Perplexed by the Code? NEC Requirements for Transformer Ventilation Clearances

Courtesy of

All questions and answers are based on the NEC 2020.

Q What are the NEC requirements for transformer ventilation clearances?

A. Transformers must be installed according to the manufacturer’s instructions and their ventilation openings must not be blocked [Sec. 110.3(B)]. The required clearances to the ventilation opening must be clearly marked on the transformer (Fig. 1).

Q What is the NEC rule for means of disconnection on the supply side of a transformer?

A. A transformer disconnect switch within sight of the transformer is required, unless the location of the disconnect switch is field marked on the transformer and the disconnect switch can be locked in the open position with provisions for the lock to remain in place, whether the lock is installed or not. not (Sec. 110.25 and Sec. 450.14), as noted in Figure 2.

“Within sight” means it is visible and no more than 50 feet from the location of the equipment [Art. 100].

Q What types of battery interconnections are permitted by the Code for storage batteries?

A. Flexible cables, as identified in Art. 400, in sizes 2/0 AWG and larger are permitted in the battery enclosure from the battery terminals to a nearby junction box where they must be connected to an approved wiring method. Flexible battery cables are also permitted between batteries and cells inside the battery enclosure. These cables must be listed and identified for the environmental conditions. Fine stranded flexible cables may only be used with terminals, lugs, devices or connectors according to Sec. 110.14 [Sec. 480.12].

These brackets are supplied to us by Mike Holt Enterprises in Leesburg, Florida. To view the Code training materials offered by this company, go to

About Donald Martin

Check Also

WA COVID wave leaves respiratory emergencies actually lower than before

The vast majority died in aged care or hospice facilities, with a median age of …