for the grounding electrode that the secondary must be connected to.

Chris

 

[wyreman]

Sorry

So, in this case, the green #12 from the 277v primary bonds
to the secondary neutral
and to the case
and to the load equipment grounds
all with #12

If a ground rod was desired, it would have to be connected to the service ground and done in the size wire of the service grounding electrode system?

 

[david luchini]

Sorry

So, in this case, the green #12 from the 277v primary bonds
to the secondary neutral

No, the secondary neutral should connect to one of the grounding electrodes in 250.30, as Chris mentioned. I'm going by memory, but I believe the electrode would be the nearest of effectively grounded building steel or water pipe.

 

[wyreman]

No, the secondary neutral should connect to one of the grounding electrodes in 250.30, as Chris mentioned. I'm going by memory, but I believe the electrode would be the nearest of effectively grounded building steel or water pipe.

what size is the grounding wire then?
sized for the 20A @277 or the 46A@120

you could drive a new ground rod if you bond it to the system grounding electrode with proper size wire..
all the way back to the grounding buss.

 

[augie47]

In terms of transformer protection (Art 450) if you are protecting the transformer 277 primary with a 20 amp breaker that's greater than 125% so you must have secondary protection at 125% {see 450.3(B)}

In terms of conductor protection, as Chris stated, you must follow 240.21(C)

The secondary must be grounded following the guidelines in 250.30.
Based on 250.30(C) and 250.66, I would say you need a #8 GEC to one of the electrodes listed in 250.30(A)(7)

 

[petersonra]

How hard is it to add some fuses if you think they are needed?

 

[mxslick]

Sorry to bust your chops, but based on these two photos, a ground wire and fusing isn't what I'd red-tag this install for:

and:

Apparently there are either no locknuts or they are not fully seated.

If you went through the trouble of replacing the transformer, why wasn't the sealtite replaced with ones of proper length to reach as well?

And I'm not 100% on this, but the other pics show no support for the short lengths of sealtite either. (IIRC if under 3 feet it does not require support.)