Smaller Size Sub-Panels Connected to Larger Transformer

Samardas

Member
I have a situation where the owner has a 480V, 3 Phase, 30KVA transformer and a 60A disconnect already installed (existing). I think the primary side disconnect is ok, although a bit higher than needed (~50A). However, he wants to install a very small sub-panel, maybe, <=20A. Is it okay to attach a <=20A sub-panel to a 30 KVA transformer which has a output capacity of ~100A? This is a light industrial/commercial facility where he wants to connect a couple of lights and a couple of receptacles (no more than 3-4 Amps) for rental purposes. The tenant will eventually install proper size panels etc when they move in. Thanks a LOT for your assistance.
 

GoldDigger

Moderator
Staff member
The wires from the transformer secondary to the main breaker of the small panel would be considered tap conductors and would have to follow one of the tap rules in sizing.

BTW, are you saying that there is no secondary protection on the transformer?
The NEC allows the primary OCPD to protect the secondary windings in some cases, but that protection does not extend to the wires which are then attached to the transformer secondary.
 

Samardas

Member
Smaller Size Sub-Panels Connected to Larger Transformer

Hello GoldDigger, Thanks for your quick response. The transformer secondary is not protected. The panel it will be connected to is MLO and is about 50 feet away in a visible area. Since this arrangement is temporary for the owner to show the property to a tenant, his intention is to attach a smallest panel size (hence conduit/cable) possible to connect a couple of receptacles and lights. Based on the tap rule (NEC 240), other than the distance limitation between the disconnects, etc., is there a secondary protection mandatory? If so, can a secondary disconnect switch, say, 20A before a 20A panel be okay (even though transformer secondary is around 100A)? Thanks & Regards, Samar
 

GoldDigger

Moderator
Staff member
Your best bet, in my opinion, would be to temporarily mount a fused disconnect right at the transformer to protect the feed to the temporary panel.
Longer term you will need permanent protection right at the transformer.
50 feet is too long for an inside tap and running full current sized wires would drill require protection at the upstream end.
 

GoldDigger

Moderator
Staff member
Your best bet, in my opinion, would be to temporarily mount a fused disconnect right at the transformer to protect the feed to the temporary panel.
Longer term you will need permanent protection right at the transformer.
50 feet is too long for an inside tap and running full current sized wires would drill require protection at the upstream end.
drill --> still
Dangnabbed ought-to-correct
 
Wouldnt 240.21 C 4 apply in this condition?

Outside transformer secondary conductors can be installed in any length if they are protected from physical damage and have an ampacity large enough to feed the load on the secondary conductors, or have a rating equal to or greater than a single overcurrent device at the termination of the secondary conductors. The single overcurrent device must be part of a disconnecting means or right next to one. The disconnecting means for the outside transformer secondary conductors must be readily accessible either outside the building or inside nearest the point of entrance.


 
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